Structural Protection

2.1. Repellent Chemicals

Chemical treatments of the soil surrounding a structure formed the basis for structural protection of homes from termite damage since the beginning of the 20th century (Randall and Doody 1934, Potter 1997). From the early 1950s through the late 1980s, cyclodiene compounds prevailed as the most used insecticides for structural protection against termites. They were effective in protecting structures because of their longevity in the soil and because they could be cheaply applied to the soil. However, concerns about the effects of these chlorinated hydrocarbons on the envi ronment and on human health led to their withdrawal from the marketplace in 1988.

Since then organophosphates and pyrethroids formed the basis of termite control products. These products were either "fast-killing" insecticides or were themselves repellent to the termites and thus a continuous soil barrier prevented the termites from entering a structure. However, members of a colony that did not come into contact with the insecticide were simply directed to forage for a source of food that was unprotected from such a barrier. Thus, although structures were protected from termite assault by these chemical barriers, colony populations were largely unaffected. Disruption of the chemical barrier through landscaping or construction activities, degradation of the barrier itself, or bridges of untreated material inadvertently placed over the chemical barrier could then allow the remaining population to enter and attack the structure. On the other hand, when applied to the soil of an infested structure, such repellent barriers could prevent any termites that had created a nest within the walls or attics of the structure from returning to the soil; they could also entrap the colony within the structure if a source of moisture was available. Organo-phosphate termiticides have recently been removed from the market as a result of the Environmental Protection Agency's (EPA) Food Quality Protection Act re-registration review, leaving pyrethroids as the main class of repellent barrier chemicals.

2.2. Non-Repellent Liquid Chemicals

Several new termiticides have recently been introduced which are marketed as "non-repellent" to the termites. Termites are believed to readily penetrate such chemical barriers and ingest or contaminate their cuticles with the toxins. On returning to the nest, the chemicals are spread to nest mates through mutual grooming or feeding others. The initial concept was that such chemicals could provide population reduction because of their slow spread throughout the colony. Potter and Hillery (2002) have reported success in killing termites at monitoring sites distal to perimeter-applied barrier bands using a non-repellent chemical. Other researchers have reported similar results (Waite et al. 2004), and EPA has approved a perimeter plus limited direct treatment application label for one of the non-repellent liquid chemicals to control subterranean termites. Since the approval for this label is so recent, data concerning the effectiveness of this treatment approach on termite populations surrounding such a structure are not available. Recent research has cast some doubt on the ability of one such non-repellent chemical to affect the entire colony of the Formosan subterranean termite (Osbrink and Lax 2003).

2.3. Baits

Termite baits are cellulose materials laced with a slow acting non-repellent toxin that when ingested can be taken back to the colony and shared with nest mates through feeding and trophallaxis. The earliest bait systems for the control of subterranean termites were arsenic dust or mirex-treated wood blocks (Esenther and Gray 1968, Esenther and Beal 1974). These were shown to reduce termite populations but were never incorporated into commercially viable products. Later bait formulations included slow acting metabolic inhibitors (stomach poisons) or relatively slow acting inhibitors of the insect nervous system. Other active ingredients include insect growth regulators such as the chitin synthesis inhibitor, hexaflumuron (Su and Scheffrahn 1998, Su 2003a). Extensive field trials using these chitin synthesis inhibitor-containing baits have demonstrated population reduction or "functional elimination" (Su and Scheffrahn 1998, Su 2003b) of the treated termite colony.

In several of the bait strategies termite populations are monitored by placing untreated cellulose (usually wood blocks) in a regular pattern surrounding a structure. These monitors are checked on a regular schedule (usually monthly) for the presence of termites. If termites are present, the wood blocks are replaced or supplemented with a toxin-containing cellulose substrate that the termites consume and distribute through the colony. In addition to the monitoring/baiting stations within the soil, above-ground bait stations may be placed in the vicinity of a termite infestation when it is discovered within a structure. In such instances, active ingredient is added immediately upon installation of the station. When sufficient toxin has been introduced into the colony the population is eliminated. Thus baits protect structures through colony elimination and ultimately through population reduction of the termites in the surrounding areas, rather than merely by protecting a single structure, which would allow the colony to discover nearby unprotected food sources.

2.4. Physical Barriers

There are several physical barriers, such as stainless steel mesh, thin metal termite shields, a pyrethroid-laced vapour barrier, and basaltic particle barriers that can help prevent termite entry into the structure (Yates et al. 2000, Wege et al. 2003). These are installed beneath a structure or incorporated into the structure during construction but have no direct effect on the colony's population and merely cause termites to search elsewhere for a food source.

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