Termites Wood Destroying Fungi and Mold


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Termites are true social insects. They not only live in colonies, but divide labor so that different forms or castes carry out prescribed duties for the good of the entire colony. No individual termite can survive alone. These insects can be considered beneficial or harmful depending upon one’s frame of reference. Worldwide, they are extremely beneficial as decomposers of dead trees, leaves and other sources of cellulose. On the other hand, they are major pests on agricultural crops, forest nursery seedlings, rangeland grasses, stored products, and household furniture. Most importantly, they are of extreme importance as structural pests worldwide.

Biology. From an evolutionary standpoint these insects are considered somewhat primitive, but social organization is well developed in termites. Along with ants and the more highly organized bees and wasps, they belong to the truly Eusocail insects.

The common traits of Eusocial insects include:

1) Cooperative caring for the young.
2) A division of labor with different forms carrying out different functions for the overall good of the colony.
3) An overlap of at least two generations so that at some point the offspring can assist their parents in colony development.

By necessity, in order to develop colonies, social insects must be long-lived. One of the limiting factors to accomplish longevity is a continuous source of food. Of course termites feed on one of the two most common sources of food in the world, namely plants (as opposed to animals) or, more specifically, cellulose. 

The two major families of termites in the U.S. (U.S.) are Kalotermitidae (drywood termites) and Rhinotermitidae (subterranean termites). Both depend on protozoan in their hindguts to digest cellulose. Termites do not possess these protozoa at birth, but acquire them by proctodeal feeding or more simply feeding on fluids from the anus of mature individuals. Because the lining of the hindgut (including the protozoans) is shed with molting, newly molted individuals reacquire the protozoans by proctodeal feeding. This feeding activity by termites in general (including feeding from another termites mouth-stomodeal feeding) is referred to as tropholaxsis. It is the main means of communication between termites in a colony. It has multiple functions including efficient use of nutrients, recognition of nest mates, distributions of chemicals involved in caste regulation and as previously indicated the transfer of microbes used to digest cellulose. As previously indicated, termites are true social insects and have a very advanced social system with different forms (castes) carrying out different functions. The castes in a mature termite colony are discussed below.

Reproductives: Queen/King. The sole function of the queen is to lay eggs. She is quite long-lived and, in some African species, the queen may lay up to 10,000 eggs a day for 30 years. She basically is a big egg laying machine with a huge abdomen and engorged ovaries. The queen termite is so large and heavy bodied that she is incapable of moving on her own. If there is a need to change her location, the workers will line up on one side and roll her like a pencil. As the eggs exude from her abdomen, they are carried away by worker termites that also continuously feed her. There is a king which is diminutive when compared to the queen. How he mates must be private as there is little in the literature about him. It must be difficult.

A queen of an African termite is capable of laying 10,000 eggs a day for 30 years. Image
courtesy of Entomart.

Supplementary or Substitute Reproductives. There are usually two types of this caste which are also known as neotenics. These may be either lightly pigmented with short wing pads (brachypterous) or very lightly pigmented with no wingpads (apterous). As their names imply, they can lay eggs under certain conditions depending on species.

Replacement reproductives of subterranean termites. Image courtesy of Jim Kalisch, Dept. Entomology, University of Nebraska.

Soldier Termites. Soldier termites are easily recognized by their relatively large heads and protruding mandibles. The soldiers functions to protect the colony against their chief enemies the ants. In the mound, building termites of Africa, Asia and other tropical countries the world, soldiers have funnel-shaped heads and can squirt a sticky substance for protection; these soldiers are called nasutes.

IMG_4385termites, Nasutitermes spp.  (Isoptera: Termitidae)
Left: A soldier subterranean termite with disproportionately large head. Image Dr. Kaae.Right: A nasute worker. Images courtesy Whitney Cranshaw,
Colorado State Univ., Bugwood.

Worker Caste. The worker caste comprises the overwhelming majority of termites in a colony. As their name implies, they perform all the work in a given colony. This can include foraging for food (cellulose), care and feeding of the young and soldiers, excavating their tunnels or galleries and ministering the queen.

An army of subterranean termite workers. Image Dr. Kaae.

Alates. IIn the spring and or fall, depending on the species, a mature termite colony may form a large number of alate (winged) termites. These are the potential kings and queens of new colonies. Very frequently, following the first seasonal rain, these alates will swarm, or fly, from the colony. The significance of swarming following the first rain of the season is several-fold. Termites are soft-bodied and susceptible to desiccation; therefore, moisture is conducive to survival. It is also important that most of the colonies in an area swarm at the same time. This insures that mates will be found, and there will be cross mating between colonies to prevent inbreeding. Finally, the rain softens the soil (in the case of subterranean termites) and enhances the chances of some species forming new colonies below ground.

Swarming termites—these potentially will become the kings and queens of new termite colonies. Image compliments of Ganesh Subramaniam - Flickr CC BY-SA 2.0

Hundreds of alates will leave a mature colony during swarming. They are weak fliers and many are lost to birds, ants and other predators. Because they are weak fliers, they do not fly great distances before settling down. Once a female alate leaves, she releases a sex pheromone from her abdomen that attracts an alate male. After mating, they both break off their wings that are no longer needed and burrow into the ground or into a wooden structure to start a new colony. Then they form a small cavity within which the female will lay several eggs. For the first year, the king and queen do not eat but dissolve their wing muscles to produce nutrients that they feed to the nymphs. Once these nymphs develop within the first year, they take on work duties. Colony growth is slow and there are no signs of this new colony in or around a structure for the first two to three years after initial infestation.

The alates are the most likely caste someone is likely to see. Therefore, it is to the advantage of the professional pest control operator to be able to distinguish and teach the differences in appearance of termite alates and carpenter. Both are similarly appearing insects that can also swarm. Even though they may seem similar, there are very distinct differences in the morphology of the two.

The order of termites is Isoptera. Iso in Latin means equal and of course ptera refers to wings. In winged termites, the first and second pair of wings is of equal size and shape, and in winged ants the second pair of wings is much smaller than the first. Also with termites there are many more veins in the wings and when at rest the wings are held flat over the back. In addition, the wings of termites are very large and extend way past the tip of the abdomen. Ant wings only project slightly past the abdomen. Ants have elbowed antennae and termites have bead-like antennae. Finally, in ants the thorax is joined to the abdomen by a narrow stalk or petiole and that of termites is broadly joined. I am sure that is more than you ever wanted to know about the morphology or structure of these two groups of insects.

A carpenter (left) ant with elbowed antennae, petiole, and unequal wings. An alate termite (right) with beadedantennae, broad waste and equal sized wings that lay flat over the back. Images courtesy of Jim Kalisch, University of Nebraska Entomology.

Drywood Termites. Drywood termites (Family Kalotermitidae) establish their colonies in non-decayed wood with relatively little moisture content (generally 2.8% to 3% water content). Unlike subterranean termites, their colonies never require contact with the ground. There are 16 species of Kalotermitidae in the U.S. The most important in the West of are Cryptotermes brevis-the powderpost termite, Marginitermes hubarderi-the desert drywood termite and Incistermes minor-the western drywood termite.

Western Drywood Termite. The western drywood termite is by far the most common and damaging of the three above mentioned species

The eggs of this species hatch in 30 to 60 days depending on temperature. There are a total of seven nymphal instars. With drywood termites, there is no worker caste and the nymphs carry out the work duties. The first three nymphal instars of drywood termites remain undetermined. Once the 4th through the seventh instars are reached, they can eventually become either replacement reproductives, or soldiers, depending on colony need.  

The drywood termite can be identified in the alate form by its reddish head and thorax and light black wings. Mature colonies of drywood termites are relatively small and rarely number more than 5,000 individuals. Their colonies are almost always are found above ground, located inside wooden sections of the house or structure and more typically in attics and upper areas. Colonies of this species develop quite slowly. A one-year old infestation typically contains 6 to 40 nymphs, one soldier and of course the king and queen. Three, four and 15-year old colonies will typically have 40 to 165 individuals with three soldiers, 70 to 700 individuals with six soldiers, and 2,350 to 2,750 individuals with 10 to 14 soldiers respectively. Alates typically are only produced after the fourth year. It is believed that the primary queen reaches her maximum egg laying capacity at about 10 to 12 years. After that point, she declines quite rapidly and secondary or replacement queens will soon take her place.

As previously indicated, colonies of drywood termites tend to be relatively small when compared to those of subterranean termites. Correspondingly, less rapid and sever damage may result from a colony of this type of termites when compared to those of subterranean termites. However, proliferation of numerous colonies in a structure can result in severe damage.

A colony of drywood termites including the red alates with dark wings. Image compliments of Department of Entomology, University of California at Riverside.

There are a number of signs or symptoms of the presence of drywood termites. Of course, the homeowner is the likely to actually only see the swarming alates. Swarming of this species typically occurs in the fall months (September, October) and more often than not on a sunny day with temperature around 80ºF. Of course, it is not uncommon for a number of these weak fliers to be caught in spider webs which can be a useful sign of an infestation.

The common indication of the presence of this termite is piled or scattered brown fecal pellets below infested wood. Other than these fecal pellets, there is little evidence of their presence on the outside of the infested wood. The pellets are kicked out of the termite galleries through small “kick holes” (about the size of a BB). The pellets typically are rectangular in shape with rounded ends and six flattened or depressed surfaces. Longitudinal ridges occur at the angle between the six surfaces

In addition, there are frequently many termite wings in an infested area. Of course, these are due to the previous presence of alates which broke off their wings soon after mating. It is quite easy to tell the type of termite that has been present by the galleries or tunnels inside the wood. Drywood termites feeding galleries tend to cross the annual growth ring of wood while subterranean termites do not.

