Abstract
Studies were conducted on population sex ratios and factors affecting adult emergence of the horn fly, Hacmatobia irritans (L.). Weekly collections were made from pastured cattle during the fly season for 2 years. A 1:1.35 male to female ratio was found from the total of 65.702 flies sampled, but a higher proportion of male horn flies was found in collections from the legs of calves. Data on adult emergence indicated that temperature controlled emergence completely, but the effects of temperature were altered by photoperiod. Females were found to emerge significantly earlier than males in various tests. Fieldreared pupae exhibited a unimodal sequence of emergence with the overt phase occurring from 4 to 8 Pm daily. The minimum threshold temperature for adult emergence was 60°F. Photoperiod had the most pronounced effect on emergence when the larval and pupal stages were reared at a 12 hour reciprocal photophase of field conditions.
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