The Contribution of Metabolism to Pyrethroid and DDT Resistance in the Horn Fly (Diptera: Muscidae)1

Pyrethroid resistance developed rapidly in a laboratory strain of the horn fly, Haematobia irritans (L.), which was selected in each successive generation with approximate LC50’s of orally-administered technical permethrin. The topical LC50’s for cis- and trans-permethrin after 7-10 generations of selection were 21.5 and 26.9 times greater, respectively, for flies from the selected strain (R) than those from an unpressured susceptible (S) strain. After 30 generations of selection, LC50’s for the permethrin isomers increased to 48.9 and 70.5 times, respectively. The R flies were cross-resistant to fenvalerate and DDT but not to stirofos or coumaphos. Studies of factors influencing development of resistance to trans-permethrin indicated no important differences between Sand R strains in rates of absorption, internal accumulation, or excretion following topical applications. Studies of the metabolism of [14C] cis- and trans-permethrin by R flies suggested that enhanced metabolic detoxification was not a contributing factor during the early stages (ca. 25-fold level) of resistance development, but was a factor at higher levels of resistance. Metabolic detoxification of DDT was significantly greater in R flies than in S flies, but the level of enhancement (ca. 2 times) was insufficient to account for the major differences between the strains in susceptibility to the compound. Coadministration of piperonyl butoxide (PBO, a microsomal oxidase inhibitor) with cis- or trans-permethrin and fenvalerate caused a major reduction in metabolic detoxification, while a hydrolase inhibitor (DEF) had only a slight effect on metabolism. Tests of the topical toxicity of trans-permethrin to R flies demonstrated that coadministration of PBO caused a 10-fold reduction in the LC50 and DEF caused a 4-fold reduction. Although there is no reason to doubt that target site insensitivity is the major factor in pyrethroid resistance in the horn fly, this study provides direct evidence that enhanced metabolic detoxification can also be a contributing factor.