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Toxicity of the phenylpyrazole insecticide fipronil was determined in laboratory and field-collected strains of the German cockroach, Blattella gennanica (L.). Fipronil effectively killed German cockroaches when applied in nanogram quantities. Orlando (insecticide susceptible), HRDC (carbamate- and organophosphorus-resistant), and Village Green (pyrethroid-resistant) strains were equally susceptible to fiproni with LD50 values between 4.6 and 5.4 nanograms per insect. The Marietta strain (pyrethroid-, organophosphorus-, and carbamateresistant) was 1.6 times more tolerant of fipronil compared with the Orlando susceptible strain. Five German cockroach strains collected from the field were considerably more tolerant of fipronil than Marietta cockroaches. Piperonyl butoxide and S, S, S-tributyl phosphorotrithioate antagonized the toxicity of fipronil in all strains evaluated. Fipronil was 2.2-3 times less toxic when cockroaches were pretreated with piperonyl butoxide or S, S, S-tributyl phosphorotrithioate compared with fipronil alone. These data suggest that fipronil is metabolically activated in German cockroaches, possibly via sulfone formation catalyzed by microsomal oxidases.
Valles et al. (Wed,) studied this question.