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Drywood Termites

Illustration by D. Kidd

The western drywood termite, Incisitermes minor, is California's second most important termite pest after the western subterranean termite. It is a native insect that has been here millions of years, mostly attacking trees along river washes and arroyos. In California drywood termites are most prevalent in southern California and the Central Valley but also can be found infesting wood along the coast, in bay areas south of San Francisco, and in the southern California desert.

DETECTION

Drywood termites are secretive insects and are difficult to detect. They live deep inside wood and, except during periods when they swarm or when repair work is being done on infested homes, they are seldom seen. Colonies are small (usually fewer than 1,000 individuals), can be widely dispersed, and take years to mature. While a homeowner may initially detect the presence of termites when they swarm or if fecal pellets are discovered, inspecting for drywood termites and determining the extent of an infestation require experience.

The minimum requirement by California state law for drywood termite inspections includes visual searches of accessible areas. However, detection of difficult-to-find infestations may require removal of walls, paneling, and stucco as well as the use of ladders and scaffolds.

During a visual inspection for drywood termites, our inspectors look for feeding damage, shed wings, termite fecal pellets, and kickout holes, which are small holes the size of BB shot through which termites push fecal pellets out of the wood.

Fecal pellets, hexagonal in shape, are diagnostic for drywood termites. However, whether the infestation is currently active or what the extent of the infestation is cannot be determined from pellets alone. Cleaning up the fecal pellets around a kickout hole and checking a few days later to see if new pellets have appeared can help to determine if an infestation is active. (Building vibrations/movements may cause some pellets to appear.) If an active infestation of drywood termites is found in your structure, you should have it treated.

ELIMINATING EXISTING INFESTATIONS

All drywood termite control methods can be categorized as either whole-structure or localized.  A whole-structure treatment is defined as the simultaneous treatment of all infestations, accessible and inaccessible, in a structure. A localized or spot treatment is more restrictive, often applied to a single board or small group of boards. Homeowners are advised to know the distinction between whole-structure and spot treatments when deciding which method to select because all treatment methods are not equivalent.

Whole-structure treatments have an advantage over spot treatments in that they can eliminate all infestations, even hidden ones. With the uncertainty of current detection methods, particularly when drywall or other wall coverings conceal infestations, there is always some doubt as to the extent of dry-wood termite colony boundaries within homes. Consequently one can never be sure that all infestations have been treated when applying spot treatments. The strengths and limitations of whole-structure and spot/localized treatments are outlined in Table 1.

TABLE 1. Summary of Commercially Available Drywood Termite Management Options.

Treatment	Efficacy in field	Strengths	Considerations/Limitations	Damage to structure
EXISTING INFESTATIONS
Whole-structure
Fumigants	up to 100% 1,2,3	hidden sites treated	correct dosage must be achieved; residents must leave house; no residual	gas pilots must be extinguished before treatment; possible damage to roof from tarpaulins or if walked on
Heat	up to 100%2	hidden sites treated	lethal temperature must be achieved in the core of all infested wood; no residual; heat sinks may affect efficacy	possible damage to roof if walked on and for some heat-sensitive household items
Localized/Spot treatments
Chemical
Chemical liquids & dusts	up to 90%3	long-term	few active ingredients commercially available; detection accuracy critical; chemical residual; results vary with active ingredient used and concentration; infestation may rebound	yes, if drill holes used
Chemical foams	no information	coverage of hidden infestation, long-term	no published efficacy studies	yes, drilling holes
Liquid nitrogen	74 to 100%2	benign material	highly dependent on dosage; detection accuracy critical; no residual	yes, drill holes
Nonchemical
Biological control	no information	no chemicals	few commercially available; research needed	don’t know
Electrocution	44 to 98%2	portable	detection accuracy critical; many disclaimers; infestation may rebound	yes, if drill holes used
Heat	up to 100%2	semi-portable	lethal temperature must be achieved in the core of all infested wood; no residual; heat sinks may affect efficacy	may be to wood or heat-sensitive household items
Microwaves	89 to 98%2	semi-portable	detection accuracy critical; highly dependent on treatment time and wattage; heat sinks may affect efficacy	may be to wood or household items
PREVENTIVE
Chemical liquids & dusts	up to 90%3	long-term	few active ingredients available; chemical residual; results vary depending on active ingredient used and concentration; infestation may rebound	yes, if drill holes used
Pressure-treated wood	no information	long-term	few active ingredients commercially available; chemical residual; results vary with active ingredient used and concentration; environmental persistence	no
Nonchemical
Barriers (screens/paint)	no information	long-term	barriers degrade & can be breached; some feeding damage may occur	no
Resistant woods	no information	long-term	efficacy highly variable depending on species of wood; costly; availability; some feeding damage may occur	no
1 - Su & Scheffrahn 1986; 2 - Lewis & Haverty 1996; 3 - Scheffrahn et al. 1997

