Horn fly susceptibility to diazinon, fenthion, and permethrin at selected elevations in Wyoming

Horn fly, Haematobia irritans (L.) (Diptera: Muscidae), populations from so utheastern Wyoming were assayed for resistance against permethrin, diazinon , and fenthion. All of the populations examined were significantly more res istant to permethrin than a susceptible strain. Eight of 25 populations exa mined were significantly more resistant ito diazinon than a susceptible str ain. Five of 15 populations examined were significantly more resistant to f enthion than a susceptible strain. No relationship was detected between ele vation and resistance levels.