Termites, ants and other wood-destroying organisms

Editor's Note: This material isexcerpted from A Guide to the Inspection of Existing Homes for Wood InhabitingInsects, written by Michael P. Levy and published by the U.S. Department of Housingand Urban Development (HUD).

Formosan subterranean termites. Photo by Scott Bauer, U.S. Department of Agriculture

Wood is a porous material and will absorb moisture from the air. Moisture is attractedto the walls of the tubes that make up the wood. As walls absorb moisture, the woodswells. If the humidity is kept at 100 percent, the walls become saturated with water. Themoisture content at which this occurs is the fiber saturation point, which isapproximately 30 percent by weight for most species used in construction. Fungi will onlydecay wood with a moisture content above the fiber saturation point. To allow a safetymargin, wood with a moisture content above 20 percent is considered to be susceptible todecay.

Wood in properly constructed buildings seldom will have a moisture content above 16 tol8 percent. Thus, wood will only decay if it is in contact with the ground or wetted by anexternal source of moisture, such as rain seepage, plumbing leaks, or condensation. Drywood will never decay. Also, the drier the wood, the less likely it is to be attacked bymost types of wood-inhabiting insects.

Wood-inhabiting fungi are small plants that lack chlorophyll and use wood as their foodsource. Some fungi use only the starch and proteins in the wood and don't weaken it.Others use the structural components, and as they grow, they weaken the wood, whicheventually becomes structurally useless. All fungi require moisture, oxygen, warmth, andfood. The keys to preventing or controlling growth of fungi in wood in buildings are toeither keep the wood dry (below a mois-ture content of 20 percent) or to usepreservative-treated or naturally resistant heartwood or selected species.

Wood-inhabiting insects can be divided into those that use wood as a food materia --termites and wood-boring beetles, for example -- and those that use it for shelter --carpenter ants and bees, for example. Damage is caused by immature termites called nymphs,by the larvae or grubs of the wood-boring beetles, and by the adults in ants and bees.

Some wood-inhabiting organisms are found in all parts of the country, others are highlylocalized. Some, although common, cause very little structural damage. The following is adescription of the major wood-inhabiting fungi and insects in the United States.

Surface molds and sapstain fungi

Surface molds or mildew fungi discolor the surface of wood, but do not weaken it. Theyare generally green, black, or orange and powdery in appearance. The various buildingcodes allow the use of framing lumber with surface molds or mildew, providing that thewood is dry and not decayed. Spores (or seeds) of surface molds or mildew fungi growquickly on moist wood or on wood in very humid conditions.

They can grow on wood before it is seasoned, when it is in the supplier's yard oron the building site, or in a finished house. When the wood dries, the fungi die or becomedormant, but they do not change their appearance. Thus, wherever surface molds or mildewfungi are observed on wood in a building, it is a warning sign that at some time the woodwas moist or humidity was high.

Surface molds and mildew fungi are controlled by eliminating the source of highhumidity or excess moisture, for example by repairing leaks, improving ventilation inattics or crawl spaces, or installing soil covers. Before taking corrective action, thesource of the moisture that allowed fungus growth must be determined. If the wood is dryand the sources of moisture are no longer present, no corrective action need be taken.

Sapstain or bluestain fungi are similar to surface molds, except that the discolorationgoes deep into the wood. They color the wood blue, black, or gray and do not weaken it.They grow quickly on moist wood and do not change their appearance when they die or becomedormant. They usually occur in the living tree or before the wood is seasoned, butsometimes they grow in the supplier's yard, on the building site, or in a finishedhouse. In the latter case, they are normally associated with rain seepage or leaks. Stainfungi are a warning sign that at some time the wood was moist. Control is the same as forsurface molds or mildew fungi.

Water-conducting fungi

Most decay fungi are able to grow only on moist wood and cannot attack adjacent drywood. Two brown-rot fungi, Poria incrassata and Merulius lacrymans, are able to conductwater for several feet through root-like strands or rhizomorphs, to moisten wood and thento decay it.

These are sometimes called water-conducting or dry-rot fungi. They can decay wood inhouses very rapidly, but fortunately they are quite rare. Poria incrassata is found mostfrequently in the Southeast and West. Merulius lacrymans occurs in the Northeast.

Both fungi can cause extensive damage in floors and walls away from obvious sources ofmoisture. Decayed wood has the characteristics of brown rotted wood except that thesurface of the wood sometimes appears wavy but apparently sound, although the inte-riormay be heavily decayed. The rhizomorphs that characterize these fungi can be up to an inchin diameter and white to black in color, depending on their age. They can penetratefoundation walls and often are hidden between wood members.

