The effects of natural enemies on the population dynamics of sugarcane soldier fly, Inopus rubriceps (Dip.: Stratiomyidae)

The soldier fly, Inopus rubriceps (Macquart), is an endemic pest of sugarcane in Queensland, Australia, and an introduced pest of pasture and lawn grasses in New Zealand and California. This paper reports a population dynamics study of the fly in Queensland grasslands and sugarcane undertaken between 1981 and 1984. Three species of parasitoid (Hymenoptera: Diapriidae) and a complex of polyphagous predators (Coleoptera) attacked the pupal stage. Density-dependent pupal mortality within generations was indicated from sampling in canefields. Densities of autumn-emerging adult flies fluctuated around 45 m-2 in grasslands. No equilibrium cane, with population growth occurring in insecticide-treated and young canefields, and population decline in old canefields (5-8 years old) during the study. Key factor analyses suggested that early stage (egg-young larvae) mortality and pupal mortality (predation plus parasitism) were responsible for determining population levels (b=0.43 and b=0.33, respectively). Egg loss and young larval mortality rate varied considerably, with greatest losses in old canefields although no density-dependent effects were apparent. Larval survival rate was apparently constant over the density range studied. Pupal mortality rate, and adult to adult population changes varied with density in grasslands but not in canefields. Predation and parasitism of pupae were responsible for the variation in pupal mortality rate. A general model, based on the responses of polyphagous predators, described population changes in grasslands and old canefields. The relatively short duration of cane crops prevents natural enemy-induced stability in soldier fly populations