Fipronil: Toxicity to Subterranean Termites and Dissipation in Soils

Fipronil (Termidor 80 WG) was applied to covered and exposed plots at one secondary and four primary USDA Forest Service termiticide test sites in the U.S. Residue analyses and bioassays of soil samples were conducted over 5 y. Fipronil had an exponential decay at all sites. The DT 50 of fipronil in a silt loam soil in Oktibbeha Co., MS in covered and exposed plots was 202 and 177 d, respectively. Fipronil dissipation appeared to be faster at the secondary site (Oktibbeha Co, MS) compared to the primary tests sites. Dissipation was faster in covered plots in AZ and MS compared to FL and SC; while that in exposed plots was similar among sites. In 7 d bioassays, termite penetration of soil cores from primary test sites was significantly greater in exposed plots (33.9 ± 1.2 mm) than that from covered plots (25.7 ± 1.2 mm). Differences in termite penetration and termite mortality between covered and exposed plots at the secondary test site were not significant. Average distance penetrated by termites, averaged over treatments and primary sites, significantly increased during the last three years (37.2 mm) of sampling compared to the first three sampling times (20.7 mm). Termite mortality averaged over sites and years for covered and exposed plots was not significantly different. Freundlich adsorption coefficients (K f), determined from adsorption isotherms, ranged from 0.14 on a gravelly sand (Pima Co., AZ) to 5.47 on a silt loam (Oktibbeha Co. MS).