Population Suppression of Subterranean Termites by Slow-Acting Toxicants

Historic background and the concept of slow-acting toxicants for population suppression of subterranean termites are reviewed. Information needed for development of bait-toxicants and studies needed to generate such informa­ tion are summarized. Current control measures for subter­ ranean termites rely exclusively on soil termiticides, primarily organophosphates and pyrethroids. They are used as toxic or repellent barriers between soil-borne termite colonies and structures requiring protection. La Fage (1986) estimated that soil termiticides are applied at a rate of ca. 390 kg/ha beneath treated structures compared to agricultural rates of 2.17 kg/ha (Pimentel and Levitan 1986). The withdrawal from use of chlorinated hydrocarbon termiticides in 1987 reflects public concern over the use of these persistent chemicals in highly populated urban environments. Despite the large quantities of pesticides used, soil treatments do not affect termite populations but only provide barriers to separate structures from soil-borne termites. Colonies of subter­ ranean termites, especially those of the Formosan subterranean termite (Coptotermes formosanus Shiraki) which may forage as far as 100 m from the central nest, remain viable near the structures even after treatment (Su and Scheffrahn 1988a). Because of the inability of current con­ trol techniques to reduce existing subter­ ranean termite populations, the severity of infestations by C. formosanus in areas such as Honolulu, Hawaii, and New Orleans, Louisiana, has increased in recent years. Presented at the Symposium on Current Research on Wood-Destroying Organisms and Future Prospects for Protecting Wood in Use, September 13, 1989, Bend, Oregon. Associate Professors, Ft. Lauderdale Research and Education Center, Institute of Food and Agricultural Sciences, Univer­ sity of Florida, Ft. Lauderdale, Fla. Researchers in the early 1900's first observed that the slow-acting arsenic dusts could be used to reduce colony populations of subterranean termites (Randall and Doody 1934). The principle of suppressing colony populations is to provide a means for individual termites to acquire a lethal dose of slow-acting toxicant at a given foraging site. The intoxicated individuals must not be so impaired at the onset of exposure that they cannot move away from the toxicant acquisition site to die. The slow-acting characteristic of a toxicant is particu­ larly important because accumulation of a large number of dead termites at the acquisition site will repel other nestmates from approaching the toxicant (Su and others 1982a). Ideally, the toxicant has to be nonrepellent to termites, or at least be masked by other agents to prevent avoidance behavior by the foraging ter­ mites. Under this premise, the toxicant can be incorporated into a bait (feeding acquisition) or tracking powder (contact and grooming acquisition). In this paper, we will primarily discuss the bait con­ cept. Beard (1974) suggested the use of bait toxicants as a possible strategy to eliminate established colonies of the subterranean termites, Reticulitermes spp. Dechlorane (mirex) baits have been used to suppress activity of field colonies of Reticulitermes in the United States (Esenther and Beal 1974, 1978), and to kill field colonies of an Australian subterranean termite, Mastotermes darwini­ ensis Frogg. (Paton and Miller 1980). Gao and others (1985) also reported successful field control of termite infestations with mirex baits in China. Laboratory studies indicated that hydramethylnon (Amdro), avermectin B1 (Su and others 1987), A-9248 (Su and Schef­ frahn 1988b), sulfluramid (Su and Schef­ frahn 1988c), and insect growth regulators (IGRs) such as methoprene, fenoxycarb, and S-31183, have shown delayed toxicity against C. formosanus and the eastern subterranean termite, Reticulitermes flavipes (Kollar) (Jones 1984, Su and others 1985, Haverty and others 1989, Su and Scheffrahn 1989). A field trial with hydramethylnon baits, however, resulted USDA Forest Service Gen. Tech. Report PSW-128. 1991. 51 POPULATION SUPPRESSION OF SUBTERRANEAN TERMITES BY SLOW-ACTING TOXICANTS LABORATORY PHASE FIELD PHASE Figure 1--Studies needed to establish information for development of bait toxicants to suppress populations of subterranean ter­ mites. only in partial control of C. formosanus colonies (Su and others 1982b). INFORMATION NEEDED FOR DEVELOPMENT OF A SLOW-ACTING BAIT Development of a bait toxicant requires numerous, interconnected laborato­ ry and field studies followed by a rigor­ ous evaluation phase (Figure 1). Laboratory Phase Toxicity and Lethal Time Oral or topical LD50 or LD95 should be calculated to estimate the gross quantity of toxicant needed for control of entire colonies. This information can also be used to compare the relative toxicity among toxicants. 52 USDA Forest Service Gen. Tech. Report PSW-128. 1991. Lethal time of a toxicant is concen­ tration dependent. A bait toxicant should be used at a concentration that causes protracted behavioral impairment and latent mortality so that termites, having fed on bait, will disperse throughout their foraging territories before succumb­ ing to the toxicant. Su and others (1987) defined an effective lethal time 90% (ELT90) as the amount of time required by a fixed concentration of a toxicant to kill at least 90 percent of termites. ELT90 can be used to assess the feasibili­ ty of a candidate compound as a bait toxicant and to predict the time frame required for control. Lethal time (ELT90) of candidate toxicants can also be used to compare toxicants.