Whole-structure Treatment

Fumigants (sulfuryl fluoride) treat all infestations simultaneously and have high levels of efficacy if correctly applied. Sulfuryl fluoride kills drywood termites in about 3 days. A monitored fumigation, which involves installing gas monitoring lines inside the structure undergoing treatment, has the highest rate of treatment success. Nonmonitored fumigation may not have enough gas concentration to kill infestations, and failures may occur. Fumigation's advantage over localized treatment is that it may 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. Residual control means long-term protection (several years or more) from drywood termite attack. (Generally, only chemicals added to or onto wood provide residual control.) It will also be necessary to vacate the structure for 2 to 3 days while it is being treated and then ventilated. Additionally, roofs may be damaged by having tarpaulins dragged across them.

Methyl bromide was another fumigant used for many decades in California to control drywood termites. However, because of environmental concerns about the atmospheric ozone layer, the strong odors of some formulations, the long aeration times for fumigated structures, and the need for extensive aeration buffer areas around structures, this fumigant has been phased out for urban use in California.

Heat is a nonchemical option for whole-structure treatments. Excessive heat kills drywood termites by disrupting cellular membranes and denaturing enzymes needed for their survival. The treatment process involves heating all wood in the structure to a minimum of 120°F and holding this temperature for at least 33 minutes. The benefit of heat treatment is the ability to treat the entire structure without the use of chemicals and the relatively short period of time the structure must be vacated (hours instead of days, as with the use of fumigants). An additional advantage is that portions of large structures can be treated separately, which is very useful in apartments and condominiums. 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, which are areas within the structure that are difficult to heat, such as wood on concrete or tile. As more powerful and efficient heaters are developed, larger homes can be efficiently treated with heat.

Other issues to consider include damage to heat-sensitive items in homes including plastics (e.g. electrical outlet covers) and cable wiring. Also, like fumigants, heat treatments have no residual control. Of course, preventive chemicals can be applied to areas treated with fumigants or heat for long-term protection (see preventive section in Table 1).

Localized or Spot Treatments

There are many localized/spot treatment methods available (Table 1) that include both chemical and nonchemical options. The chemical options include aerosol pyrethrum and aerosol and liquid pyrethroids (cyfluthrin, permethrin, bifenthrin), liquid imidacloprid, liquid nitrogen, and liquid and dust formulations of disodium octaborate tetrahydrate. Chemicals that have been phased out of commercial use include organophosphates, carbamates, silica-gel, and dri-die. For liquid and dust insecticides to be effective, termites must touch or ingest them. Spot treatments should be applied only by licensed applicators. Home use products are not effective.

Depending on the chemical used for spot treatments, laboratory studies have shown a variation of 13% to 100% in their effectiveness in controlling drywood termites. However, many of these chemicals have not been tested in large-scale field-tests. A newer insecticide (imidacloprid) with very good lab and field results is available and will be used increasingly in California. Botanical-based products (orange oil and neem oil) have been tried, but there are no published studies that verify the efficacy of these materials in controlling drywood termites. Recent experiments evaluating surface or gallery injections of aqueous disodium octaborate tetrahydrate did not effectively control a closely related species of drywood termites, Incisitermes synderi (Scheffrahn et al. 1997).

