Field observations of the host-parasite relationship associated with the common horse bot fly, Gasterophilus intestinalis.

Behavior of Gasterophilus intestinalis (Diptera: Gasterophilidae) was observed in the laboratory and field over a period of 10 years. Mating occurred in a frenzied manner as flies emerged from pupae. Males attempted to copulate with females that had not fully emerged from the pupal case. Mating was never observed in the field or near the horse. Observations suggest that fecal piles of the horse are used as the mating site for the newly hatched flies. Activation of host-seeking behavior was found to occur early and remain throughout the life span of the adult as it sought multiple hosts for oviposition. Evidence is presented supporting the use of olfaction and vision in finding the host. Interest by flies in shadows and linear high-contrast objects was observed. Methods used by the flies to maintain close proximity with the host are discussed including flight with the host and avoidance of host defenses. Recapture methods were successful in determining the time taken to reestablish at the host, and extent of travel to the host. The presence of kairomone(s) within horse secretions is suspected.