Enhanced Diazinon Susceptibility in Pyrethroid-Resistant Horn Flies (Diptera: Muscidae): Potential for Insecticide Resistance Management

Dose-mortality data from 40 horn fly, Haematobia irritans (L.), populations revealed that susceptibility to permethrin and diazinon was negatively correlated (i.e., as susceptibility to permethrin decreased, toxicity of diazinon increased). Permethrin-resistant horn fly populations exposed for 18 wk to cattle tagged with ear tags impregnated with 20% (AI) diazinon resulted in a 2- to 6-fold reduction in this resistance. These populations also had LC50s for diazinon that were two to five times lower than those of populations susceptible to permethrin. Frequency of homozygous permethrin-resistant individuals decreased as much as 57% after exposure to diazinon during this period.