RE‐EXAMINATION OF COMPETITION BETWEEN MUSCA VETUSTISSIMA WALKER (DIPTERA:MUSCIDAE) LARVAE AND SEASONAL CHANGES IN FAVOURABILITY OF CATTLE DUNG

Intraspecific competition between different numbers of larvae of bush fly, Musca vetustissima Walker, in one litre pads of decreasing quality of cattle dung collected monthly from October to February was measured in the laboratory at 25°C. Larval size, estimated from the headwidths of emerging flies, showed the same maximum in dung collected from October to January. For a family of curves only one parameter, the ‘exponent, was needed to describe the effect of dung quality on the relation between dung supply/egg and size. Dung supply/egg at which competition occurred increased from 1 mL in October to 13 mL in January. In February dung all flies were smaller and survival was lowest. Survival in any month was not correlated with dung supply/egg, but was positively correlated with adult size up to a threshold of 2 mm headwidth. When bush fly populations in the field are increasing and dung quality is decreasing, the fall in size, and thus fecundity, is predicted to occur earlier in the season because of these interactions between dung supply/egg and dung quality than would be predicted from laboratory bioassays which assume that no interactions occur.