Cross section of a 2 x 4 with western drywood termite galleries crossing the annual growth rings. In addition, galleries are clean and free of soil. Image Dr. Kaae

Powder-post termites (Cryptotermes brevis). This species is well established in Hawaii, Florida and Louisiana. Like other species of drywood termites, they can infest wood with very low moisture content and require no contact with the ground. Powder-post termites can infest homes but are best known for their infestations in furniture and other small wooden products. Although the pest control operator is not likely to encounter these in California, it is worthwhile to be aware of their existence as occasionally they are imported from tropical countries in wooden objects.

Desert drywood termite (Marginitermes hubbardi). This termite replaces the western drywood termite in the more arid desert regions of southeastern California, Arizona and northern Mexico. The alates are yellow to brown and about the same size as their more common counterpart. The alates emerge at night, usually just after a rain and are attracted to lights in huge numbers.

Drywood Termite Control. There are several techniques that are used to treat infestations of the western drywood termite. These can be broadly categorized as whole structure treatment and local treatment. The advantage of whole structure treatment is that it greatly increases the chances of total elimination of drywood termites from a structure. If properly applied, fumigation and heat treatment will typically result in 100% control of drywoods and many other insect pests in the structure. Of course, both of these treatments are considerably more expensive than any of the local treatments. Also, neither of these gives any residual control.

Local treatments are less expensive than whole structure treatment and can vary considerably in effectiveness. One limitation of these types of treatment is that even with great inspection techniques, it is quite possible that deep seated infestations cannot be found and therefore not treated.

Microwaves. Microwave generators are mounted against walls on a pole. As with the common microwaves that are found in almost every home, these machines produce a penetrating heat which in this case is lethal to termites (basically the waves cook the termites inside the wood). These machines typically are used for local infestations with each treatment limited to small areas. As a result, the poles and microwave machines are moved to the next area after each treatment. A disadvantage is that detection is critical to success and in some situations it is very difficult to locate deep seated infestations. In addition, microwave treatment may damage certain types of surfaces and other materials. This is especially true with high voltage machines. The main advantage when compared to localized chemical control of course is that there are no pesticides used. A factor that is attractive to some individuals.

Electricity. High voltage electricity or electrocution is an additional nonchemical option for controlling drywood termites. In this case, infested damaged wood is exposed. Then an Electro-Gun is placed on one side and ground on the other side of the infested timber. With use of electrical shock of low current (~0.5 amps), high voltage (90,000 volts), and high frequency (60,000 cycles), it passes through termite galleries and ultimately ends at the ground. Death results from electric shock or delayed mortality from the destruction of the termites' intestinal protozoa. Besides the limitations of all localized treatments, the efficiency of this type of treatment may be reduced by interference of a number of standard building materials such as glass, concrete and metal. In addition, if drill holes are required to reach infested, wood damage may occur to wall coverings and structural wood.

Freezing. Liquid nitrogen is pumped into the infected area chilling it down to -20 degrees (F) and resultantly freezing the termites. This is not practical for treatment of large areas, or near window glass (can shatter glass). Tarps are used for larger areas like porches. Liquid nitrogen has no residual activity when used alone. Minor damage to the structure occurs from the holes drilled for spot liquid nitrogen insertion.

Drill and Treat with Chemicals. This type of application has been a standard for nearly 100 years in the U.s. In this case, insecticide is injected into small holes that have been drilled through the infested into termite galleries. As a result, the pesticide is subsequently delivered directly to the pest population. This is the simplest and most direct method of treatment. The amount of drilling required and the effectiveness of this treatment depend on the chemical used and the nature of the infestation. Most chemicals will remain active in the wood after treatment to protect against resurgent colonies. This is the most effective of the local means of treating drywood termites.

Heat Treatment. Heat treatment is a source of partial or complete building control for drywood termites. In the case of complete control, nylon tarps are used to tent the building. Normally, materials that are not heat resistant are removed from the building and water is left running to protect plastic pipes. With this technique, propane heating units blow hot air in and around to heat the structure to 120ºF several hours to 130ºF for a few hours. Whole structure heat treatment is attractive to certain homeowners who object to the use of chemicals. In addition, the process can be accomplished in a short period of time (hours) instead of days as seen with fumigation. Parts of structures (e.g.-apartments and condominiums) can be treated as opposed to fumigation where this cannot be accomplished. The major drawbacks of heat treatments include the difficulty in raising the internal core temperature of large structural beams that are infested and heat sinks. These are areas within the structure that are difficult to heat, such as wood on concrete or tile. In addition, materials such as plastics (e.g. electrical outlet covers), cable wiring and other sensitive material may be harmed by high temperatures.

Tent Fumigation. Sulfuryl fluoride (Vikane) is the highly toxic, colorless, odorless gas that is used for fumigation. Since it is odorless, a low percentage of chloropicrin (tear gas) is added to the main fumigant as a marker or indicator of the presence of the gas. Fumigation (tent fumigation) with this material is highly effective when properly applied. An important aspect which increases the chances of successful fumigation is monitoring the distribution of the gas within the treated structure. This is accomplished by installing gas monitoring lines inside the structure undergoing treatment. Non-monitored fumigation may not have enough gas concentration to kill infestations and failures may occur.

Human deaths have been known to occur every year as a result of this practice. Vandals, thieves, and the homeless are the most common victims. A tented home with nobody home is a prime target for burglars. This procedure presents virtually no threat to the homeowner because Vikane has a very short residual and is gone within a short amount of time. Tear gas (choloropicrin) is mixed with Vikane as a warning agent and it will quickly alert the homeowner if any residual gas remains. Tear gas has a longer residual activity than Vikane.

Home under tent fumigation. Image courtesy of https://upload.wikimedia.org/wikipedia/commons/thumb/5/58/Tent_fumigatio... CC BY-SA 3.0

Fumigation has distinct advantages over localized treatment mainly in that it will typically eliminate infestations that are hidden from view. Major issues to consider with the use of fumigants include the difficulty of installing tarpaulins, the difficulty in determining the proper dosage, the need to protectively bag food items and the lack of residual control. In addition, the structure must be vacated for 2 to 3 days while it is being treated and subsequently ventilated. Certain types of roofs (especially tile) may be damaged by having tarpaulins dragged across them. Of course there is always the reluctance of certain individuals to use pesticides in general. It should be noted even though sulfuryl fluoride is a highly toxic pesticide, that after the required three day treatment and ventilation period there is essentially no toxic residue of the chemical left. 

Termite Inspections. Anytime a house or other structure is sold and is financed by conventional means such as FHA, VA or banks, a termite inspection is required by the lending institution. The lender needs to be sure the structure is not totally riddled with these pests, as it legally owns the house until the loan is paid off (which in many cases take as long as 40 years). If a home is inspected and termites are found, they must be controlled before the loan is granted. Any termite damage or violations of the structural building code that may lead to termite infestation must also be corrected. In some cases, the total cost of this work can be in the tens of thousands of dollars.

Subterranean Termites

Western Subterranean Termite (Reticulitermes hseperus)

This is the principle subterranean termite in the western U.S. ranging from British Columbia to Western Mexico and California to eastern Idaho and Nevada. The alates are distinguished from other common species by their black to dark brownish coloration.  Swarming typically occurs in later-winter or spring although timing can vary considerable depending on area and weather. As the name implies, colonies of these insects typically are found below ground. In some cases, the colonies may extend as much as 30 feet below the soil surface. Mature colonies tend to be very large and can contain as many as 300,000 individuals. Soil moisture is a critical factor in the environment of these insects. Subterranean termites typically are more of a problem in clay compared to sandy soil. Clay has more of a capacity to hold moisture over a longer period of time than sand.

A black subterranean termite alate. Image Dr. Kaae.

As with most termite colonies, growth is slow. Only a few eggs are deposited the first year and require an average of 50 days to hatch. Even under the best of conditions, alates do not appear in a colony until the 3rd or 4th year. The supplementary reproductive become quite prevalent and greatly accentuate growth of the colony. However, since colony growth is slow, signs of a new infestation in a building will take three to four years.

Unlike drywood termites, subterranean termite colonies typically require ground contact. Apparently, if there is sufficient food, water and warmth colonies can survive above ground, but this is rare. In most areas, the ground serves as a protective barrier against extreme temperatures and as a reservoir of moisture. Unlike drywood termites, subterranean termites are quite susceptible to dry conditions. Colonies of subterranean termites will readily change soil depth in order to seek favorable temperatures and moisture conditions.

Signs of an Infestation. In the U.S., if subterranean termites are found below a structure, they can gain access to the house by building earthen tubes (shelter tuber) or by working their way through inter-veining wood. They feed in the wood of the house but will also return periodically to the colony via the earthen tubes to replenish the moisture content of their bodies. If a house is built on a slab-type foundation, these termites can gain access to the home via cracks or holes in the slab for sewer inlets and gas fixtures. The shelter tubes also serve to protect these termites from ant predators. These shelter tubes are constructed out of sand or dirt, bits of wood and even fecal pellets. These are bonded by a glue-like material that is secreted by the worker termites.

The western subterranean termite uses at least three types of shelter tubes. Utility tubes are used to gain access to the house and run from the soil to the first wooden member of the structure and return to the soil to replenish the termites lost body moisture. These tubes are typically flattened and wide in appearance. Exploratory or migratory tubes are similar to utility tubes but typically do not extend to the wooden parts of the structure and are less sturdy and have small exit holes. As their name implies, they are used to seek out new locations of food. Suspend or drop tubes extend down from the wooden structure and may or may not reach the ground depending on their stages of construction. They are normally lighter in color than the other types of shelter tubes since more wood fiber is used in their construction. They are used by stranded termites in attempt to return to the soil.

