Physiological Age Composition and Reproductive Biology of Horn Fly Populations, Haematobia Irritans Irritans (Diptera: Muscidae), in Iowa, USA

Abstract

Age-grading techniques were applied to natural populations of the horn fly, Haematobia irritans irritans, sampled from beef cattle in 1979 and 1980 in Ames, Iowa, USA. Rate of insemination was measured against female physiological age, and a maximum of 78% inseminated was detected among gravid flies. Once begun, vitellogenesis proceeded simultaneously, but not synchronously, in up to 6 follicles per ovariole. It was suggested that oviposition occurs daily among horn flies 3.5 calendar days of age and older. The seasonal mean number of functional ovarioles per fly was 18.4 ± 2.7 and showed no diminution with physiological age. Real differences in the age structure and sex ratio of horn flies were found among samples from cattle backs and bellies. Males and nulliparous females were most frequent on host backs; parous flies were most frequent on the bellies. Average frequency of gravid horn flies was 30% in 1979 and 22% in 1980. Mean frequency of teneral flies was 12.4%, and mean proportion parous was 57.5%. On the principal assumption that risk of mortality among adult flies was independent of age, the probability of survival through 1 day was estimated to be ca. 86%. A calculated mean number of 78 eggs was produced by an adult horn fly over her average expected lifetime of 6.6 days.