Wood is a porous material and will absorb moisture from the
air. Moisture is attracted to the walls of the tubes that
make up the wood. As walls absorb moisture, the wood swells.
If the humidity is kept at 100 percent, the walls become saturated
with water. The moisture content at which this occurs is the
fiber saturation point, which is approximately 30 percent
by weight for most species used in construction. Fungi will
only decay wood with a moisture content above the fiber saturation
point. To allow a safety margin, wood with a moisture content
above 20 percent is considered to be susceptible to decay.
Wood in properly constructed buildings seldom will have a
moisture content above 16 to l8 percent. Thus, wood will only
decay if it is in contact with the ground or wetted by an
external source of moisture, such as rain seepage, plumbing
leaks, or condensation. Dry wood will never decay. Also, the
drier the wood, the less likely it is to be attacked by most
types of wood-inhabiting insects.
Wood-inhabiting fungi are small plants that lack chlorophyll
and use wood as their food source. 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, which eventually becomes structurally useless. All
fungi require moisture, oxygen, warmth, and food. The keys
to preventing or controlling growth of fungi in wood in buildings
are to either keep the wood dry (below a mois-ture content
of 20 percent) or to use preservative-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 highly localized. Some, although common,
cause very little structural damage. The following is a description
of the major wood-inhabiting fungi and insects in the United
Surface molds and sapstain fungi
Surface molds or mildew fungi discolor the surface of wood,
but do not weaken it. They are generally green, black, or
orange and powdery in appearance. The various building codes
allow the use of framing lumber with surface molds or mildew,
providing that the wood is dry and not decayed. Spores (or
seeds) of surface molds or mildew fungi grow quickly 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 or on the building site, or in a
finished house. When the wood dries, the fungi die or become
dormant, but they do not change their appearance. Thus,
wherever surface molds or mildew fungi are observed on wood
in a building, it is a warning sign that at some time the
wood was moist or humidity was high.
Surface molds and mildew fungi are controlled by eliminating
the source of high humidity or excess moisture, for example
by repairing leaks, improving ventilation in attics or crawl
spaces, or installing soil covers. Before taking corrective
action, the source of the moisture that allowed fungus growth
must be determined. If the wood is dry and the sources of
moisture are no longer present, no corrective action need
Sapstain or bluestain fungi are similar to surface molds,
except that the discoloration goes 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 become dormant. They usually
occur in the living tree or before the wood is seasoned,
but sometimes they grow in the supplier's yard, on the building
site, or in a finished house. In the latter case, they are
normally associated with rain seepage or leaks. Stain fungi
are a warning sign that at some time the wood was moist.
Control is the same as for surface molds or mildew fungi.
Most decay fungi are able to grow only on moist wood and
cannot attack adjacent dry wood. Two brown-rot fungi, Poria
incrassata and Merulius lacrymans, are able to conduct water
for several feet through root-like strands or rhizomorphs,
to moisten wood and then to decay it.
These are sometimes called water-conducting or dry-rot
fungi. They can decay wood in houses very rapidly, but fortunately
they are quite rare. Poria incrassata is found most frequently
in the Southeast and West. Merulius lacrymans occurs in
Both fungi can cause extensive damage in floors and walls
away from obvious sources of moisture. Decayed wood has
the characteristics of brown rotted wood except that the
surface of the wood sometimes appears wavy but apparently
sound, although the inte-rior may be heavily decayed. The
rhizomorphs that characterize these fungi can be up to an
inch in diameter and white to black in color, depending
on their age. They can penetrate foundation walls and often
are hidden between wood members.
The source of moisture supporting the fungal growth must
be found and eliminated to control decay. Common sources
include water leaks and wood in contact with or close to
the soil: for example, next to earth-filled porches or planters.
Where the fungus grows from a porch, the soil should be
removed from the porch next to the foundation wall to prevent
continued growth of the fungus into the house. Poria incrassata
normally occurs in new or remodeled 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 grow only on moist wood. The fungi
can be present in the wood when it is brought into the house
or can grow from the spores that are always present in the
air and soil. Wood attacked by these fungi should not be
used in construction. Wood decayed by brown-rot fungi is
brittle and darkened in color. As decay proceeds, the wood
shrinks, twists, and cracks perpendicular to the grain.
