Onset of resistance to fenvalerate, a pyrethroid insecticide in Argentine horn flies (Diptera: Muscidae).

Haematobia irritans (L.) horn flies recently have immigrated into Argentina from Brazil or Paraguay. Bioassays were conducted with fenvalerate to develop baseline information on pyrethroid susceptibility. Four populations tested in 1994 and 1995 were more susceptible than a laboratory strain of horn fly not exposed to insecticides since 1970, but 3 populations from Corrientes Province tested in 1996 were moderately resistant to fenvalerate, with resistance ratios ranging from 3 to 26. Argentine horn fly populations were exposed to pyrethroid treatments of pour-on, spray, and dip formulations for 4 yr before developing resistance. Horn flies in the United States developed pyrethroid resistance after 2 yr of treatment with pyrethroid ear tags.