Toxicity and neurophysiological effects of fipronil and fipronil sulfone on the western corn rootworm (Coleoptera : Chrysomelidae)

Fipronil is a member of the relatively new phenylpyrazole insecticide class that is active at the neuro-inhibitory gamma amino butyric acid (GABA)-gat ed chloride channel/ionopore complex. The toxicity and neurophysiological e ffects of fipronil and its oxidative sulfone metabolite [5-amino-1-(2,6-dic hloro-4-trifluoromethylphenyl)-3-cyano-4-trifluoromethylsulfonylpyrazole] w ere investigated by using an insecticide-susceptible western corn rootworm population In topical bioassays using adult rootworms, fipronil was toxic a t very low doses (LD50 = 0.07; LD90 = 0.33 ng/mg). At the LD90, pre-treatme nt with the cytochrome P450 monooxygenase inhibitor piperonyl butoxide led to mild antagonism of fipronil toxicity (LD90 = 0.42 ng/mg), while the sulf one analog had greater toxicity (LD90 = 0.22 ng/mg). In neurophysiological studies of spontaneous electrical activity, adult and larval rootworms were equally affected by fipronil and the sulfone analog at 10 mu M (in the pre sence of 5 mill GABA) in comparison to GABA-treated baselines. Using larval rootworms, insensitivity of the GABA receptor to binding by picrotoxinin o r dieldrin (10 mu M) was not apparent in the presence of 5 mM GABA. Further neurophysiological investigation using a range of concentrations (0.625-20 .0 mu M) on larval rootworms indicated concentration-dependent effects on b ursting activity for both fipronil and the sulfone analog; however, subtle differences were observed between these two compounds. Results indicate tha t both fipronil and its oxidative sulfone metabolite have similar toxicolog ical and neurological effects on rootworms. (C) 1999 Wiley-Liss, Inc.