There have been 15 species of termites found in California but only two species need concern homeowners, the subterranean and the drywood termites. Wood is their food source. Both can do great damage to the structural timbers in a residence but the two have different methods of attack. Although the focus of this article is on the drywood termite, familiarity with the subterranean termite is needed.
The subterranean termite must live in moist soil or where there is a continuous source of moisture. It reaches wood not in direct contact with the soil by building soil tubes up the foundation walls. Thus protected from the desiccating elements it can forage for lignin products on which to feed. These tunnels are easy to spot. Termite inspectors use powerful lights to illuminate the darker areas under the structure to see the tubes. However, foundation cracks, voids in hollow cement blocks and unbended stucco can provide the termites with hidden accesses to their source of food, in this case, the structure.
Subterranean termite colonies are huge, can cover areas a block in size and the colony can number in the millions, all hidden from easy view.
Control of the subterranean termite consists primarily of destroying the tubes and then preventing the termites from reaching the foundation by physical and/or chemical barriers. Fumigation is never used as a control.
Drywood termites do not need protection from the elements as they live entirely inside a piece of wood, such as the lumber in subfloors, walls, attics, roofs and shingles. They are difficult to detect. The rate of destruction is low. Colonies start with a single king and queen and develop slowly over the years. It might take seven years to grow to a population of l,OOO nymphs, soldiers and the king and queen. A colony of this size might eat only one-half pound of wood in a year. It takes many years before drywood termites do significant harm to a structure.
Evidence of infestation can be visual. Piles of frass (the insects excrement) indicate their presence. Noting entrance and exit holes is another. Termites swarm in the fall, to mate and seek new colony sites. Other methods of detection currently in use include the use of sound detection devices, fiber optic scopes for viewing wall cavities and dogs trained to smell the presence of the insects. However, as these insects can spend years inside wood without ever being seen or leaving any evidence of their presence, detection is difficult. They can infest timbers in inaccessible places such as walls, attics and wood foundations.
Once they are positively detected, several methods of control are available. Fumigation has been the primary method. The structure is tented or "bagged" and chemicals are released inside. After 24 hours the structure is allowed to air out. More and more Californians, citizens as well as governmental bodies, are asking for control methods not involving fumigation of the infested structure. Some, such as school districts, place an outright ban on this method of control, citing potential harm to students from any residual fumigants.
ALTERNATIVE METHODS OF CONTROLLING DRYWOOD TERMITES
The primary methods of control are of three types: 1) killing termites harbored in wood; 2) physical and chemical barriers and 3) removal of infested wood.
Methods used to control (kill) termites in existing structures.
Termites can only survive in a narrow range of temperatures. They are protected from wide variations while inside their micro-habitats. Methods have been developed that heat the infested wood above the level in which the termites can survive. The structure may be "bagged" or tented, or an individual room sealed off. In order to be effective the lethal temperatures must be maintained long enough to kill the insects. The method relies on the use of propane burners to heat the air that is then blown into the structure. The goal is to heat the structure so that the temperature inside the infested wood is heated to 120 degrees and maintained for at least 30 minutes. Usually the structure must be heated to 170-180 degrees from several hours up to a day to ensure reaching the optimum temperature inside the wood where the termites are hidden. Some difficulties with this method are: Loss of heat adjacent to concrete (mudsills, for example), large timbers located behind wall board or insulation not getting hot enough to kill the existing colony, damage to articles within the structure, difficulty in measuring the heat levels with probes (the probes may themselves conduct heat) and ensuring a careful analysis of structural limitations, monitoring devices to determine correct temperatures, proper adjustment of equipment and determining the size and number of pieces of equipment needed for the job (heaters, generators, fans, probes), and the length of time needed to raise the temperature high enough to effect control of the invading termites. Toxic gases are not released by the propane heaters but toxic gases can be released from building materials and furniture inside the structure. These types of gases released are, as of this writing, unknown. Better results are obtained when using this method in smaller structures, individual rooms within the structure or an individual wall.
Chilling, the reverse of heat, is used as a method to kill drywood termites. Cold, by its very nature, is limited in usage to small areas. Liquid nitrogen is used to lower the temperatures to 20 degrees below zero long enough to kill the termites inside the wood. The liquid nitrogen is pumped into the infested areas, turns into a gas, and chilling the surrounding area. This method of treatment does not release any gases not already found in the atmosphere. The wood must be chilled to -20 degrees for thirty minutes to kill the termites. The area to be treated may be covered with insulating mats to increase efficiency. Many of the concerns regarding the effectiveness of cold are similar to those listed for heat fumigation. Efficacy in control of drywood termites is directly related to finding the nests or colonies. If a colony goes undetected the application of cold will not control them. Applicators using liquid nitrogen must be licensed. Potential problems using cold are asphyxiation, damage to water pipes and other materials and the different materials in a structure will have an effect on the cooling rate.
Commercial microwave devices are used to kill termites inside infested wood. The microwaves heat the termites faster than the wood. Lethal doses range from 10 second to 10 minute exposures and power usages range from 700 to 2000 watts. Exposure times and power requirements are relative to the thickness of the wood and the accessibility of the infested wood. The operator sets the device on a tripod and it is left on for the prescribed amount of time, then moved to the next site. Microwaves will not bubble the paint or heat nails in the structure, a potential fire hazard. Residents of the structure being treated are evacuated. Technicians wear exposure meters while operating the equipment, leaving the immediate area when the microwave is turned on. No removal of plants, food or heatvulnerable articles is necessary. Microwaves operate on a logarithmic scale. Energy from the device decreases logarithmically as the waves move away from the source. This allows control of the high energy zone. Conversely, this limits the effectiveness of microwaves in controlling termite infestations as accurate detection of the colony is absolutely necessary.