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From left to right: Utility, exploratory and drop tubes. Image courtesy Umberto lopez.

Besides the earthen tubes, additional signs of an infestation of subterranean termite infestation in a structure include the presence of alates, wings that have broken off the swarming alates, and the presence of dark areas in infested wood and blistering of the floor. Sometimes the distinct sound of soldiers can be heard by knocking or tapping infested wood. When disturbed, this caste will violently jerk its head against the roof of the gallery and make a distinct almost ticking sound. This is thought to serve as an alarm of possible danger to other soldiers. 

Subterranean termites feed primarily on soft spring growth. As a result, the tunnels or galleries of these insect have more of a layered appearance as opposed to the random indiscriminant burrowing across and the grain of infested wood by the drywood termites. In addition, these termites tend to bring small amounts of soil into their galleries and even pack chewed wood and other sources of cellulose into the unused portions of their galleries. Because colonies of these insects are so much larger than those of drywood termites, damage can be far more rapid with subterranean termites than with drywoods.

Wood with subterranean termite gallery with layered tunnels limited to soft spring growth and packed with chewed food. Image Courtesy of University of Nebraska Entomology-Jim Kalisch.

Subterranean Termite Control

Soil Barrier Termiticides. Soil treatments rely on creating a toxic chemical barrier in the soil between a potential nest and wooden parts of the structure. Many of the older chemicals have repellent characteristics and termites avoid treated soil. To achieve termite control for long periods of time, such termiticides must be applied as a continuous barrier in the soil next to and under the foundation. If there are untreated gaps in the soil, termites may circumvent the chemical treatment. Hence, such treatments during preconstruction can provide for more uniform coverage. Once a home is constructed, the chemical has to be injected through drill holes and trenching around the foundation. This process can result in less accurate coverage.

Spot treatment refers using barrier termiticides only in those areas of the structure where termites have been detected. Many pest management firms avoid or will not guarantee such treatments since termites are capable of finding other untreated points of entry into the structure. Localized spot treatments are considered risky except in re-treatment situations.

Some of the newer termiticides are marketed as non-repellent to termites with delayed toxicity. As termites penetrate the "treated zone", they contact the active ingredient possessing delayed mortality. Since the termites are not immediately killed, the toxicant is thought to be passed to nest mates through grooming activities and social food exchange (trophallaxis). With this type of activity, control usually is achieved within three months. Non-repellent termiticides include fipronil (Termidor®), imidacloprid (Premise®), and chlorfenapyr (Phantom®).

Baits. Termite baits rely on wood or a cellulose matrix that is attractive to termites that are impregnated with a slow-acting pesticide. The theory is that termite workers will feed upon the bait and transfer it by trophallaxis to other colony members. The aim is to eventually reduce or eliminate the entire colony. Another advantage of using slow acting active ingredients is that termites tend to avoid sites where sick and dead termites accumulate.

Typically, in-ground stations are inserted in the soil next to the structure and near known or suspected sites of termite activity. These stations often initially contain untreated wood or other attractive materials that serve as a monitoring device. The monitoring wood is replaced with toxicant materials once termites have been detected. In addition, above ground stations may be installed inside or on the structure in the vicinity of damaged wood and shelter tubes. Above ground stations also initially contain non-toxic bait.

It is very important that bait systems are properly installed and diligently serviced. Monthly inspections of a baiting system usually are necessary with the exception of during inclement winter weather. Successful termite baiting necessitates proper monitoring and maintenance of the stations. Baits work much more slowly than soil termiticides, and the homeowner should be aware of the possibility of a lengthy baiting process. Several months or more may elapse before the termites locate stations. Termites must feed on sufficient amounts of the toxicant for the desired effect.

An often-cited advantage of termite baits is that they are "environmentally-friendly" because they use very small quantities of chemical and decrease the potential for environmental contamination. In addition, bait application causes little disruptive noise and disturbance compared to soil treatments. Furthermore, baits can be used in structures with wells or cisterns, sub-slab heating ducts, and other features that may preclude a soil treatment. Baits are often used in sensitive environments.

A number of baits have been marketed to control termites. Bait products that are available for licensed pest management professionals include the Sentricon® Termite Colony Elimination System (hexaflumuron [Recruit® II bait] or noviflumuron [Recruit® III bait]), FirstLine® Termite Defense System (sulfluramid), Exterra® Termite Interception and Baiting System (diflubenzuron [Labyrinth® bait]), Subterfuge® Termite Bait (hydramethylnon) and Outpost® Termite Bait Response (diflubenzuron). Not all of these bait systems are equally effective. It is advisable to review any independent research that has been conducted on particular baits. Some of these products have been evaluated much more rigorously than others.

Physical Barriers. Different physical barriers are particularly appropriate during the preconstruction phase to provide protection of the structure from subterranean termites. One such physical barrier is stainless-steel wire mesh (TermiMesh®) that is fitted around pipes, posts, or foundations. The newest physical barrier, Impasse® Termite System, contains a liquid termiticide (lambda-cyhalothrin) locked in between two layers of heavy plastic that is installed before the concrete slab is poured. It is supplemented with Impasse® Termite Blocker, which uses special fittings around plumbing and electrical pipes and conduits.

Biological Control Agents. A couple species of round worms (nematodes) parasitize and kill soil inhabiting termites. They are commercially available and marketed by a relatively few companies. Although they have shown some promise in laboratory condition, control in field conditions have been quite variable. This apparent lack of success may be attributed to ability of termites to recognize and wall-off infected individuals and as a result limit the spread of these predators throughout the colony. Furthermore, soil moisture and soil type appear to limit the nematode’s ability to move in the soil and locate termites.

The fungus Metarhizium anisopliae (Bio-Blast®) is a biological termiticide that requires special application and handling techniques. It is labeled for above ground application to termite infestations in structures, but it is not labeled for application to the soil. Spray effectiveness is enhanced when applied to many foraging termites because infected termites can pass on the fungus to nest mates. However, it is difficult to infect a large enough number of termites for the infection to spread throughout the colony. Furthermore, it provides no long-lasting residual activity, and the fungal spores die with the dead termites. Insufficient research has been conducted to indicate whether this is an effective method for controlling termites.

Building construction. In addition to the use of chemical barriers, proper building construction is essential to curtail infestations of these pests. States have strict building codes that are to reduce infestations of termites. A primary example of a situation that might lead to subterranean termite attack would be if any wooden part of the house were to come in direct contact with the earth. Additionally, situations such as improperly vented crawl spaces, leaking plumbing, grading of the earth that leads to water accumulation next to the foundation, or any other situations which might lead to excessive moisture below a house typically are prohibited by law. 

Mound Building Termites (Termitidae)

Mound building termites are found throughout the tropical areas of the world. At maturity, a primary queen has a great capacity to lay eggs. The queen is widely believed to be a primary source of pheromones useful in colony integration, and these are thought to be spread through shared feeding (trophallaxsis).

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A Gigantic Queen Termite. Image Courtesy of Brutalux.

The king does not grow significantly after mating but amazingly (consider in the size of the queen) continues to mate with the queen for life. This is quite different from ant colonies where the queen mates once with the male(s) dying soon after mating. The queen in this case stores the sperm for life.

The worker castes carry out the various labors including brood and nest care, food storage, foraging, and in some species defense of the colony. As with other termites, the soldier cast primarily functions to protect the colonies against attack of ants and other potential predators. Most species of Termitidae have unique soldiers (nasutes) that exude noxious liquids through either a horn-like head (nasus) or simple hole in the head (fontanelle).

termites, Nasutitermes spp.  (Isoptera: Termitidae)
Nasute Soldier Termites. Image Courtsey Whitney Cransahaw.

In order to defend against the attack of ant or other predators, soldiers frequently block tunnels in order to prevent these insects from reaching the main areas of the colony. This defense has significant built-in backups with soldiers standing behind soldiers so once the first line falls, others will take their place. In situations where an attack is coming from a gap that is larger than the individual soldier's head, defense requires numerous soldiers around the breach. An additional self-sacrificing defense is performed by South-East Asian species referred to as the tar-baby termites. In this case, the soldiers commit suicide by rupturing a large gland close to the surface of the exoskeleton. As a result, a dense, sticky, yellow fluid is released entangling ants or other insects invading their colonies.

As previous indicated, termite workers build and maintain these giant colonies. These are massive, elaborate structures built from of combination of mud, soil, saliva, feces and macerated wood/cellulose. These structures serve a variety of functions including providing a protected relatively safe location, and a source to collect water via condensation. They consist of a maze of tunnel-like galleries that additionally serve as air conditioning and control the CO2/O2 balance.

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Termite Mounds. Images from left to right Courtesy of Yewenyi at the English language Wikipedia.

Colonies are typically constructed around or on top of large pieces of wood, inside fallen trees or on top of living trees. Large above ground mounds or termitaria are frequently constructed when an above-ground nest grows beyond its initially concealing surface. In Africa and Australia, they are commonly falsely referred to as anthills.

In tropical savannas of Africa, these termiteria can reach a height of 30 feet; however, six to nine feet would be more common for the largest mounds in many savannas. The shape of these mounds vary from somewhat asymmetrical cones which are covered by grass and/or woody shrubs to well-sculptured hard earth mounds or a combination of both. Typically species identification can be determined by the shape of these mounds. In some areas of African, a high density of these mounds dominates the landscaping. As an example, in the Busanga Plain area of Zambia, relatively smaller mounds of approximately 3 feet in diameter reach a density of approximately 300 per acre.  