Finally, it becomes dry and powdery. Brown-rot is the commonest
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 until decay
These fungi can be controlled by eliminating the source
of moisture that allows them to grow, for example by improving
drainage and ventilation under a house, repairing water
leaks, 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 be
eliminated, all the decayed wood should be replaced with
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 found
in softwood lumber from the West Coast.
Subterranean termites normally damage the interior of wood
structures. Shelter tubes are the most common sign of their
presence. Other signs include structural weakness of wood
members, shed wings or warmers, soil in cracks or crevices,
and dark or blister-like areas on wood.
The major characteristics of infested softwood when it is
broken open are that damage is normally greatest in the
softer springwood and that gallery walls and inner surfaces
of shelter tubes have a pale, spotted appearance like dried
oatmeal. The galleries often contain a mixture of soil and
digested wood. Termites usually enter houses through wood
in contact 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 can survive 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 during the first five to 10 years
of a building's life, although their attack may start as
soon as it is built.
Subterranean termites can be controlled most effectively
by the use of chemicals in the soil and foundation area
of the house, by breaking wood-soil contact, and by eliminating
excess moisture in the house. When applied properly, these
chemicals will prevent or control termite attack for at
least 25 years.
Formosan subterranean termites
Formosan subterranean termites are a particularly vigorous
species of subterranean termite that has spread to this
country from the Far East. They have caused considerable
damage in Hawaii and Guam and have been found in several
locations on the United States mainland.
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. The
most obvious characteristics that distinguish Formosan subterranean
termite swarmers from those of native species are their
larger size (up to16 mm compared to 9 to 13 mm) and hairy
wings (com-pared with smooth wings in other subterraneans).
Soldiers have oval shaped heads, as opposed to the oblong
and rectangular heads of native soldiers. Formosan termites
also produce a hard material called carton, which resembles
sponge. This is sometimes found in cavities under fixtures
or in walls adjacent to attacked wood. Other characteristics-
and control methods-are similar to those for native subter-ranean
However, Formosan subterranean termites are more vigorous
and can cause extensive damage more rapidly than do native
species. For this reason Formosans should be controlled
as soon as possible after discovery.
It is quite common for buildings to be infested by drywood
termites within the first five years of their construction
in southern California, southern Arizona, southern Florida,
the Pacific area, and the Caribbean. Swarmers generally
enter through attic vents or shingle roofs, but in hot,
dry locations, they can be found in crawl spaces. Window
sills and frames are other common entry points.
Drywood termites live in wood that is dry. They require
no contact with the soil or with any other source of moisture.
The first sign of drywood termite infestation is usually
piles of fecal pellets, which are hard, less than 1 mm in
length, with rounded ends and six flattened or depressed
sides. The pellets vary in color from light gray to very
dark brown, depending on the wood being consumed. The pellets,
eliminated from galleries in the wood through round kick
holes, accumulate on surfaces or in spider webs below the
kick holes. There is very little external evidence of drywood
termite attacks in wood other than the pellets. The interior
of damaged wood has broad pockets or chambers that 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 the termites to cause
serious weakness in house framing. Damage to furniture,
trim, and hardwood floors can occur in a few years. The
choice of control method depends on the extent of damage.
If the infestation is widespread or inaccessible, the entire
house should be fumi-gated. If infestation is limited, spot
treatment can be used or the damaged wood can be removed.
Dampwood termites of the desert Southwest and southern
Florida are rarely of great danger to structures. Pacific
Coast dampwood termites can cause damage greater than subterranean
termites if environmental conditions are ideal.
Dampwood termites build their colonies in damp, sometimes
decaying wood. Once established, some species extend their
activities to sound wood. They do not require contact with
the ground, but do require wood with a high moisture content.
There is little external 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 dampwood termites depends on the amount of decay
present. In comparatively sound wood, galleries follow the
springwood. In decayed wood, galleries are larger and pass
through both springwood and summerwood. Some are round in
cross section, others are oval. The surfaces of the galleries
have a velvety appearance and are sometimes covered with
dried fecal material.
