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)-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 μM (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 μM) was not apparent in the presence of 5 mM GABA. Further neurophysiological investigation using a range of concentrations (0.625-20.0 μM) 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.