True/False Questions.

1. Once the sixth instar of a drywood termite is reached, it can only turn into a soldier termite.

2. Alate swarming following the first seasonal rain increases the chance of finding a mate from another colony thus mixing of gene pools.3.. Drywood termite galleries typically do not cross the annual growth rings of the wood they infest.

3. With the drill and treat method for control of the drywood termite, chemicals that have been phased out of commercial use include organophosphates, carbamates, silica-gel, and dri-die.

4. Whole house heat treatment generally takes about three days for completions and does fumigation of a house.

5. Based on relative residual activities, when Vikane and chloropicrin are simultaneously applied to a structure, if there is no smell of tear gas there should not be any Vikane left in the structure.

6. Subterranean termite colonies tend to be much larger than those of drywood termites and they cannot withstand as dry of conditions as drywood termites.


Wood Destroying Fungi

Wood Destroying Fungi .

Wood is subject to attack by both fungi and insects. these organisms infest wood in a variety of ways, including as a source of food, shelter, and nesting sites for raising their young. It is advantageous to pest control technicians to to recognize and understand the nature of these wood-attacking organisms; some are major problems and others are of minor concern. Unlike other plants, fungi are heterotrophic; this simply means they must eat, rather than manufacturing their own food via photosynthesis.

The fundamental structures of fungi are called hyphae. They are thread-like in appearance and are not individually visible to the naked eye. Deadwood conks and mushrooms are fungal fruiting bodies that produce and disseminated spores. All fungi produce spores that are distributed by wind and water and can infest moist wood during storage, processing and use.

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Fungal hyphae and conk or fruiting body. Courtesy of CDC.

Some fungi merely discolor wood, but wood-decaying fungi are capable of changing its physical and chemical properties, ultimately reducing its strength. There are many species of wood-inhabiting fungi. These are divided into 2 major groups based on the type of damage they cause; these are wood-decaying fungi  and wood-staining fungi.

All fungi have certain basic requisites and conditions under which they can become established and grow. One is favorable temperatures that usually range between 50ºF and 90ºF. The typical optimum temperature for establishment of wood infesting fungi is about 70ºF to 85ºF. There are also temperatures under which fungi cannot become established and survive in or on wood. Wood is basically safe from decay when exposed to temperatures below 35ºF and above 100ºF.

Adequate moisture is needed for fungi to grow and survive. Wood decaying fungi require a wood moisture content of around 30%. This is close to the accepted saturation point of wood. Air-dried wood typically has a water content under 20%, and lumber that is kiln-dried has a content of 5% or lower. With lumber and other types of wood, the favorable environmental  conditions discussed are considered safe from fungal attack. Fungi require adequate oxygen. This is not normally a major problem. However, when wood becomes totally saturated with water, there is inadequate oxygen available for these pests to survive.

Sapwood and heartwood from the majority of trees are both susceptible to wood decaying fungi. The majority of damage caused by these types of fungi occurs in the interior of the wood or lumber. However, on the surfaces of infested wood, there are typically a variety fungal fruiting bodies that are fan-shaped, cottony growths or root-like in appearance. The coloration of these fruiting bodies varies from white through light brown and yellow to dark brown. These spore-producing structures include mushrooms, shelf-like brackets, or flattened crust-like organs. Fine threadlike fungal strands called mycelia grow throughout the wood and digest it. Ultimately, the strength and other properties of wood are destroyed.

Once decay starts, the rate of deterioration of the host depends on the existence of favorable or unfavorable conditions for growth of the fungi. Decay stops when temperature of the wood are too low or too high and when the moisture content is lower than that required by the fungus. If prevailing temperatures and moisture levels become favorable, fungal growth will again start. Early decay is more readily visible on newly exposed surfaces of unseasoned wood, than when it has been exposed and discolored by weather. Wood decaying fungi are general classified into 3 categories, namely soft, brown and white rots.

Brown Rot (Poria monticola). Cellulose and lignin are the major building blocks of wood. Fungi that cause brown rot primarily attack the cellulose component of lumber (wood), leaving a brown residue of lignin. This activity can greatly reduce the strength of the host, even before decay is visible. The final stage of brown rot is characterized by  a dark brown color of the host, excessive shrinkage, cross-grain cracking and the ease with which dry wood can be crushed. In the U. S, brown rot is typically considered the most economically important of the wood rotting fungi; it commonly attacks the softwoods that are used in above ground building construction. Once wood infected with brown rot dries, it is frequently referred to as dry rot.

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Dry rot characterized by cross grain cracking and brown color. Image courtesy Mätes II. CC BY_SA 3.0

Serpula lacymans. In temperate areas of the world, this species is considered one of the most damaging dry rot infestations of indoor construction wood. It is most commonly found in temperatures of 70º F to 72º F but can survive any temperature from 37º F to 79º F. It is not known how much light is needed to promote growth of this species. In terms of aeration, this fungus frequently grows near ventilation shafts. This may indicate a need for concentrated oxygen. Moisture content of 30% to 40% is optimum for fruiting body development. Serpula lacymans apparently requires an environment where both inorganic and organic materials are present. The fungus uses calcium and iron ions extracted from plaster brick and stone to aid in the break-down of its host.

Serpula lacrymans in a crawl space. Image courtesy of http://www.zimmerei-sachverstaendiger.de/19501.html. GNU Free Documentation icense

Water Conducting Fungi. This type of brown rot has the ability to conduct  moisture from 30 feet or more away from its location. This is accomplished via rhizomorphs; these are dense masses of filaments forming root-like structures. If the environment  of an  infestations falls below the moisture content needed for survival, rhizomorphs seek other locations to bring water into its immediate environment. There are 2 common species, namely Porria incrassate which is found primarily along the Pacific Coast and in the southeastern U.S., and Merulius lacryamans (also called teardrop fungus)  which is found primarily in the Northeastern States and across Europe. Both species attack softwood, such as pine, spruce, and fir. They are usually found in protected locations in structures, such as dirt filled porches and crawl spaces.

Water conducting fungi are typically found in areas of low temperature and can withstand long periods of drought. The minimum growth temperature of these species is 54º F,  with an optimum of 77ºF. The fungi will die if exposed to temperatures of 95ºF or more and will only survive 10 days at a relative humidity of 90%. In addition, these fungi require a wood moisture content of 22% to 32%. They do not commonly occur in homes, but when present can cause significant damage. Most infestations occur in newer structures or in those with recent additions or modifications. As might be expected, the most effective way to prevent water conducting fungi infestations is to reduce moisture in and around the house. Fans and commercial dehumidifiers can be used reduce relative humidity.

White Rot (Phellinus megaloporus, Poria contigua). White rot fungi attack both the lignin and cellulose component of wood, and their presence results in a bleaching effect with wood that is whiter than normal. Affected wood usually does not collapse or crack across the grain, as seen with brown rot infestations. Infested wood gradually loses its strength until it becomes spongy to the touch. Sometimes white rot fungi infestation results in thin, dark lines that form around decayed areas. These are referred to as zone lines. The wood does not shrink until decay is advanced. White rot fungi usually attack hardwoods, but several species can also infest softwood.

White rot fungi. Image courtesy of Sten, GNU Free Documentation lLcense 1.2

Soft Rot. Soft rot fungi usually attack green (high moisture) wood which typically results in a gradual and shallow softening from the surface inward. Soft-rot fungal infestations normally occur in moist situations. Common locations are fence posts, telegraph poles, wooden window frames, timber of cooling towers, and wood in estuaries or marine environments. The soft-rot fungi have little or no effect on lignin, which remains more or less intact. All the soft-rot fungi need relatively high nitrogen levels (typically 1%) for decay of wood decay. If this is unavailable in the wood itself, nitrogen can be recruited from the environment, such as the soil at the bases of fence posts..

Wood Staining Fungi. This type of fungus is typically visible as a discoloration of wood. It is commonly bluish/black and is normally of minor concern when compared to other wood rots. The presence of this fungus is an indication that conditions are within the limits for potential growth of other damaging fungi. However, if logs are cut and left unprotected for less than a week during warm, moist conditions, a number of sap staining fungi will will likely infest,  leaving bluish steaks and thus reduce the value of the end product, namely lumber. White pine and sugar pine are 2 of the most common types of woods that are susceptible to attack of these fungi. The name comes from the fact that these fungi only attack sapwood.

The more common species of blue staining fungi belong to the genus Opiostoma, although there are other genera that produce similar symptoms. Unlike other fungi that decompose wood, staining fungi do not derive nutrients from cellulose or lignin. Instead they feed on fats, sugars, protein, and starches that are stored in sapwood and bark of the tree. It is not exactly known why, but these fungi synthesize melanin, thus leaving a dark stain trail as they grow through wood. Melanin is a pigment that naturally occurs in most organisms.  Even though these stain fungi feed on the wood i, they do so at a very slow rate and do not cause much damage. Depending on the ultimate use of the lumber, the staining may be objectionable, but some lumber stores will sell lumber with blue stains for a higher price due to a perceived decorative value. Blue stain is not just a problem for lumber. When pulp destined for paper making is badly stained, it requires significant bleaching procedures that are expensive and detrimental to the environment.

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Blue stain fungi infesting cut timber. Image courtesy of USDA.

