Toxicity and neurophysiological effects of fipronil and fipronil sulfone on the western corn rootworm (Coleoptera: Chrysomelidae)
1999
Scharf, M.E. | Siegfried, B.D.
Fipronil is a member of the relatively new phenylpyrazole insecticide class that is active at the neuro-inhibitory gamma amino butyric acid (GABA)-gated chloride channel/ionopore complex. The toxicity and neurophysiological effects of fipronil and its oxidative sulfone metabolite [5-amino-1-(2,6-dichloro-4-trifluoromethylphenyl) 3-cyano-4-trifluoromethylsulfonyl pyrazole] were investigated by using an insecticide-susceptible western corn rootworm population. In topical bioassays using adult rootworms, fipronil was toxic at very low doses (LD50 = 0.07; LD90 = 0.33 ng/mg). At the LD90, pre-treatment with the cytochrome P450 monooxygenase inhibitor piperonyl butoxide led to mild antagonism of fipronil toxicity (LD90 = 0.42 ng/mg), while the sulfone 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 micromolar (in the presence of 5 mM GABA) in comparison to GABA-treated baselines. Using larval rootworms, insensitivity of the GABA receptor to binding by picrotoxinin or dieldrin (10 micromolar) was not apparent in the presence of 5 mM GABA. Further neurophysiological investigation using a range of concentrations (0.625-20.0 micromolar) on larval rootworms indicated concentration-dependent effects on bursting activity for both fipronil and the sulfone analog; however, subtle differences were observed between these two compounds. Results indicate that both fipronil and its oxidative sulfone metabolite have similar toxicological and neurological effects on rootworms.
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