Liquid nitrogen is different from other spot treatment methods in that its mode of action is thermal; it causes a sudden drop in temperature, which kills the termites. Laboratory studies have shown drywood termites are killed after momentary exposures of temperatures in the range of -5.8°F to 1.4°F when temperatures were lowered from room temperature at a rate of 33.8°F per minute (Rust et al. 1997). Studies on liquid nitrogen show that dosages exceeding 30 pounds per enclosed wall space between 2 by 4s achieve high levels of effectiveness. Although most chemicals used for spot treatments give long-term control, liquid nitrogen has no residual activity when used alone. Minor damage to the structure occurs from the holes drilled for spot treatments of chemicals and for liquid nitrogen insertion. For all chemical spot treatments, including liquid nitrogen, it is critical that all infestations in a structure are detected so that they all receive treatment.

There are four nonchemical options for drywood termite control with spot or localized application (Table 1), including heat, which is used for both spot and whole-structure treatments. The advantages and disadvantages discussed for heat as a whole-structure treatment also apply to spot treatments. Microwave devices are also available for drywood termite control. Microwaves kill termites by causing fluids inside their cells to boil, which destroys cell membranes; in short, the termites are cooked inside the wood. There are a number of firms now offering microwave treatments. One advantage of microwaves is their relative portability; another advantage is that they leave no chemical residue. When using microwaves, however, detection accuracy is critical to success. Both microwaves and heat treatments may damage the surface or interior of wood boards, depending on the power of the device. (The wattage or power of microwave or heating devices may vary from several hundred to more than 10,000 watts.) Lab studies revealed no relationship between increasing microwave wattage and drywood termite mortality (Lewis et al. 2000). As with heat treatments, it may be difficult to heat areas with heat sinks to high enough temperatures with microwaves for effective control.

High voltage electricity, or electrocution, is another nonchemical option for controlling drywood termites. The device currently marketed uses high voltage (90,000 volts) but low current (less than 0.5 amps). Death to drywood termites occurs by electric shock, although delayed mortality may also occur from the destruction of intestinal protozoa. The advantage of electrocution is that the equipment is portable. The limitations include detection accuracy and the possible reduced efficacy from the interfering actions of common building materials, for example metal, concrete, and glass. If drill holes are used to enhance the flow of current into wood, damage occurs to wall coverings, walls, and structural wood members.

Wood replacement is another remedial treatment option. However, similar to other spot treatments, its effectiveness is highly dependent on detection accuracy and extent and location of the infestation, and it may be expensive to accomplish.

There is little research on biological control of drywood termites. Biological control is the use of other life forms (e.g., insects, nematodes, or microbes) to control pest insects. Although predators, parasites, and pathogens have been shown to control other insect pests, their efficacy for drywood termite control has not been explored.

LONG-TERM PREVENTIVE TREATMENTS

Although chemicals are commercially available in California for long-lasting prevention against infestation, there is little data on their effectiveness against the drywood termites that occur in California. Recent research from the University of Florida demonstrates that new colony establishment by another species of drywood termite, Cryptotermes brevis, could be prevented using dust formulations of commercially available disodium octaborate tetrahydrate (Scheffrahn et al. 2001). Drawbacks with some chemical preventive treatments include damage from drill holes and unsightly appearance from dusts.

Pressure-treated wood (chemically treated wood that is green in color) for drywood termite prevention can be effective for species that occur in California. However, the use of most wood preservatives has been restricted. Painting of wood with enamel, shellac, or varnish gives very little protection against drywood termite feeding.

Integrating nonchemical and chemical treatments to ensure that termites are not able to colonize over the long term is a strategy used by some pest control professionals. Nonchemical, long-term preventive methods include physical barriers, such as metal screens. Resistant woods can reduce but do not eliminate damage. There are few studies that demonstrate the efficacy of combinations of methods or of nonchemical, long-term preventive treatments directed against drywood termites.

DID I MAKE THE RIGHT CHOICE?