Fecal pellets are about 1 mm long and colored according
to the kind of wood being eaten. 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 the sides of the galleries.
Dampwood termites must maintain contact with damp wood.
Therefore, they can be controlled by eliminating damp wood.
Treatment of the soil with chemicals can also be used to
advantage in some areas.
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 in houses.
Normally they do not cause extensive structural damage.
Most species start their nests 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 13 mm long, inside the house. Damage
occurs in the interior of the wood. There may be piles or
scattered bits of wood powder (frass), which are very fibrous
and sawdust-like. If the frass is from decayed wood, pieces
tend to be darker and more square ended. The frass is expelled
from cracks and crevices, or from slit-like openings made
in the wood by the ants. It is often found in basements,
dark closets, attics, under porches, and in crawl spaces.
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 if they had been sandpapered,
and are clean. The most effective way to control carpenter
ants is to locate the nest and kill the queen in colonies
in and near the house with insecticides. It is sometimes
also helpful to treat the voids in walls, etc. For current
information 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 these cause considerable damage
if not controlled quickly. Others are of minor importance
and attack only unseasoned wood. Beetles, bees, and wasps
all have larval, or grub, stages in their life cycles, and
the mature flying insects produce entry or exit holes in
the surface of the wood. These holes, and sawdust from tunnels
behind the holes, are generally the first evidence of attack
that is visible to the building inspector.
Correct identification of the insect responsible for the
damage is essential if the appropriate control method is
to be selected. The characteristics of each of the more
common groups of beetles, bees, and wasps are discussed
in the following table which summarizes the size and shape
of entry or exit holes produced by wood-boring insects,
the types of wood they attack, the appearance of frass or
sawdust in insect tunnels, and the insect'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 to those listed in the table;
note whether the damaged wood is a hardwood or softwood
and whether damage is in a new or old wood product (evidence
of inactive infestations of insects that attack only new
wood will often be found in old wood; there is no need for
control of these). Next, probe the wood to determine the
appearance of the frass. It should then be possible to identify
the insect type. It is clear from the table that there is
often considerable variation within particular insect groups.
Where the inspector is unsure of the identity of the insect
causing damage, a qualified entomologist should be consulted.
How to identify common beetles, bees and wasps that attack
Shape and Size (inches) of Exit/ Entry Hole Wood Type Age
of Wood At- tacked Appearance of Frass in Tunnels Insect Type
Round 1/50 - 1/8 Softwood & hardwood New None Present
Ambrosia beetles No
Round 1/32 - 1/16 Hardwood New & old Fine, flour-like,
loosely packed Lyctid beetles No
Round 1/16 - 3/32 Bark/ sapwood interface New Fine to coarse,
bark colored, tightly packed Bark beetles No
Round 1/16 - 1/8 Softwood & hardwood New & old Fine
powder and pellets, loosely packed; pellets may be absent
and frass tightly packed in some hardwoods Anobiid beetles
Round 3/32 - 9/32 Softwood & hardwood (bamboo) New Fine
to coarse powder, tightly packed Bostrichid beetles Rarely
Round 1/6 - 1/4 Softwood New Coarse, tightly packed Horntail
or woodwasp No
Round 1/2 Softwood New & old None Present Carpenter bee
Round-oval 1/8 - 3/8 Softwood & hardwood New Coarse to
fibrous, mostly absent Round- headed borer No
Oval 1/8 - 1/2 Softwood & hardwood New Sawdust-like, tightly
packed Flat-headed borer No
Oval 1/4 - 3/8 Softwood New & old Very fine powder &
tiny pellets, tight Old house borer Yes
Flat oval 1/2 or more or irregular surface groove 1/8 - 1/2
wide Softwood & hardwood New Absent or sawdust-like, coarse
to fibrous; tightly packed Round- or flat-headed borer, wood
machined after attack No
(New wood is defined as standing or freshly felled trees and
unseasoned lumber. Old wood is seasoned or dried lumber.)
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 wood until
it disintegrates. Lyctids range from 3 to 7 mm in length
and are reddish-brown to black. The presence of small piles
of fine flour-like wood powder (frass) on or under the wood
is the most obvious sign of infestation.