The use of electrical shock (Electro-Gun) to kill termites has been available for several years. With 90,000 volts of power the Electro-gun sweeps across areas suspected of harboring drywood termites. The shock kills termites within the enclosed galleries. This device is good for spot treatments only. The Electro-Gun uses high frequency, but low amperage. If the Gun accidental contacts a human the current sweeps over the skin and does not penetrate at all. This eliminates the concern of electrocution or heart fibrillation. The Electro-Gun does not raise the temperature of the wood to the point of ignition. As with other methods, accurate detection of the colony(s) is needed for effectiveness. Complete knowledge of the efficiency of electrical shock treatments in structures is lacking.
These dusts abrade the waxy surface of the insect, drying out the termite and killing it. The dusts usually contain a stomach poison, that if eaten, kills the termite. These dusts can be injected into termite galleries under pressure or applied by hand.
Aerosol chemicals. The chemicals used (carbamates and organophosphates) are "contact" poisons. They work when the termite contacts them or inhales them. They attack the nervous system of an insect. These chemicals are also used as a preventative technique as the insecticide repels the invading termites.
This is an EPA registered chemical that is odorless, colorless, water-soluble and has a neutral P. It is a persistent chemical, only leached from wood by continuous contact from flowing water. Liquid borate sprays are effective when termites ingest the borate. There is a repellent effect when borate is used in higher dosages and higher concentrations of borate in the wood. The spray works on the principle that termites live as close under the surface of the wood as is practical, thus the chemical gets down into the wood far enough to be an effective killer. Applicators spray bare wood, insert in holes drilled for the purpose, and if the wood is exposed to the elements, sealed after application to prevent escape of the chemical. Borate cannot penetrate sealed or painted wood.
Factors to be considered when treating wood with borate include the wood species, age of the tree when cut for lumber, location of the board within the tree, geographic location of the tree, humidity, moisture content of the wood to be treated as the drier wood accepts more borate but the moist wood allows the borate to penetrate deeper and surface texture. Repeated aqueous solutions mean more loading of borate in the wood and deeper penetration.
Borate treatments differ in the type of construction used in the structure. Slab structures, sealed walls, hidden sill plates or studs, all affect the course of treatment. The most effective treatments are noted in areas with large amounts of bare wood, such as attics, garages, crawl spaces and unfinished basements. The applicators must treat not only the areas of suspected termite infestations but surrounding areas as well to prevent termite migrations. The more surface area treated, the greater control and the increased possibility of prevention.
Multiple applications of borate, with adequate drying times in between applications is recommended to get a high borate concentration in the wood. Annual re-applications are recommended in order to get the borate to the center of the wood.
To control active galleries of termites and termite access points, the borate must be applied under pressure to flood the area under attack, surface applications will not work. The larger the beams infested, the more pressure must be used to get the solution into the infected wood.
This promises to be in widespread use in 3-5 years. The baits allow a smaller, micro-amount of pesticide to be used. The baits can be removed when control of the colony is reached. The only caveat could be that the baits themselves could attract termites. Research on the attractants and termiticides is currently underway.
Pressure-treated wood, impregnated with CCA (chromates copper arsenate) and ACZA (ammoniacal copper zinc arsenate), work as stomach poisons to feeding insects.
PHYSICAL AND CHEMICAL BARRIERS TO PREVENT TERMITE INFESTATIONS REPELLENTS
These consist of sprays applied to the surface of the wood, the use of pressure-treated wood in housing construction and the use of silica argyle dusts (Dri-die 67) blown into attics, voids and between walls. The argyle dust is an abrading dust that desiccates the insect by scraping the surface of the insects skin, allowing bodily fluids to escape.
REMOVAL AND REPLACEMENT OF INFESTED WOOD
In areas of the structure easily accessible this may be a practical solution.
DIFFICULTIES ENCOUNTERED USING ALTERNATIVE CONTROL METHODS
All of the above alternative control methods, except heat and cold, are spot treatments. They try to eradicate individual termite galleries. Here are some points to consider when deciding whether to use an alternative method to control drywood termites. 1) Failure to detect ALL invading colonies of drywood termites in the infested structure and difficulties in determining accurately the size of the colony. 2) Failure to completely eradicate all the termites leaving some behind to renew the colony. 3) Aerosols, dusts and sprays may not penetrate the infested wood sufficiently to reach all galleries or parts of galleries. 4) The removal of infested wood and replacement has obvious limitations. Accessibility, cost, destruction of part of the structure and deserter during the process of replacement are but some of the difficulties with this approach.
Consumers wishing to use any of the above methods to control drywood termites should consider these four points:
1) When using commercial firms for alternative controls of drywood termites the owner should insist on a written guarantee that the control method used will eradicate the termites for a period of at least two years. Regular checkups are needed to ensure complete eradication of the colonies.
2) Get quotes from three firms.
3) Ask for referrals to previous users of the alternate methods of control.
4) Secure the service record of the prospective firm(s) from the Structural Pest Control Board.
For completely up-to-date information about pest termite control, please see Pests in Homes, Gardens, Landscapes and Turf.