When planning treatment for drywood termites, consider whether the whole structure is to be treated or just localized areas. Localized/spot treatment methods make it more difficult to ensure complete control because of the difficulty in determining the extent of a drywood termite infestation. There also appears to be considerable variation in effectiveness of various techniques from applicator to applicator. Read your guarantee carefully; you may wish to consider an annual inspection service. Also important is a company's reputation. There are thousands of pest control companies in the state. They don't all have the same services or performance. Obtain at least three vendor bids before you decide. Check the reliability of the vendor by asking for client referrals and check the status of its business license and consumer complaints with the California Department of Consumer Affairs, Structural Pest Control Board, in Sacramento and with your local Better Business Bureau. For added information on safety of chemicals to humans and structures, request the Material Safety Data Sheets or equivalent information for nonchemical control methods from the pest control company.

In summary, research indicates that if you correctly locate the colony and get the chemical or nonchemical treatment directly onto the termites, the effectiveness of control will be high (90%). For failed treatments, an additional callback treatment may lead to better results.

Subterranean Termites

Subterranean termites live in underground colonies or in moist secluded areas aboveground that can contain up to 2 million members. They build distinctive "mud tubes" to gain access to food sources and to protect themselves from open air. Termite colonies are organized into castes depending on tasks -- workers, soldiers and reproductives. The characteristics of a subterranean termite are dependent on the termite's role in the colony. Cream-colored Worker subterranean termites are 1/8 to 3/8's of an inch in length. Soldier subterranean termites are of a similar body length, but are distinguished by their powerful mandibles. Solider termites have cream-colored bodies and brown heads. Reproductive subterranean termites are approximately one inch long.

Habits

Subterranean termites live underground and build tunnels, referred to as mud tubes, to reach food sources. Like other termite species, they feed on products containing cellulose. Subterranean termites swarm in the spring -- groups of reproductive termites go off to start new colonies.

Habitat

Subterranean termites need contact with the soil to survive and live underground. They can build tunnels through cracks in concrete.

Threats Subterranean termites are by far the most destructive species. They can collapse a building entirely, meaning possible financial ruin for a homeowner. The hard, saw-toothed jaws of termites work like shears and are able to bite off extremely small fragments of wood, one piece at a time.

Subterranean termites, in natural settings, work as beneficial insects by breaking down cellulose-containing materials, such as dead trees. They live in the soil and must maintain contact with the ground or some other moisture source to survive. Termites become a problem to humans when structures containing cellulose are built over or near their colonies in the ground. They are able to find weakened areas in the structure, or areas of direct wood-to-ground contact, and feed on the cellulose. Termites build earthen shelter tubes from the ground into the structure for protection from predators and to help maintain a moist environment. Many times these tubes are built on inside walls, porches or chimneys where they cannot be seen.
 
In some rare situations, if water and wood are available from a source other than the soil, subterranean termites can establish a colony with no ground contact. Isolated, above-ground infestations may occur in buildings where termites have access to water from condensation, leaking pipes, roofs or other sources.
 
TERMITE COLONY
 
Termites are social insects that live in highly organized colonies. Like many insects, termites have an egg, an immature and an adult stage. There are three main types of adult colony members, or castes: reproductives, workers and soldiers. The reproductives include the king and queen, and in large colonies, supplementary reproductives that produce eggs. Workers are usually the most numerous individuals in the colony. They are small, wingless and whitish and may be found in damaged wood. Workers care for all of the other termites and forage for food (wood). The soldiers protect the colony from attackers such as ants.
 
Soldiers fit the same description as workers, but have long, dark mouthparts protruding from their large heads. Soldiers may also be found in damaged wood. Termites are able to digest wood with the help of microorganisms which live in the termite gut.
 
When a colony is several years old and relatively large, it may produce another form of adult termite called a "swarmer." Swarmers have four wings, are often brown or black and range in size from approximately 3/8 to ¼ inch. Swarmers are the termite’s way of sending out new kings and queens to start colonies. In the spring, great numbers of swarmers can fly from a single colony. Male and female swarmers pair up, shed their wings and tunnel into the ground. The pair then prepare a chamber near a wood source where the female will begin to lay eggs. These eggs are cared for by the king and queen and will develop into worker termites. The workers take over care of the young from the queen and king. Once enough workers are established, soldiers and other castes will develop from eggs produced by the queen. Two or three years after the establishment of the colony, secondary reproductives are produced. These greatly increase the egg-laying activity and population of the colony. Normally at least three to four years or more will pass before any swarming of winged termites from the colony occurs.
 