Blue stain fungi, as with other wood rotting fungi, are more of a problem under certain conditions. If the tree bark is removed due to any of a number of conditions, this leaves an unlimited number of entry points for the fungal spores. Under these conditions, the fungi can become established in a few weeks. A moisture content of 22%  is ideal; however, if the logs are saturated, the fungus cannot become established. Typically in the log industry, sprinklers are placed on cut logs until they are milled. A constant spray of water deprives the staining fungi of needed oxygen and lowers the temperature of the logs. Colder temperatures also reduce the ability of these fungi to attack cut trees.

Blue stain fungi spores are carried by wind, but the more likely means of transportation from trees to tree is via bark beetles. Bark beetles, as their name implies, typically feed and establish their galleries under the bark of trees. A healthy tree has an inborn mechanism of retarding attacks by these beetles. With a healthy tree, once a bark beetle bores into bark, it will normally be killed by the tree’s sap flow. However, if a tree is cut or is unhealthy due to disease or drought, its sap flow is greatly reduced. In addition, a weakened or cut tree typically gives off chemical signals of its condition. In essence, the bark beetles and the blue stain fungi work together. The disease weakens the tree and its defensive mechanism, and the beetles deliver the fungi spores to the trees.

Because the blue stain fungal spores are extremely common, it is very difficult to control this species. Pentachlorophenol used to be a standard; however, because it has been classified as a carcinogen and extremely harmful to the ecosystem, it is no longer readily available. Recently, an albino strain of Ophiostoma piliferum has been isolated, cultivated, and marketed. When used to inoculate wood, this variant out-competes the melanin-producing version of these fungi and prevents them from becoming established. The albino strain, as its name suggest, does not produce staining.

Preventing attack of this fungus includes keeping the wood dry, protecting the wood from mountain pine beetles, and maintaining temperatures above or below ideal growing temperatures. The spread of blue stain fungus can only be controlled by protecting the trees from the mountain pine beetle.

Mold Fungi (Fusarium spp. and  Penicillium spp.) Mold fungi initially  appear as brown, black, yellow  or green, fuzzy or powdery growths on the surfaces of wood. Initially these spores typically can be washed off wood surfaces. However, once establish, the mycelia from these surface molds may cause stains that are too deep to be removed easily on open pored hardwoods. Freshly cut or seasoned wood that is stored during warm, humid weather may be significantly discolored with by these molds in a week or more.. These molds do not reduce the strength of wood, but can increase the ability of wood to absorb moisture; this can result in an increase in the potential of infestation by decaying fungi

Chemical Stains.Chemical stains are similar in appearance to blue or brown stains but are not caused by fungi. They result from chemical changes in softwoods and hardwoods. Staining typically occurs in logs or in lumber and may be confused with a brown sap stain fungi. The most important chemical stains are brown stains that can downgrade lumber. They typically can be prevented by rapid air drying or by using relatively low temperatures during kiln-drying.

Insects Associated with Wood Destroying Fungi. Many insect pests are attracted to structures with excessive moisture. Termites, especially the dampwood termites and subterranean termites, require moisture in their environment. Subterranean termites increase moisture in structural wood by bringing water and moist soil up from their colonies and transfer it to their mud tubes and above ground galleries. Some colonies of subterranean termites occur above ground, especially if sufficient moisture is present from water leaks and other sources of moisture.

The presence of moisture is not the only significant water-related factor in termites. The warm, moist conditions that occur in closed system of their galleries create an ideal site for the growth of microorganisms, especially fungi that  provide termies a source of protein and vitamins. These are essential for termite survival. The build-up of termite feces in termite galleries, in turn, helps to establish the growth of the fungi. It should be noted that termites, carpenter ants, and some wood beetles do not cause wood decay. These insects are attracted to wood that has been softened by decay.

Control and Management of Wood Infesting Fungi.

Inspection. The pick test is of valued in identifying the presence of wood decay resulting from  at least 5% to 10% loss of wood weight. In this test, a pointed object, such as an ice pick, is used to pry up a segment of wood. In decayed wood, the pried-up section will break abruptly, directly over the tool; on the other hand, in sound wood, the break will occur at a point away from the tool. This test is very subjective, but it is possible to detect very early stages of decay by both brown rot and white rot.

Molds and stain fungi infestations develop more rapidly than decay fungi and frequently appear on wood during construction. This type of growth cases to  continue once construction is completed, as soon as the wood dries out. However, the presence of these fungi are an indication that conditions at one time were suitable for decay. As a result ,a moisture meter should be used to determine if the wood still contains enough moisture  to support moredamaging decay fungi.

The moisture meters is base on the principle that electric resistance of wood declines as  moisture content increases. The meter determines the resistance between 2 needles that are  inserted into wood. The higher a meter reads, the higher is the moisture content in the wood. The effectiveness of these meters can be affected by the wood species involved, moisture distribution, grain direction, chemicals in the wood, and weather. As a result, directions and information supplied with the meter must be understood and followed.

Several families of beetles are also commonly associated with high levels of moisture in structures, but it is the fungus associated with moisture that attract them. These beetles include minute fungus beetles, silken fungus beetles, minute brown scavenger beetles, darkling beetles and the flat bark beetles. The adults and larvae of these beetles feed on fungal  growth on wood. These beetles are not wood-damaging but are associated with moisture and are a good indication that moisture problems are present.

Carpenter ants are another group of insects that are attracted to moist wood. They prefer to tunnel in softened wood. They do not feed on the wood but merely excavate large chambers to create a suitable location to inhabit and rear their larvae. The control of moisture sources will minimize these insects from infesting structures. In addition to the beetles and ants, a number of other pests are attracted to moisture conditions in buildings. Most are merely nuisances that cause no problem beyond their mere presence. Some of these are springtails, silverfish, mites, millipedes, fungus gnats, and booklice.

Some common sources of moisture in structures are listed below. These areas should be inspected for signs of wood-decaying fungi and moisture above 20%:

  • Water vapors from the combustion of natural gas that improperly vent into the attic or other enclosed areas.

  • Condensation of water on windows .

  • Moisture from crawl spaces and the dirt below

  • Improperly placed or absence ofdrain pipes, downspouts.

  • Leaking roofs.

  • Improperly constructed side wall construction. and sealed foundations, basement walls.

  • Presence of foundation cracks.

  • Direct contact of wood with soil or concrete, allowing “wick” action that pulls water into wood.

  • Improper drainage of water away from structure or out of crawl spaces.

  • Improperly fitted flashings at roof lines or shingles

  • Poorly constructed overhang.

  • Poorly installed or lack of moisture barriers under stucco, shingles.

  • Sweating water pipes.

  • Leaking air conditioners and swamp coolers.

  • Various leaking plumbing, appliances, toilets, shower stall pans.

  • Improper seals or caulk around bathtubs and showers.

  • Plugged or leaking downspouts from roof gutters.

Water Condensation. Condensation is free water or ice extracted from the atmosphere and deposited on any cold surface. The term relative humidity is a means of describing the amount of water vapor held by air. Warm air holds more water vapor than cold air. As air cools, the amount of water vapor it holds decreases until it reaches the dew or saturation point: at that point little droplet or dew develop on surrounding surfaces.

In recent years, the shift in building practices to larger airtight homes has led to additional condensation problems. Energy conservation practices have increased the air tightness of buildings. Also, emphasis has been placed on the installation of humidifiers in heating units to create a more comfortable environment. Their use also increases the likelihood of moisture problems in wood.

There are numerous sources of water vapor in buildings. Washing clothes, showering and bathing, cooking, mopping floors, leaking faucets, washing dishes, baking, and others are said to introduce an estimated pound of water daily into the air of an average home. A poorly ventilated crawl space may produce huge amounts of water. All these conditions are favorable for the reproduction and survival of fungi, termites, and other moisture-loving insects.

Prevention. Maintenance by fixing leaky pipes and faucets, fixing  leaky roofs, and correcting other moisture problems is often all that is required to prevent or control wood-destroying fungi. Prevention begins even prior to the maintenance stage. Structure must initially be built properly. When wood is used in the construction, it should be well-seasoned. Wood used for these purposes should be dry enough to minimize or prevent the establishment of these pests. In very wet or humid areas, construction lumber is commonly treated with preservative chemicals to avoid fungal damage. Water should drain away from a properly constructed building. Proper grading should be established before construction. It is important that condensation (e.g., from air conditioners) be properly drained. Indoors, dehumidifiers should be used where moisture in the air is likely to be a problem.

Adequate ventilation in crawl spaces can be developed by using 1 square foot of openings for each 25 linear feet of walls. These should be positioned so to maximize cross-ventilation. These openings should not be blocked by plants and any other item. Where screening, wire mesh, or louvers are used, the total opening should be greater than 1 square-foot per 25 feet of wall.

Attic vents are normally constructed at the rate of 1 square ft of vent for every 150 to 300 square ft. of attic floor space. Vents should be constructed near the ridge and at the eaves to maximize airflow.

Vapor barriers are usually applied to the subareas of buildings. Installation of a vapor barrier on the soil surface will cause soil moisture to condense on the barrier and return to the soil rather than condense on the floor and joists above. Covering the soil with roofing paper or 4-mil to 6-mil polyethylene sheets can make adequate barriers. Proper installation of these barriers is essential; a small portion of the soil surface should be left uncovered. Leaving spaces between strips, for example, allows the subarea to “breathe” better and provide a drain source for standing water. This is particularly important if the subarea is very wet prior to installation. This will also allow wood in the crawl space to dry slowly, minimizing warping and cracking. An inspection 1 to t3 weeks after installation will allow for proper adjustments of the vapor barrie,r so that the wood can slowly recover from excess moisture.

