Dividing the Pie: Differential Dung Pat Use by Horn Flies and Face Flies

Horn flies (Haematobia irritans) and face flies (Musca autumnalis) are serious pests of cattle. 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 in Pomona, California. Pat surface area, pat estimated mass, and emerging fly densities were recorded for each whole pat. Horn flies colonized twice as many pats and used a greater range of dung pat sizes and masses than face flies. Fly-positive pats revealed that face fly-only pats had a greater average surface area and mass than horn fly-only pats, while horn and face fly-occupied pats were of intermediate surface area and mass. The number of horn flies per positive dung core were unaffected by the presence/absence of face flies, while 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 contributed to their success in an arid pasture. The causal mechanisms (e.g. selective adult oviposition, larval competition) behind differential size-based pat use remain to be determined.