Abstract
The pyrethroids permethrin and cypermethrin were mixed in selected ratios with a synergist (piperonyl butoxide [PB]) and an organophosphorus insecticide (chlorpyrifos) to determine their efficacy on susceptible and resistant populations of horn flies, Haematobia irritans (L.), in the laboratory and field. The addition of either PB or chlorpyrifos to the pyrethroids increased toxicity 1.4- to 4.2-fold and did not change resistance ratios compared with the pyrethroids alone. The pyrethroid/chlorpyrifos/synergist mixture changed toxicity 8.6- to 0.4-fold and generally reduced the level of resistance, compared with the pyrethroids. Field evaluations revealed that no mixture maintained resistant horn fly populations below economically damaging levels, but the addition of PB or chlorpyrifos or both generally improved control. Based on available information concerning toxicity, resistance, and long-term considerations, pyrethroids should be excluded from use in ear tag mixtures for horn fly control.
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