Even a slight jarring of the wood makes the frass sift
from the holes. There are no pellets. The exit holes are
round and vary from 1 to 1.5 mm in diameter. Most of the
tunnels are about 1.5 mm in diameter and loosely packed
with fine frass. If damage is severe, the sapwood may be
completely converted to frass within a few years and held
in only by a very thin veneer of surface wood with beetle
The amount of damage depends on the level of starch in
the wood. Infestations are normally limited to hardwood
paneling, trim, furniture, and flooring. Replacement or
removal and fumigation of infested materials are usually
the most economical and effective control methods. For current
information on the use of residual insecticides, the inspector
should contact the extension entomologist at his nearest
land grant university or a reputable pest control company.
The most common anobiids attack the sapwood of hardwoods
and softwoods. They reinfest seasoned wood if environmental
conditions are favorable. Attacks often start in poorly
heated or ventilated crawl spaces and spread to other parts
of the house. They rarely occur in houses on slab foundations.
Anobiids range from 3 to 7 mm in length and are reddish-brown
to nearly black.
Adult insects are rarely seen. The most obvious sign of
infestation is the accumulation of 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 numbers of holes and the
powder is bright and light colored like freshly sawed wood,
the infestation is both old and active. If all the frass
is yellowed and partially caked on the 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 or more years before
the number of beetles infesting wood becomes large enough
for their presence 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
extension entomologist 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 rarely attack and reinfest seasoned wood.
Bostrichids range from 2.5 to 7 mm in length and from reddish-brown
to black. The black polycaon is an atypical bostrichid and
can be 13 to 25 mm in length. The first signs of infestation
are circular entry holes for the egg tunnels made by the
females. The exit holes made by adults are similar, but
are usually filled with frass. The frass is meal-like and
contains no pellets. It is tightly packed in the tunnels
and does not sift out of the wood easily. The exit holes
are round and vary from 2.5 to 9 mm in diameter. Bostrichid
tunnels are round and range from 1.5 to 10 mm in diameter.
If damage is extreme, the sapwood may be completely consumed.
Bostrichids rarely cause significant damage in framing lumber
and primarily affect individual pieces of hardwood flooring
or trim. Replacement of structurally weakened members is
usually the most economical and effective control method.
Old house borer
This beetle infests the sapwood of softwoods, primarily
pine. It reinfests seasoned wood, unless it is very dry.
The old house borer probably ranks next to termites in the
frequency with which it occurs 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 house borer may be the sound of larvae
boring in the wood. They make a rhythmic ticking or rasping
sound, much like a mouse gnawing. In severe infestations
the frass, which is packed 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 the North), small piles of frass may appear
beneath or on top of infested wood. The exit holes are oval
and 6 to 10 mm in diameter. They may be made through hardwood,
plywood, wood siding, trim, sheetrock, paneling, or flooring.
The frass is composed of very fine powder and tiny blunt-ended
If damage is extreme, the sapwood may be completely reduced
to powdery frass with a very thin layer of surface wood.
The surfaces of the tunnels have a characteristic rippled
pattern, like sand over which water has washed. Control
can be achieved by both chemical and non-chemical methods.
For current information on control of the old house borer,
the inspector should contact the extension entomologist
at his nearest land grant university or a reputable pest
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,
is preferred. 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
on surfaces 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 increases the damage to the surface of
the wood. The carpenter bee tunnels turn at a right angle
after extending approximately an inch across the grain of
the wood, except when entry is through 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 structural failure to occur. However,
damaged wood is very unsightly, particularly if woodpeckers
have 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 be
plugged with dowel or plastic wood. Prevention is best achieved
by painting all exposed wood surfaces.
Other wood-inhabiting insects
There are several other species of insects that infest
dying or freshly felled trees or unseasoned wood, but that
do not reinfest seasoned wood. They may emerge from wood
in a finished house or evidence of their presence may be
observed. On rare occasions, control measures may be justified
to prevent disfigurement of wood, but control is not needed
to prevent structural weakening.
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.
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.
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.
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 recommending
Small samples of damaged wood, with any frass and insect
specimens (larvae or grubs must be stored in vials filled
with alcohol), should be sent for identification to the
entomology or pathology department of the state land grant