Chemical Control. Spraying pesticides will not control these type of fungi. It is more significant to eliminate moisture sources and replaced decayed wood with pressure-treated lumber. The use of chemicals may be of use in situations where wood cannot be easily dried or replaced. Under certain conditions, chemical wood preservatives are an effective means of preventing wood decay. Pressure treatment with preservatives such as creosote, zinc chloride, pentachlorophenol, and/or copper naphthenate has been used extensively. The pest management professional needs to be aware of the high toxicity of these chemicals. Pentachlorophenol, for example, is no longer readily available to the consumer in either the ready-to-use (5%) or the concentrated (40%) formulation because of its high toxicity and status as a carcinogen. Pest management professionals should be careful when handling pretreated wood. Wear rubber gloves and long-sleeved clothing and wash thoroughly after handling. Never dispose of preservative-treated wood by domestic incineration or use as a fuel in fireplaces or wood-burning stoves. Treated wood, end pieces, wood scraps, and sawdust should be disposed of at a sanitary landfill. Small quantities may be disposed of with household trash.

Less toxic, more environmentally friendly fungicides than the pressure-treated wood preservatives are commercially available. These fungicides are often borate based. To control fungi on existing wooded structures, the wood should be kept clean with periodic high-pressure washings and a fungicide application to kill remaining fungal spores to prevent reinfestations. It is most important to point out that the application of fungicides or insecticides to fungus-infested wood or soil will not stop the wood decay. Only by eliminating the moisture source can wood decay be completely controlled. Therefore, the application of chemicals by pest management professionals is of minor importance in fungus control work.

Borates as Fungicides. A number of boron-containing products are available and referred to generically as “borates.” The borate known as disodium octaborate tetrahydrate (DOT) is actually a combination of several borates. Borates are well suited to fungus control because they are low hazard, easy to apply, long lasting, and quite effective against both fungi and wood-destroying insects. Part of their success as a wood treatment can be attributed to their high solubility in water. They are easy to mix in a water carrier and are carried through the wood by water diffusion.

They are available in a variety of formulations that allow spraying, brush-on, gel, and foam applications. There is also a formulation available consisting of solid rods that are inserted into holes drilled into the wood. These are designed for use in wood with high moisture content that cannot be dried easily.

True/False Questions.

77. The typical optimum temperature for establishment of wood infesting fungi is about 70ºF to 85ºF.

78. Wood is basically safe from decay when exposed to temperatures below 35 and above 100ºF.

79. The final stage of brown rots can be identified by dark brown color of the wood, excessive shrinkage, cross-grain cracking and the ease with which the dry wood can be crushed.

80. Porria incrassate is found primarily along the Pacific Coast, the southeastern U.S. and is a common example of a water conducting fungi.

81. Stain fungi are usually visible as a discoloration of the wood, often bluish, and are typically of little significance as wood rots.


Mold in Structures and Construction

Growing public concern about the potential health effects of mold in homes and structures has been heightened by media reports and litigation. There can be no disputing that mold litigation is a growing facet of the legal landscape that is not going away any time soon. One commentator estimated that more than 10,000 mold cases were pending in the United States in the early part of this decade.

Claimants have brought mold exposure cases in a variety of contexts, with mixed results. One scenario is the individual homeowner claiming personal injuries, property damage, or both as a result of the presence of mold. Often, these claims are presented against the injured party’s own homeowner’s insurance policy. In one such case, a Texas homeowner was awarded in excess of $32 million, later reduced to $4 million on appeal, in a property damage claim. In another residential mold claim,a California jury awarded an individual $18.5 million in an insurance coverage dispute that arose out of mold contamination.


Unlike wood destroying fungi, molds do not typically cause decay.   In addition there are many materials in and around structures (including homes) that can support growth of mold.  Molds can grow on any organic material used in the construction of structures, including drywall paper, wood panels, lumber and carpet backing. On the other hand they may equally develop on inorganic materials such as concrete, glass or plastics, provided they have sufficient nutrients on their surfaces.

  http://www.epa.gov/mold/moldcourse/images/big_pic49.jpgMold on wooden paneling.

Mold on organic matter-drywall on wood paneling. Images courtesy EPA


Mold on suitcase, painted wall and shower curtan-all inorganic materials  Image courtesy EPA.

Molds and Disease.. The term "toxic mold" is not accurate. While certain molds are toxigenic, meaning they can produce toxins (specifically mycotoxins), the molds themselves are not toxic, or poisonous. Hazards presented by molds that may produce mycotoxins should be considered the same as other common molds which can grow in your house. There is always a little mold everywhere - in the air and on many surfaces.  Many common molds can produce mycotoxins. Those that arbitrarily have been cited as "toxic molds" include Stachybotrys chartarum, and various species of Aspergillus, Fusarium and Penicillium

Stachybotrys chartarum (also known by its synonym Stachybotrys atra) is a greenish-black mold. It can grow on material with a high cellulose and low nitrogen content, such as fiberboard, gypsum board, paper, dust, and lint. Growth occurs when there is moisture from water damage, excessive humidity, water leaks, condensation, water infiltration, or flooding. Constant moisture is required for its growth. It is not necessary, however, to determine what type of mold is present. All molds should be treated the same with respect to potential health risks and removal.

Mold in structures is nothing new considering molds have been on the earth far before prehistoric humans decided to move indoors. However the general public became concerned about this potential problem after several articles on the subject appeared in scientific journals. One of the most widely publicized articles was written by researchers from the U.S. Centers for Disease Control (CDC). They reported that in 1993, there were 10 cases of acute ideopathic pulmonary hemorrhage/hemosiderosis (AIPH) in infants, some of whom died. These cases were thought to be linked to the mold Stachybotrys chartarum (also known as toxic mold).

This report resulted in great concern across the country. However, upon closer scrutiny of the article and its conclusions, a CDC expert panel and an outside expert panel both seriously questioned the initial reported results. Both panels agreed in that there was no reliable scientific evidence that Stachybotrys (toxic mold) caused the health problems in these infants. Unfortunately the initial article was widely publicized while the revised findings received little publicity. As a consequence, there continues to be the misperception that there is scientific proof Stachybotrys chartarum causes serious health problems in infants. However the CDC concludes: "At present there is no test that proves an association between Stachybotrys chartarum and particular health symptoms."

Comprehensive reviews of scientific literature by the Institute of Medicine, the American College of Occupational and Environmental Medicine and the Texas Medical Association have similarly concluded that an association has not been shown to exist between the presence of mold or other agents in damp indoor spaces and AIPH.

In its extensive analysis, the Institute of Medicine did not conclude that any adverse health outcomes are caused by the presence of mold or other agents in damp indoor environments. The Institute did document that there is an association between certain symptoms (upper respiratory tract symptoms, cough, hypersensitivity pneumonitis in susceptible persons, wheeze, and asthma symptoms in sensitized persons) and mold or damp indoor environments, but the Institute makes it clear that "associated with" does not mean "caused by." The Institute also found that the evidence is not sufficient to document even an association between the presence of mold and any other symptom such as  airflow obstruction, lower respiratory illness in otherwise healthy adults, shortness of breath, mucous membrane irritation syndrome, inhalation fevers, cancer, asthma, long term obstructive pulmonary disease, skin irritations, gastrointestinal tract problems, fatigue, neuropsychiatric symptoms and rheumatologic and other immune disease.

The ACOEM generally agrees with the IOM report in that the health effects from mold in indoor environments are limited, in most instances, to allergic health effects in sensitive persons. Two other articles have reinforced the concern about molds and health problems, specifically in individuals working in office buildings. These studies claimed a link between working in the buildings and symptoms such as headache, fatigue and cough, as reported. The authors of these studies concluded that mold producing mycotoxins were the root of the health symptoms. Again when these reports were scrutinized, they were found to have many limitations and the data did not support such conclusions.

In addition to the above studies, there are some widely cited anecdotal reports of acute, or sudden, health effects attributed to mycotoxins after exposure to extremely moldy conditions. There is little information in these reports to relate the health effects to mycotoxins and no measurement of exposure. In all of them, the individuals were likely exposed to high concentrations of mold spores but recovered after they were removed from exposure.

Other symptoms, such as nervous disorders, memory loss and joint pain, are attributed to molds. However, there is no credible evidence in the medical and scientific literature that supports the link between molds and these health problems. Other Health Effects of Molds and Their Byproducts.

The musty odor associated with mold comes from volatile compounds generated as the mold reproduces.  Although these compounds may be annoying and irritating they do not contain mycotoxins nor are they highly toxic. The concentration of the volatile compounds is usually too low in buildings (even in damp and moldy buildings) to cause sensory irritation to eyes and upper airway.  However their presence can be quite bothersome as their presence is quite noticeable even at low concentrations.

Humans are exposed constantly to molds in their everyday environment, whether indoors and outdoors. Fortunately we have well develop defense systems (immune system, respiratory clearance systems) that protect us from potential health effects of airborne borne mold spores.   However problems can arise when the immune system is compromised or suppressed by condition such as HIV infection, cancer treatment, over-responsive (allergy) or when exposures are exceedingly high (irritation and mycotoxin effects). Some people with compromised immune function may be at increased risk for opportunistic infections from common molds both indoors and outdoors.

Mold infections are possible in people with immune system suppression, but this has not been reported to occur due to mold in residential settings. About 5 % of the American population experiences allergic symptoms (Asthma and hay fever symptoms) due to molds and other fungal antigens. The most common allergies are to the abundant outdoor molds, such as Cladosporium, Aspergillus and Penicillium.

