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This study originated from the knowledge that Haematobia irritans (L.), which inhabit the feet and lower legs of cattle, are predominantly males. When the feet and lower legs of steers infested by horn flies were sprayed twice weekly for 5½ weeks with a residual insecticide the entire horn fly population was eliminated. After 2 and 4 weeks of treatment, reductions were noted in male-to-female ratio and percentage of inseminated females. A similar treatment with dairy herds for 4 weeks held horn fly numbers to a level comparable with that attained by conventional chemical treatments. Male-to-female ratio and percentage of inseminated females also were reduced.