The source of moisture supporting the fungal growth must be found and eliminated tocontrol decay. Common sources include water leaks and wood in contact with or close to thesoil: for example, next to earth-filled porches or planters. Where the fungus grows from aporch, the soil should be removed from the porch next to the foundation wall to preventcontinued growth of the fungus into the house. Poria incrassata normally occurs in new orremodeled houses and can cause extensive damage within two to three years.

Brown-rot and white-rot fungi

The fungi often produce a whitish, cottony growth on the surface of wood. They growonly on moist wood. The fungi can be present in the wood when it is brought into the houseor can grow from the spores that are always present in the air and soil. Wood attacked bythese fungi should not be used in construction. Wood decayed by brown-rot fungi is brittleand darkened in color. As decay proceeds, the wood shrinks, twists, and cracksperpendicular to the grain. Finally, it becomes dry and powdery. Brown-rot is thecommonest type of decay found in wood in houses.

Wood decayed by white-rot fungi is fibrous and spongy and is bleached in color.Sometimes it has thin, dark lines around decayed areas. The wood does not shrink untildecay is advanced.

These fungi can be controlled by eliminating the source of moisture that allows them togrow, for example by improving drainage and ventilation under a house, repairing waterleaks, or preventing water seepage. When the wood dries, the fungi die or become dormant.Spraying wood with chemicals does not control decay. If the moisture source cannot beeliminated, all the decayed wood should be replaced with pressure-treated wood.

White-pocket rot

White-pocket rot is caused by a fungus that attacks the heartwood of living trees.Decayed wood contains numerous small, spindle- shaped white pockets filled with fungus.These pockets are generally 3 to 13 mm long. When wood from infected trees is seasoned,the fungus dies. Therefore, no control is necessary. White-pocket rot generally is foundin softwood lumber from the West Coast.

Subterranean termites

Subterranean termites normally damage the interior of wood structures. Shelter tubesare the most common sign of their presence. Other signs include structural weakness ofwood members, shed wings or warmers, soil in cracks or crevices, and dark or blister-likeareas on wood.

The major characteristics of infested softwood when it is broken open are that damageis normally greatest in the softer springwood and that gallery walls and inner surfaces ofshelter tubes have a pale, spotted appearance like dried oatmeal. The galleries oftencontain a mixture of soil and digested wood. Termites usually enter houses through wood incontact with the soil or by building shelter tubes on foundation walls, piers, chimneys,plumbing, weeds, etc.

Although they normally maintain contact with the soil, sub-terranean termites cansurvive when they are isolated from the soil if they have a continuing source of moisture.Heavy damage by subterranean termites (except Formosans) does not normally occur duringthe first five to 10 years of a building's life, although their attack may start assoon as it is built.

Subterranean termites can be controlled most effectively by the use of chemicals in thesoil and foundation area of the house, by breaking wood-soil contact, and by eliminatingexcess moisture in the house. When applied properly, these chemicals will prevent orcontrol termite attack for at least 25 years.

Formosan subterranean termites

Formosan subterranean termites are a particularly vigorous species of subterraneantermite that has spread to this country from the Far East. They have caused considerabledamage in Hawaii and Guam and have been found in several locations on the United Statesmainland.

It is anticipated that they could eventually become established along southern coasts,the lower East and West Coasts, in the lower Mississippi Valley, and in the Caribbean. Themost obvious characteristics that distinguish Formosan subterranean termite swarmers fromthose of native species are their larger size (up to16 mm compared to 9 to 13 mm) andhairy wings (com-pared with smooth wings in other subterraneans).

Soldiers have oval shaped heads, as opposed to the oblong and rectangular heads ofnative soldiers. Formosan termites also produce a hard material called carton, whichresembles sponge. This is sometimes found in cavities under fixtures or in walls adjacentto attacked wood. Other characteristics- and control methods-are similar tothose for native subter-ranean termites.

However, Formosan subterranean termites are more vigorous and can cause extensivedamage more rapidly than do native species. For this reason Formosans should be controlledas soon as possible after discovery.

Drywood termites

It is quite common for buildings to be infested by drywood termites within the firstfive years of their construction in southern California, southern Arizona, southernFlorida, the Pacific area, and the Caribbean. Swarmers generally enter through attic ventsor shingle roofs, but in hot, dry locations, they can be found in crawl spaces. Windowsills and frames are other common entry points.

