Dividing the pie: differential dung pat size utilization by sympatric Haematobia irritans (Diptera: Muscidae) and Musca autumnalis (Diptera: Muscidae)

Horn flies (Haematobia irritans) and face flies (Musca autumnalis) are serious pests of cattle. Horn flies significantly decrease feed conversion rates, grazing time, cattle weaning weights, and milk production, while face flies mechanically transmit the bacterium, Moraxella bovis (bovine pink eye), which can cause blindness. Both species exclusively survive on bovines, with bovine dung being their natural, sole oviposition and larval development site. However, no research has documented how they utilize a shared resource. Dung pats (n=350) were core-sampled during the summers of 2012 and 2013. Surface areas and estimated weights were recorded for each whole pat and the emerging flies from cores were identified. In Pomona, California, 42.0% of cores yielded neither species, 29.7% yielded only horn flies, 12.9% yielded only face flies, and 15.4% yielded both species. Fly-positive pats revealed that face fly-only pats had a greater surface area and mass than horn fly-only pats, while horn and face fly occupied pats were of intermediate surface area and mass. Horn flies per positive dung core were unaffected in the presence/absence of face flies, while less than half as many face flies emerged from pats shared with horn flies. Horn flies tolerate much lower dung moisture content than face flies, which likely allows horn flies to colonize more pats and a greater range of dung pat sizes and masses than face flies in a dry, southern California habitat. The causal mechanisms (e.g. adult oviposition, larval competition) behind differential size-based pat use, or its consequences, remain to be determined.