Cross‐resistance potential of fipronil in Musca domestica

The toxicity of fipronil to insecticide‐susceptible houseflies and the cross‐resistance potential of fipronil were determined for six insecticide‐resistant laboratory housefly strains by topical application and feeding bioassay. The insecticide‐resistant strains represented different levels and patterns of resistance to pyrethroids, organophosphates, carbamates and organochlorines. Five strains were almost susceptible to fipronil in feeding bioassay with resistance factors at LC 50 between 0.36 and 3.0. Four of these strains were almost susceptible to topically applied fipronil (resistance factors at LD 50 were 0.55, 0.83, 3.3 and 2.5, respectively), whereas one strain was 13‐fold resistant to topically applied fipronil. A highly γ‐HCH‐resistant strain, 17e, was 430‐fold resistant to fipronil in topical application bioassay and 23‐fold resistant in feeding bioassay at LD 50 /LC 50 . We also tested the toxicity of fipronil in a feeding bioassay and γ‐HCH in topical application bioassay on thirteen housefly field populations. Eleven of the field populations had resistance factors for fipronil ranging from 0.98 to 2.4 at LC 50 , whereas two populations were 4.0‐ and 4.6‐fold resistant to fipronil. The resistance level to γ‐HCH at LD 50 in the field populations ranged from 1.8‐ to 8.1‐fold. The two strains showing fipronil resistance were 3.4‐ and 8.1‐fold resistant to γ‐HCH. Fipronil and γ‐HCH toxicities were positively correlated in the field populations. Biochemical assays of esterase, glutathione S ‐transferase and cytochrome P450 monooxygenase indicated that the low fipronil resistance observed in laboratory and field strains could be caused by elevated detoxification or be due to a target‐site resistance mechanism with cross‐resistance to γ‐HCH. Copyright © 2004 Society of Chemical Industry