Drywood termites live in wood that is dry. They require no contact with the soil orwith any other source of moisture. The first sign of drywood termite infestation isusually piles of fecal pellets, which are hard, less than 1 mm in length, with roundedends and six flattened or depressed sides. The pellets vary in color from light gray tovery dark brown, depending on the wood being consumed. The pellets, eliminated fromgalleries in the wood through round kick holes, accumulate on surfaces or in spider websbelow the kick holes. There is very little external evidence of drywood termite attacks inwood other than the pellets. The interior of damaged wood has broad pockets or chambersthat are connected by tunnels that cut across the grain through springwood and summerwood.The galleries are perfectly smooth and have few, if any, surface deposits.

There are usually some fecal pellets stored in unused portions of the galleries.Swarming is another sign of termite presence. It normally takes a very long time for thetermites to cause serious weakness in house framing. Damage to furniture, trim, andhardwood floors can occur in a few years. The choice of control method depends on theextent of damage.

If the infestation is widespread or inaccessible, the entire house should befumi-gated. If infestation is limited, spot treatment can be used or the damaged wood canbe removed.

Dampwood termites

Dampwood termites of the desert Southwest and southern Florida are rarely of greatdanger to structures. Pacific Coast dampwood termites can cause damage greater thansubterranean termites if environmental conditions are ideal.

Dampwood termites build their colonies in damp, sometimes decaying wood. Onceestablished, some species extend their activities to sound wood. They do not requirecontact with the ground, but do require wood with a high moisture content. There is littleexternal evidence of the presence of dampwood termites other than swarmers or shed wings.They usually are associated with decayed wood. The appearance of wood damaged by dampwoodtermites depends on the amount of decay present. In comparatively sound wood, galleriesfollow the springwood. In decayed wood, galleries are larger and pass through bothspringwood and summerwood. Some are round in cross section, others are oval. The surfacesof the galleries have a velvety appearance and are sometimes cov-ered with dried fecalmaterial.

Fecal pellets are about 1 mm long and colored according to the kind of wood beingeaten. Found throughout the workings, the pellets are usually hard and round at both ends.In very damp wood, the pellets are often spherical or irregular, and may stick to thesides of the galleries.

Dampwood termites must maintain contact with damp wood. Therefore, they can becontrolled by eliminating damp wood. Treatment of the soil with chemicals can also be usedto advantage in some areas.

Carpenter ants

Carpenter ants burrow into wood to make nests, but do not feed on the wood.They commonly nest in dead portions of standing trees, stumps, logs, and sometimes wood inhouses. Normally they do not cause extensive structural damage. Most species start theirnests in moist wood that has begun to decay. They attack both hardwoods and softwoods.

The most obvious sign of infestation is the large reddish-brown to black ants, 6 to 13mm long, inside the house. Damage occurs in the interior of the wood. There may be pilesor scattered bits of wood powder (frass), which are very fibrous and sawdust-like. If thefrass is from decayed wood, pieces tend to be darker and more square ended. The frass isexpelled from cracks and crevices, or from slit-like openings made in the wood by theants. It is often found in basements, dark closets, attics, under porches, and in crawlspaces. Galleries in the wood extend along the grain and around the annual rings.

The softer springwood is removed first. The surfaces of the galleries are smooth, as ifthey had been sandpapered, and are clean. The most effective way to control carpenter antsis to locate the nest and kill the queen in colonies in and near the house withinsecticides. It is sometimes also helpful to treat the voids in walls, etc. For currentinformation on control, an entomologist should be contacted.

Wood-boring beetles, bees, and wasps

There are numerous species of wood-boring insects that occur in houses. Some of thesecause considerable damage if not controlled quickly. Others are of minor importance andattack only unseasoned wood. Beetles, bees, and wasps all have larval, or grub, stages intheir life cycles, and the mature flying insects produce entry or exit holes in thesurface of the wood. These holes, and sawdust from tunnels behind the holes, are generallythe first evidence of attack that is visible to the building inspector.

Correct identification of the insect responsible for the damage is essential if theappropriate control method is to be selected. The characteristics of each of the morecommon groups of beetles, bees, and wasps are discussed in the following table whichsummarizes the size and shape of entry or exit holes produced by wood-boring insects, thetypes of wood they attack, the appearance of frass or sawdust in insect tunnels, and theinsect's ability to reinfest wood in a house.

