Parahost behavior of adult Gasterophilus intestinalis (Diptera: Gasterophilidae) in Delaware.

Parahost behavior of adult Gasterophilus intestinalis (DeGeer) was observed during the summers of 1978 and 1979 near pastured horses on two farms near Newark, Del. Adult fly occurrence, although sparse in early summer, indicated a gradual increase, with three surges in emergence. In the laboratory, males lived 1-3 d and females 1-2 d. In the field, marked flies were not seen on days following their release. Adult behavior indicated an urgency in mating and oviposition dictated by a short life span. Male flight patterns were characterized by in-flight searches for females and intraspecific conflicts with other males. This behavior appeared to be density dependent. Horses grazing singly or in groups were aggressively defended by hovering males. Following midair coupling, mating took place near horses. Female horse bot flies oviposited in a variety of environmental and parahost conditions. The rate of oviposition varied, but a female could deposit her total complement of 900 eggs in less than 1 h. Egg counts on hosts indicated that horses can ingest greater than 90% of the infective stage larvae through self and mutual grooming. The hanging ovipositor profile identified an ovipositionally spent female. The effective life span of an ovipositing female was 1 d.