2 SUBTERRANEAN TERMITES AND THEIR CONTROL

2.1 ECONOMIC IMPORTANCE OF SUBTERRANEAN TERMITES AND CONVENTIONAL CONTROL MEASURES

Subterranean termites are important structural pests in the United States, costing the public approximately $1.2 billion each year. Historic buildings and structures are particularly vulnerable to subterranean termite damage, given the traditional use of wood as a building material. Termite damage to historic buildings is both costly and irreversible and can diminish the integrity of a structure.

Conventional methods for the control of termite infestations rely heavily on the use of organic insecticides to provide a barrier for the exclusion of soil-borne termites from a structure. Typically, large quantities of liquid insecticide are applied to the soil beneath and surrounding an infested building. The insecticide barrier technique is the most common method for subterranean termite control. Creating an uninterrupted barrier of treated soil beneath an existing structure, however, is extremely difficult (Frishman and Bret 1991). A single colony of subterranean termites may contain 100,000 to 1 million workers that forage up to 100 m in search of food (Su et al. 1993). Because the soil treatment only deters termite attack, the vast majority of subterranean termites are unaffected (Su and Scheffrahn 1988). If gaps in the soil barrier occur, subterranean termites may eventually find the untreated soil and make their way back into the structure, causing more damage and necessitating further treatment.

Soil treatments often require drilling of the foundation floor before liquid insecticides are injected into subfoundation soil, an unacceptable practice for many historic structures. Another disadvantage of soil treatment is the risk of ground-water contamination in historic landscapes. This concern is especially valid in the Statute of Liberty, as the monument foundation is less than 100 m away from New York Harbor.

2.2 SENTRICON TERMITE COLONY ELIMINATION SYSTEM

In recent years, baits containing slow-acting toxicants, such as the insect growth regulator hexaflumuron, have been used as alternatives to conventional soil insecticides. Hexaflumuron inhibits the synthesis of chitin, which is essential for the formation of insect exoskeleton but is virtually harmless to vertebrates. Because of its low mammalian lethal effect (LD50:>5,000 mg/kg), it is registered in the least toxic category, “caution.” Using a monitoring and baiting procedure, hexaflumuron is delivered by foraging termites to eliminate the entire colony populations of several million individuals (Su 1994). The procedure is currently marketed as the Sentricon Termite Colony Elimination System (Dow Agro-Sciences, Indianapolis, Indiana) to authorized pest control applicators. Studies using the in-ground Sentricon system or its commercial prototypes confirmed that termite colonies of several million individuals could be suppressed to the point of inactivity (or observed elimination) using less than 1 g of hexaflumuron (Chambers and Benson 1995; DeMark et al. 1995; Su et al. 1995; Grace et al. 1996). Moreover, elimination of colony populations created a zone of termite-free soil surrounding a building for several years (Su and Scheffrahn 1996).

The in-ground Sentricon system employs a cyclical process of monitoring and baiting for termite activity. First, Sentricon stations containing monitoring devices are installed in the soil surrounding a structure. When termite activity is discovered in a station, the monitoring device is replaced with bait containing 0.5% hexaflumuron (Recruit II, Dow AgroSciences). Hexaflumuron kills termites only when they molt, or every 1–2 months. During this period, the bait is thoroughly distributed throughout the colony population by foraging termites that feed upon the baits and by trophallaxis (food exchange among nest mates). It may take several months to achieve the colony elimination, but the result is sweeping. Once the colony is eliminated, a return to monitoring continues to detect further termite activity.

Because the in-ground baiting system is difficult to use in places without soil access, an above-ground baiting procedure was also developed to deliver hexaflumuron through stations placed directly over active termite infestations (Su et al. 1997). The above-ground baiting system provides another opportunity for bait application to control structure-infesting populations of subterranean termites. Because the objective of this project is to eliminate termite populations from Liberty Island, we used all methods possible to deliver baits containing hexaflumuron.