To use the table, match the size and shape of the exit or entry holes in the wood tothose listed in the table; note whether the damaged wood is a hardwood or softwood andwhether damage is in a new or old wood product (evidence of inactive infestations ofinsects that attack only new wood will often be found in old wood; there is no need forcontrol of these). Next, probe the wood to determine the appearance of the frass. Itshould then be possible to identify the insect type. It is clear from the table that thereis often considerable variation within particular insect groups.

Where the inspector is unsure of the identity of the insect causing damage, a qualifiedentomologist should be consulted.

Lyctid powder-post beetles

Lyctids attack only the sapwood of hardwoods with large pores: for example, oak,hickory, ash, walnut, pecan, and many tropical hardwoods. They reinfest seasoned wooduntil it disintegrates. Lyctids range from 3 to 7 mm in length and are reddish-brown toblack. The presence of small piles of fine flour-like wood powder (frass) on or under thewood is the most obvious sign of infestation.

Even a slight jarring of the wood makes the frass sift from the holes. There are nopellets. The exit holes are round and vary from 1 to 1.5 mm in diameter. Most of thetunnels are about 1.5 mm in diameter and loosely packed with fine frass. If damage issevere, the sapwood may be completely converted to frass within a few years and held inonly by a very thin veneer of surface wood with beetle exit holes.

The amount of damage depends on the level of starch in the wood. Infestations arenormally limited to hardwood paneling, trim, furniture, and flooring. Replacement orremoval and fumigation of infested materials are usually the most economical and effectivecontrol methods. For current information on the use of residual insecticides, theinspector should contact the extension entomologist at his nearest land grant universityor a reputable pest control company.

Anobiid beetles

The most common anobiids attack the sapwood of hardwoods and softwoods. They reinfestseasoned wood if environmental conditions are favorable. Attacks often start in poorlyheated or ventilated crawl spaces and spread to other parts of the house. They rarelyoccur in houses on slab foundations. Anobiids range from 3 to 7 mm in length and arereddish-brown to nearly black.

Adult insects are rarely seen. The most obvious sign of infestation is the accumulationof powdery frass and tiny pellets underneath infested wood or streaming from exit holes.The exit holes are round and vary from 1.5 to 3 mm in diameter. If there are large numbersof holes and the powder is bright and light colored like freshly sawed wood, theinfestation is both old and active. If all the frass is yellowed and partially caked onthe surface where it lies, the infestation has been controlled or has died out naturally.Anobiid tunnels are normally loosely packed with frass and pellets. It is normally 10 ormore years before the number of beetles infesting wood becomes large enough for theirpresence to be noted. Control can be achieved by both chemical and non-chemical methods.For current information on control of anobiids, the inspector should contact the extensionentomologist at his nearest land grant university or a reputable pest control company.

Bostrichid powderpost beetles

Most bostrichids attack hardwoods, but a few species attack softwoods. They rarelyattack and reinfest seasoned wood. Bostrichids range from 2.5 to 7 mm in length and fromreddish-brown to black. The black polycaon is an atypical bostrichid and can be 13 to 25mm in length. The first signs of infestation are circular entry holes for the egg tunnelsmade by the females. The exit holes made by adults are similar, but are usually filledwith frass. The frass is meal-like and contains no pellets. It is tightly packed in thetunnels and does not sift out of the wood easily. The exit holes are round and vary from2.5 to 9 mm in diameter. Bostrichid tunnels are round and range from 1.5 to 10 mm indiameter.

If damage is extreme, the sapwood may be completely consumed. Bostrichids rarely causesignificant damage in framing lumber and primarily affect individual pieces of hardwoodflooring or trim. Replacement of structurally weakened members is usually the mosteconomical and effective control method.

Old house borer

This beetle infests the sapwood of softwoods, primarily pine. It reinfestsseasoned wood, unless it is very dry.

The old house borer probably ranks next to termites in the frequency with which itoccurs in houses in the mid-Atlantic states. The beetle ranges from 15 to 25 mm in length,and is brownish-black in color. The first noticeable sign of infestation by the old houseborer may be the sound of larvae boring in the wood. They make a rhythmic ticking orrasping sound, much like a mouse gnawing. In severe infestations the frass, which ispacked loosely in tunnels, may cause the thin surface layer of the wood to bulge out,giving the wood a blistered look.

When adults emerge (three to five years in the South, five to seven years in theNorth), small piles of frass may appear beneath or on top of infested wood. The exit holesare oval and 6 to 10 mm in diameter. They may be made through hardwood, plywood, woodsiding, trim, sheetrock, paneling, or flooring. The frass is composed of very fine powderand tiny blunt-ended pellets.

