Monooxygenases play only a minor role in resistance to synthetic pyrethroids in the cattle tick, Boophilus microplus

We investigated the role of monooxygenases in resistance to synthetic pyret hroids (SPs) in the cattle tick, Boophilus microplus. We found that monooxy genases play only a minor role in resistance to SPs in both resistant and s usceptible strains of B. microplus. We blocked the monooxygenases with pipe ronyl butoxide (PBO) and simultaneously applied the SPs, flumethrin and cyp ermethrin to larval B. microplus. PBO increased the effect of flumethrin (s ynergism ratios 2.7-8.9) more than it increased the effect of cypermethrin (synergism ratios 1.9-3.1). Of the four strains tested, Parkhurst, which is resistant to SPs, was the least affected by the addition of PBO (synergism ratios after cypermethrin was applied 1.9; after flumethrin 2.7) whereas N .R.F.S., the strain susceptible to SPs, was the most affected by synergism between PBO and SPs (synergism ratio after cypermethrin was applied 3.1; af ter flumethrin 8.9). We hypothesize that B. microplus lacks monooxygenases capable of conferring resistance to SPs because it and its recent ancestors were blood-feeders rather than herbivores.