Increased Susceptibility of Pyrethroid-Resistant Horn Flies (Diptera: Muscidae) to Chlorfenapyr

Chlorfenapyr (AC 303630) was found to be 5 times as toxic to pyrethroid-resistant horn flies, Haematobia irritans (L.), as to susceptibles. We hypothesize this difference occurs because higher levels of mixed function oxidases in the resistant flies convert more chlorfenapyr (a pro-insecticide) to the insecticidal form. This negative cross-resistance between chlorfenapyr and pyrethroids offers new resistance management possibilities.