Re‐examination of Competition Between Musca Vetustissima Walker (Diptera:muscidae) Larvae and Seasonal Changes in Favourability of Cattle Dung

Abstract

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.

Stages of bush fly, Musca vetustissima (Diptera: Muscidae), killed by scarabaeine dung beetles (Coleoptera: Scarabaeidae) in unfavourable cattle dung

“…Dung for the experiments with Onitis alexis was collected in January, and headwidths of M. vetustissima emerging from control pads averaged 1.87 mm, while dung for experiments with Onthophagus binodis was collected in February, and headwidths of flies emerging from control pads averaged 1.81 mm. In other experiments carried out under the same conditions, where similar numbers of eggs were laid, the size of M. vetustissima emerging from dung collected at the same site, and in the absence of dung beetles, in January 1983 averaged 1.94 mm , and in January and February 1985 was 1.75 mm (Ridsdill-Smith & Hayles, 1989). Dung collected from annual pastures in January and February was unfavourable for bush fly development in all of these years, and the data in this paper appear to be consistent with data from other years, although some year to year variation occurs.…”

“…Rate of oviposition by Onthophagus binodis and Onitis alexis is high in favourable dung and low in unfavourable dung (Ridsdill-Smith, 1986). Fewer M. vetustissima oviposit on unfavourable than favourable dung (Ridsdill-Smith & Hayles, 1989), but their main response is that adults emerging from larvae that have fed on unfavourable dung are smaller than those from favourable dung (Matthiessen & Hayles, 1983;Ridsdill-Smith & Hayles, 1989;Sands & Hughes, 1987). In the laboratory, the same number of Onthophagus binodis kill more M. vetustissima in unfavourable dung than in favourable dung (Ridsdill-Smith et ah, 1986).…”