Abstract
The susceptibilities of Haematobia irritans (L.) from four locations were compared by exposure of adults to the pyrethroids fenvalerate and permethrin. Flies were collected from dairy cattle at two locations in Okeechobee County, Fla. One group (FLA) was collected from breeding dairy heifers treated the year before, the other (FLB) was collected from untreated yearling dairy heifers ca. 1 km from the breeding heifers. Collections in Texas were made from beef cattle in Bexar County (TX) and from a laboratory strain (LAB) reared in Kerrville. Flies were exposed to each toxin in the cloth covers of plastic cups. Differences in susceptibility between sexes within a strain were smaller than differences among strains. Comparisons of overall responses (sexes combined) for strains showed that, at LC90, the FLA strain was 48-fold more tolerant to fenvalerate and 34-fold more tolerant to permethrin than the TX strain. These results, coupled with observations that control in Okeechobee County has become less effective, suggest that FLA-strain H. irritans have become resistant to the two pyrethroids. The FLB strain also appears to be resistant to permethrin and may also be resistant to fenvalerate.
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