If damage is extreme, the sapwood may be completely reduced to powdery frass with avery thin layer of surface wood. The surfaces of the tunnels have a characteristic rippledpattern, like sand over which water has washed. Control can be achieved by both chemicaland non-chemical methods. For current information on control of the old house borer, theinspector should contact the extension entomologist at his nearest land grant universityor a reputable pest control company.

Carpenter bees

Carpenter bees usually attack soft and easy-to-work woods, such as California redwood,cypress, cedar, and Douglas fir. Bare wood, such as unfinished siding or roof trim, ispreferred. The only external evidence of attack is the entry holes made by the female.These are round and 9 mm in diameter. A rather course sawdust-like frass may accumulate onsurfaces below the entry hole. The frass is usually the color of freshly sawed wood.

The presence of carpenter bees in wood sometimes attracts woodpeckers, which increasesthe damage to the surface of the wood. The carpenter bee tunnels turn at a right angleafter extending approximately an inch across the grain of the wood, except when entry isthrough the end of a board. They then follow the grain of the wood in a straight line,sometimes for several feet.

The tunnels are smooth-walled. It takes several years of neglect for serious structuralfailure to occur. However, damaged wood is very unsightly, particularly if woodpeckershave followed the bees. The bees can be controlled by applying five to 10 percent carbaryl(Sevin) dust into the entry holes. Several days after treatment, the holes should beplugged with dowel or plastic wood. Prevention is best achieved by painting all exposedwood surfaces.

Other wood-inhabiting insects

There are several other species of insects that infest dying or freshly felled trees orunseasoned wood, but that do not reinfest seasoned wood. They may emerge from wood in afinished house or evidence of their presence may be observed. On rare occasions, controlmeasures may be justified to prevent disfigurement of wood, but control is not needed toprevent structural weakening.

• Ambrosia beetles. These insects attack unseasoned sapwood and heartwood of soft-wood and hardwood logs, producing circular bore holes 0.5 to 3 mm in diameter. Bore holes do not contain frass, but are frequently stained blue, black, or brown. The insects do not infest seasoned wood.
• Bark beetles. These beetles tunnel at the wood/bark interface and etch the surface of wood immediately below the bark. Beetles left under bark edges on lumber may survive for a year or more as the wood dries. Some brown, gritty frass may fall from circular bore holes 1.5 to 2.5 mm in diameter in the bark. These insects do not infest wood.
• Horntails (wood wasps). Horntails generally attack unseasoned softwoods and do not reinfest seasoned wood. One species sometimes emerges in houses from hardwood firewood. Horntails occa-sionally emerge through panel-ing, siding, or sheetrock in new houses; it may take four to five years for them to emerge. They attack both sapwood and heartwood, producing a tunnel that is roughly C-shaped in the tree. Exit holes and tunnels are circular in cross-section and 1.5 to 7 mm in diameter. Tunnels are tightly packed with course frass. Frequently, tunnels are exposed on the surface of lumber by milling after the development of the insect.
• Round-headed borers. Several species are included in this group. They attack the sapwood of softwoods and hardwoods during storage, but rarely attack seasoned wood. The old house borer is the major round-headed borer that can reinfest seasoned wood. When round-headed borers emerge from wood, they make slightly oval to nearly round exit holes 3 to 10 mm in diameter. Frass varies from rather fine and meal-like in some species to very course fibers like pipe tobacco in others. Frass may be absent from tun-nels, particularly where the wood was machined after the emergence of the insects.
• Flat-headed borers. These borers attack sapwood and heartwood of softwoods and hardwoods. Exit holes are oval, with the long diameter 3 to 13 mm. Wood damaged by flat-headed borers is generally sawed after damage has occurred, so tunnels are exposed on the surface of infested wood. Tunnels are packed with sawdust-like borings and pellets, and tunnel walls are covered with fine transverse lines somewhat similar to some round-headed borers. However, the tunnels are much more flattened. The golden buprestid is one species of flat-headed borer that occurs occasionally in the Rocky Mountain and Pacific Coast states. It produces an oval exit hole 5 to 7 mm across, and may not emerge from wood in houses for 10 or more years after infestation of the wood. It does not reinfest seasoned wood.

If signs of insect or fungus damage other than those already described are observed,the inspector should have the organism responsible identified before recommendingcorrective measures.

Small samples of damaged wood, with any frass and insect specimens (larvae or grubsmust be stored in vials filled with alcohol), should be sent for identification to theentomology or pathology department of the state land grant university.