Parasitization of house fly pupae (Diptera: Muscidae) by Spalangia nigroaenea (Hymenoptera: Pteromalidae) in cattle feedlot environments

Spalangia nigroaenea Curtis, a parasitoid that attacks filth fly pupae, has been mass-released for biological control in western Kansas. The influence of three different variations of the parasitoid-to-host ratio on parasitism of house fly, Musca domestica L., pupae was studied under feedlot conditions from 1993 to 1995. When a single parasitoid female was introduced to containers with varied host densities, the highest percentage of parasitization occurred at densities of 15 (1993 and 1994) and 10 (1995) puparia. When the ratio of parasitoid-to-host was changed, increasing densities from 30 to 300 for the host and from 3 to 30 for the parasitoid did not significantly affect performance of the parasitoid. Finally, parasitoid densities were varied, while host densities were held constant. Although significantly more offspring were produced as the parasitoid-to-host ratio changed from 1:5 to 1:50, the percentage parasitism actually decreased. The third measure of parasitoid activity, parasitoid-induced mortality (PIM), also was examined. In 1994, PIM did not differ significantly with the changing parasitoid-to-host ratios. However, in 1995 a direct correlation occurred between PIM and parasitoid-host-ratios, During the fly breeding season (May to October in 1994), the daily maximum temperature was correlated negatively with parasitism. Clearly physical factors (mainly temperature) and biological factors (such as interspecific competition) are important in determining the success of mass release programs. The conflict between the efficiency and the effectiveness of biological control of filth flies using pteromalid parasitoids is discussed. (C) 1998 Academic Press.