Swarmers are the most visible form of termite. These termites can be confused with many ants that also swarm in the spring. However, swarming ants have elbowed antennae, a narrow waist and front wings that are longer than the back wings. Swarming termites have straight antennae, a thick waist and all wings the same length.
 
TYPES OF TERMITES

There are two major types of subterranean termites: several species of the native subterranean termites (Reticulitermes species) and the imported Formosan subterranean termite (Coptotermes formosanus).  In a mature native termite colony there may be several hundred thousand workers. A mature Formosan subterranean termite colony may have millions of workers. The sheer numbers of Formosan subterranean termites usually make this type the most destructive. Formosan subterranean termites are also the most likely subterranean termites to establish above-ground infestations. Because much of the termite’s food source is removed during clearing of a site for construction, it is easy to see why homes are often attacked.
 
PREVENTION
 
The best way to protect a home against termite attack is with a combination of prevention and inspection.
Subterranean termites feed on cellulose-containing materials, but are most destructive to structural wood. Termites find moist or decayed wood more attractive than dry wood, therefore the most extensive damage is usually found in areas surrounding structural or plumbing leaks.
Home and building owners should be alert to areas of moist or weakened wood. Several procedures which help reduce the risk of termite infestation include:
 
Remove of any cellulose debris in or near the structure. This includes debris in crawlspaces, wood piles and thick mulch. In new homes, wood stakes from foundations and porches should be removed before concrete is poured. Provide adequate ventilation in crawlspaces, basement and between plants and exterior walls. This prevents excess moisture buildup. Eliminate any wood-to-round contact, including wooden steps and support posts. Some pressure-treated lumber should not contact soil. The tag on the lumber will specify proper use. Termites also can tunnel into a structure through foam board insulation that is in contact with the ground. Remove dense vegetation growing close to the structure’s foundation or siding. Heavy vegetation traps moisture, which creates a better habitat for termites. Shrubs, trellises and other vegetation make termite inspection more difficult and block ventilation of the structure. Use mulch sparingly close to the structure. Mulch is made up of cellulose and holds water, which can attract termites. Mulch should never be in contact with wood siding or framing of doors or windows. Provide proper drainage. Water must flow away from the structure. Keep gutters clean and in good shape. Eliminate areas in the landscaping where water is allowed to stand near the structure. When building a new home, have a termite pre-treatment by a licensed South Carolina pest control operator. These treatments may include a liquid chemical treatment (termiticide) to the soil or the installation of termite monitoring/bait stations. Termiticide treatments around new homes are generally done in two steps: after construction of the foundation footings and shortly after completion of the structure. Termite monitoring/bait stations are usually placed around the structure after completion.
 
CONTROL
 
For many infestations, insecticide treatments must be used to control established termite colonies. On occasion, fumigation is needed when infestations are extensive. However, most subterranean termite control is done by applying either termiticides to the soil or termite monitoring/bait stations in or around the structure.
 
Liquid termiticide treatment involves applying termiticide to the soil underneath and adjacent to a building to create a barrier. These barriers are not applied to eliminate the termites nesting in the ground, but rather to kill any termites that would tunnel up to it, thereby protecting the structure. For optimum protection, a complete barrier should be established around and under the structure. Trenching around the structure and drilling into the slab must be done to establish a continuous barrier. The actual length of time a termiticide treatment remains effective around a structure depends on thoroughness of the application, termite foraging intensity, conducive conditions and environmental conditions. Effectiveness of liquid termiticides varies due to soil and climatic differences.
 
Termite baits do not leave a residual chemical in the soil. However, they can reduce and may even eliminate a termite colony. They can be used in several different strategies, whether used alone or in combination with other treatment forms, and are especially useful in situations where other treatments have not been successful. Different systems are being developed for both below- and above-ground baiting. Below-ground baiting usually involves installing bait stations every 10 to 20 feet around the building. The stations are then monitored for termite activity on a regular schedule. Above ground baiting involves installing bait stations directly over mud tubes or infested wood found in the structure. Currently there are a few professional-use termite baits on the market.

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