Routes of exposures to mold include dermal, inhalation and ingestion. The route of exposure has a profound effect on the amount of material or toxin absorbed by the body.  For example dermal exposure occurs when the skin is in contact with mold spores. The spores do not pass through the skin, but may cause irritation if there is contact with excessive quantities of spores or moldy material. The irritation may be from reaction to allergenic compounds or chemicals, including mycotoxins, and possibly from rubbing against the spores themselves. Skin is generally a good barrier against particles. Because mycotoxins are not volatile and stay with the spores (particles), the skin is not a significant route of exposure for mycotoxins.

Mold spores or particles are readily airborne and can be inhaled easily. Although mycotoxins can be inhaled, mold spores are small and the amount of toxin in each is tiny. For most people, the airborne concentration needed to get to a toxic dose is in the range of many hundreds of millions of spores per cubic meter of air. Ingestion is a more direct exposure route for mold.  Human poisonings from eating of moldy foods is well documented. In these cases large amounts of molds and mycotoxins are injested as compared to the amount that can normally be inhaled.

Framing lumber in a newly finished house is typically encased by panels or siding on the outside and drywall or panels on the inside. As such, there is virtually no chance for occupants in a home to be exposed to any mold on the wood through skin contact or ingestion. Inhalation exposure to mold on framing lumber in a finished home is possible, but not very likely. Mycotoxins are not volatile, so they cannot "off-gas" into the environment or migrate through walls or floors independent of a particle. Since particles cannot move through solid objects, mycotoxins in molds contained inside a wall or floor cavity will stay there unless disturbed.

Molds and Construction Wood.Wood consists primarily of cellulose, lignin and hemicelluloses and a variety of other minor ingredients such as starches, sugars, proteins, lipids and fatty acids. Celluose and lignin give wood its strength and other characteristics that make it an excellent construction material. The minor ingredients  are present in the storage tissues of the living tree and are essential for a variety of other functions. Once a tree is harvested or cut into lumber all these materials remain in the wood and can provide the initial food source for mold fungi.

Under the proper conditions, wood may be colonized by a variety of fungi. A recent study at Oregon State University revealed that Douglas fir sapwood was colonized by over 45 species of fungi within six weeks after sawing. Most of these fungi are common to many other materials, while a few were specialized and only grow on wood.  


Mold of wood. Image courtesy US EPA
Both molds and stain fungi discolor wood and it is difficult, if not impossible, to distinguish one from the other by the naked eye. Molds are typically characterized by the presence of pigmented spores.  These spores range in coloration from yellow, green, orange, black and an array of other colors. As previously indicated, discoloration seen with molds is usually confined to the wood surface. Stain fungi discolor the wood more deeply and are more difficult to remove than molds.

   Blue Stain Fungus - Chlorociboria aeruginascens

Blue stain fungus with fruiting bodies (mushrooms). Image courtesy of WiseAcres Gardens.

As previously discussed wood decaying fungi attack wood that has been exposed to excessive moisture, such as what occurs with plumbing leaks or seeping from outdoor water sources. Unlike molds, decay fungi penetrate the surface of the wood attacking cellulose, lignin and other components ultimately reducing its strength. As with some mold infested wood, decayed wood may be discolored, but spores of the decay fungus are not typically found on the surface. Spores of most species are produced on more complex fruiting structures that can produce billions. Such fruiting bodies do not occur with molds

  File:Echter Hausschwamm 5516a.jpg

Deterioration from dryrot fungus. Left, image courtesy of  Mätes II. . CC BY-SA 3.0

Frequently molds that grow on wood can also be found on almost any material (including plant leaves, bread and other food) that contains sugars or starches. They can grow on a microscopically thin layer of organic material, even forming on common household dust. These fungi have evolved to rapidly colonize a substrate and utilize the stored sugars as quickly as possible, but they lack the ability to cause significant effects on the wood structure. The most common effect of mold attack on wood is an increase in permeability, which can lead to an increase in moisture or paint uptake

Identification of Fungi and Molds Species and Infestation Levels. Proper identification of molds to species requires a professional with formal training in mycology (study of fungi).  Even though some species of mold produce distinctive structures or colors, even the experts require the use of a microscope for identification which can take a few days or several weeks.  Mold and stain fungi identification is based on the structure of spores and bodies which produce them. Samples can be taken by smoothing a piece of clear tape on the wood surface, then mounting the tape on a microscope slide or by placing a piece of the wood on a growth or nutrient media.

Individuals who inspect and test homes for mold should have the appropriate education and experience. A certified industrial hygienist (CIH) with experience in sampling for molds is generally qualified to inspect a home for the presence of visible mold, and to collect samples for mold if necessary and to help with interpretation of the results. Industrial hygienists have training in exposure assessment and methods of controlling exposures to molds and other dusts. They can also provide advice on how to control exposure and contamination during clean up of mold.

The American Board of Industrial Hygiene sets standards for certifying hygienists. These standards require at least a bachelor's degree with a minimum 30 semester hours of science and specific industrial hygiene coursework, a minimum of four years of professional level-1 industrial hygiene experience and successful completion of a comprehensive one-day examination.

Location of Mold in Buildings.. The presence of molds in our everyday environment means they can grow any place under the proper conditions. In all cases, moisture is the essential element for mold growth in buildings. There are many potential sources for unwanted moisture in buildings. For example, improperly maintained air conditioning systems that create excessive condensation can be a breeding ground and distribution mechanism for mold particles. Mold growth may be found in walls, ceilings and floor cavities when standing water is produced in a building or gets in and stays for more than a few days. Sources for water to support molds and other fungi in homes include plumbing leaks, gaps in roofs, siding or masonry, poorly sealed windows, porous slabs and foundations, inadequate drainage, and faulty roof drains and downspouts. Mold may be hidden in places such as the back side of dry wall, wallpaper, or paneling, the top side of ceiling tiles, the underside of carpets and pads, etc. Other possible locations of hidden mold include areas inside walls around pipes (with leaking or condensing pipes), the surface of walls behind furniture (where condensation forms), inside ductwork, and in roof materials above ceiling tiles (due to roof leaks or insufficient insulation).

mold growing on the back side of wallpaperhttp://www.epa.gov/mold/moldcourse/images/big_pic41.jpg

Examples of Hidden Mold. Left-Behind wallpaper. Right below bathtub. Images courtesy EPA.

Poor ventilation and/or air circulation combined with high indoor humidity from showers, cooking or other activities can result in condensation that promotes mold growth on cooler surfaces. Poorly insulated walls may also provide a surface for condensation and mold growth in buildings that do not have general humidity problems.

Surface moisture on unseasoned framing lumber, appearing as the wood dries, may create conditions for mold growth. However, once the moisture content of the wood falls below 20 percent, mold growth can no longer be supported. Depending on the climate, framing lumber will dry to below 20 percent moisture content during the construction and before the building is enclosed. In instances where wood is chronically exposed to water, wood decay fungi can invade. Decay fungi can penetrate more deeply and attack the structural polymers in the fiber, reducing the strength of the wood.

Other conditions can increase the amount of mold spores in the indoor air of buildings. Homes with exposed-dirt crawl spaces and basements tend to have more airborne mold spores than homes without. With the right humidity conditions, some molds can grow on house dust. It is not surprising, then, that poor housekeeping and high indoor humidity are both associated with increased levels of airborne mold spores. The biggest source of indoor mold spores is often the outdoor air (Higher levels of indoor mold spores tend to be found in homes with yards having dense and overgrown landscaping. Indoor mold levels are generally lower in buildings with forced-air heating systems (as opposed to window ventilation) and lower still when these systems include a well-maintained and properly functioning air conditioning system.

Mold spore levels outdoors vary with the season and weather. They may be quite high in the growing season or approach zero when snow covers the ground.  Except for the snow-cover situation, mold spores in normal indoor environments are usually between 20-50 percent of the outdoor levels.

Conditions Leading to and Prevention of Excess Humidity in a Structure and Subsequent Mold Infestations.

  • Leaks and seepage of moisture should be corrected. If water is entering a house or other structure from the outside, correction options may range from simple landscaping to extensive excavation and waterproofing. (The ground should slope away from the house.) It is much simpler and much less costly to accomplish such excavation prior to building a structure.  Water in the basement can result from the lack of gutters or a water flow toward the house. Water leaks in pipes or around tubs and sinks can provide a place for molds to grow.

  • Plastic covers can be placed over soil in crawlspaces to prevent evaporation moisture from entering a structure from the ground.  Crawlspaces should be well-ventilated following building code requirements.  Remove any vegetation or other material blocking crawl space vents.

  • Excessive clutter in structures can trap moisture and lead to development of molds.

  • Exhaust fans should be used or fixed if broken in bathrooms and kitchens to remove moisture to the outside (not into the attic).  Clothes dryer exhaust should be vented to the outside.

  • Certain appliances (such as humidifiers or kerosene heaters) should be turned off if moisture on windows and other surfaces is present.

  • Use dehumidifiers and air conditioners, especially in hot, humid climates, to reduce moisture in the air, but be sure that the appliances themselves don't become sources of biological pollutants.

  • It is important to maximize air circulation in a structure. Open doors between rooms (especially doors to closets which may be colder than the rooms) to increase circulation. Circulation carries heat to the cold surfaces. Increase air circulation by using fans and by moving furniture from wall corners to promote air and heat circulation.  The availability of a fresh air source in a structure will normally reduce humidity.

  • Carpets on concrete floors can absorb moisture and serve as a place for mold to grow. Area rugs which can be periodically removed and cleaned. In high humidity climates, it will help if a vapor barrier is installed between a rug and concrete floor.   This can be accomplished by placing plastic sheeting over the concrete and then covering that with sub-flooring (insulation covered with plywood).

  • Moisture problems and their solutions differ from one climate to another. The Northeast is cold and wet; the Southwest is hot and dry; the South is hot and humid; and the Western Mountain states are cold and dry. All of these regions can have moisture problems. For example, evaporative coolers used in the Southwest can encourage the growth of biological pollutants, including mold. In other hot regions, the use of air conditioners which cool the air too quickly may prevent the air conditioners from running long enough to remove excess moisture from the air. The types of construction and weatherization for the different climates can lead to different problems and solutions.

  • Relative humidity should generally be maintained at levels below 65% to inhibit mold growth.  Short-term periods of higher humidity would not be expected to result in mold growth. However, condensation on cold surfaces could result in water accumulation at much lower relative humidity levels.  Relative humidity should be kept low enough to prevent condensation on windows and other surfaces.  Prevention of Mold on Lumber. As with wood decaying fungi, molds have four basic requirements for growth in the infestation of structures: suitable temperature, oxygen, food and moisture. Eliminating one of these requirements can prevent fungal growth.

Mold infestations can develop over a fairly large range of temperatures but most grow best between 70 and 85 degrees F. Most fungi require oxygen to survive. Of course this is typically available in most situations.  One exception is that when wood is submerged in water which fills the wood cells with water and limits the availability of oxygen. Lumber and wood product mills often utilize this method by spraying log decks with water or storing logs in ponds at the plant.

It is typically more practical to reduce moisture content in wood than to control temperature and oxygen requirements for mold control. Reducing the moisture content of lumber to less than 20 percent will significantly decrease the opportunities for mold to form on the wood.

Drying lumber reduces the likelihood of mold formation. But it does not guarantee the wood will remain free of mold, as lumber that is exposed to moisture after it has been dried will support mold growth. Dry lumber can become wet through direct sources, such as rainfall or condensation. Even dry lumber contains some moisture. Therefore, wet pieces inside wrapped bundles of lumber could create conditions for mold growth. Exposing the bundle to direct sunlight, for example, could heat the lumber and the wrapping may trap the evaporating moisture. This trapped moisture can be sufficient to support mold growth.

Green lumber is commonly used in construction and has a moisture content which is conducive to mold infestation.  This is frequently avoided because it dries naturally, usually during the framing stages of building a house. Many mills reduce the risk of mold and stain on green lumber by applying antistatic, or sap stain treatments, which are thin coatings of fungicides on the wood surface.  When applied these chemicals produce a microscopic barrier against fungal attack that lasts for 3 to 6 months.  The residual activity of these chemicals depends on a number of factors, namely the product and concentration used, wood species and prevailing climate.

Mold Cleanup. the decision to remove mold from enclosed cavities must be made after considering how much mold is likely to be present and how likely it is that opening an enclosed area to remove mold may actually spread the mold to other areas of the structure. In buildings where mold removal from enclosed cavities is not desirable or feasible, sampling can be conducted to monitor the level of mold spores in the occupied spaces. High indoor mold spore levels are sometimes found when walls and floors containing mold are opened or disturbed, and when visible mold growth is present on exposed surfaces. The decision to clean mold from lumber depends on the amount of mold present and how likely it is to be disturbed. In nearly all cases, mold cleaning should be undertaken only after any moisture problems are resolved.

When removing mold from a structure where a large amount of dust may occur, personal protective equipment should be used.  This should include eye protection, rubber gloves and a high quality pollen or dust mask. When cleaning mold from small areas special equipment may not be required.

Non-porous materials (e.g. metals, glass, and hard plastics) can almost always be cleaned.  Semi-porous and porous structural materials, such as wood and concrete can be cleaned if they are structurally sound.  Porous materials, such as ceiling tiles and insulation, and wallboards (with more than a small area of mold growth) should be removed and discarded.  Wallboard should be cleaned or removed at least six inches beyond visually assessed mold growth (including hidden areas) or wet or water-damaged areas. A professional restoration consultant should be contacted to restore valuable items that have been damaged. 

The molds seen on lumber are largely a collection of fungal spores on the surface of the wood. As such, wet wiping or scrubbing the lumber will remove the mold. Simply wiping the wood, however, can release those spores into the surrounding air. A better approach is to gently spray or wet down the mold prior to removal. Once the mold has been wetted, it can be removed by wet-wiping the surfaces with a water and detergent solution, scrubbing if necessary.  If commercial products are used for cleaning mold, be sure to follow the manufacturer's instructions for use. Common bleach also can be used, particularly to clean the discoloration caused by mold fungi. The U.S. Centers for Disease Control (CDC) recommends using a solution of 10 parts water to one part chlorine bleach to clean mold from surfaces. When using bleach and other cleaning chemicals indoors, make sure there is adequate ventilation and wear personal protection equipment outlined previously. Never mix bleach with ammonia.

Disinfectants are seldom needed to perform an effective remediation because removal of fungal growth remains the most effective way to prevent exposure.  Disinfectant use is recommended when addressing certain specific concerns such as mold growth resulting from sewage waters.  If disinfectants are considered necessary, additional measures to protect workers and occupants may also be required.  Disinfectants must be registered for use by the United States Environmental Protection Agency (EPA).  Any antimicrobial products used in a HVAC system must be EPA-registered specifically for that use.

The use of gaseous, vapor-phase, or aerosolized (e.g. fogging) biocides for remedial purposes is not recommended.  Using biocides in this manner can pose health concerns for people in occupied spaces of the building and for people returning to the treated space.  Furthermore, the effectiveness of these treatments is unproven and does not address the possible health concerns from the presence of the remaining non-viable mold. 

Mold spores are present on surfaces in all structures, so cleaning will not prevent re-growth of mold. Even if a building is stripped of all components and every spore is killed or removed, normal background mold spores from outdoors or on replacement parts have the potential to grow. The most important objective in any mold removal is to remove or repair any sources of moisture. Should the wood framing in a house become wet, through leaks or flooding, it is imperative that the area be dried as soon as possible. In many climates, this drying will occur naturally once any standing water is removed. In other climates where the relative humidity is higher, it may be necessary to bring in portable fans to increase airflow or to use the existing heating system or portable electric heaters to encourage faster drying.

Dust Mites, Dampness and Allergies. The body of a house dust mite is just visible against a dark background in normal light. A typical house dust mite measures 0.4 millimeters (0.016 in) in length and 0.25–0.32 millimeters (0.010–0.013 in) in width. Both male and female adult house dust mites are creamy blue and have a rectangular shape. The body of the house dust mite also contains a striated cuticle. Like all acari, house dust mites have eight legs (except 3 pairs in the first instar). The average life cycle for a male house dust mite is 10 to 19 days. A mated female house dust mite can last up to 70 days, laying 60 to 100 eggs in the last 5 weeks of her life. In a 10-week life span, a house dust mite will produce approximately 2,000 fecal particles and an even larger number of partially digested enzyme-covered dust particles.


Housedust Mites. Image Courtesy 

  Gilles San Martin from Namur, Belgium CC BY-SA 2.0

The close association between damp indoor environments and dust mites is of significance when considering the health effects of dampness and mold. The house dust mite (sometimes referred to by allergists as HDM) is a cosmopolitan guest in human habitation. Dust mites feed on organic detritus such as flakes of shed human skin and flourish in the stable environment of dwellings. House dust mites are a common cause of asthma and allergic symptoms worldwide. The mite's gut contains potent digestive enzymes (notably proteases) that persist in their feces and are major inducers of allergic reactions such as wheezing. The mite's exoskeleton can also contribute to allergic reactions.

House dust mites are arachnids, and many different species have been identified, however, only a few are of concern with respect to damp indoor environments. Because natural food sources of house-dust mites includes skin scales, nutrition is abundantly available, particularly in mattresses and carpets or rugs. Laboratory studies have shown that most dust mites require a relative humidity in excess of 45–50% for survival and development, but they feed and multiply more rapidly at higher relative humidity. Indoor humidity is therefore the main influence on the presence and propagation of house dust mites, as confirmed in several field studies. Damp houses, therefore, significantly increase exposure to dust-mite allergens, at least in populations living in mild and temperate climates.

Dust mites produce the predominant inhalation allergens in many parts of the world. The most common mite species that produce allergens are Dermatophagoides pteronyssinus and Dermatophagoides farinae. The major allergens produced by D. pteronyssisus (called Der p I and Der p II) are proteases, which are present in large amounts in fecal pellets. The major allergen produced by D. farinae is Der f I. Elevated levels of these allergens have been detected in house dust, mattress dust and bedding in damp houses.  Typical symptoms of house dust mite allergies are itchiness; sneezing; inflamed or infected eczema skin; watering/reddening eye; sneezing repeatedly and frequently, e.g., on waking up or sneezing 10 or more times; runny nose; and clogging in the lungs.

At present, the best form of treatment for dust mite allergies is avoidance of dust mites and their allergens combined with medication such as antihistamines, corticosteroids or Salbutamol. The environment of bedding is optimal for most dust mites, and comparative studies have shown that the density of dust mites in mattresses to be on average greater than 2500/gram of dust. Cleaning beds with most vacuum cleaners will not remove dust mite allergens, but instead throw them into the air and increase their volatility. Some polyethylene bedding is beneficial as it makes the environment difficult for the dust mites. This bedding should also be breathable and be able to withstand frequent washing. A home allergen reduction plan has been recognized as being an essential part in the management of asthma symptoms and therefore all aspects of the home environment should be considered (proper vacuuming, use of air cleaners, etc.).

Sources. Facts about Stachybotrys chartarum and other Molds.  CDC.  Mold Remediation in Schools and Commercial Buildings. EPA