Sublethal effects of a neonicotinoid on the filth fly parasitoid Spalangia endius

Pesticide treatments can impact non-target insect species by affecting their fitness, and thus selection on them, as well as by affecting the composition of communities. Spalangia endius is a small wasp that parasitizes the pupal stage of pest flies such as house flies, which pupate in decaying organic matter such as manure. This wasp and related species occur naturally and are released as biological control agents. Neonicotinoids are often very effective against pest flies, but how neonicotinoids affect natural enemies of these flies is little known. This study examined the effect of the neonicotinoid imidacloprid on S. endius’s longevity, parasitization, offspring sex ratio, and ability to dig through substrate and locate buried fly pupae. Females that had been exposed to a high dose of the neonicotinoid (an LC₅₀ exposure) and a low dose (an LC₁₀ exposure) were compared with a solvent-only control and a clean control. Only the high dose showed an effect. Significantly fewer wasps and more flies emerged from the high dose treatment than from the other three treatments when the fly pupae were not buried. In contrast, the treatments showed no significant differences in sex ratio, in life span or in the number of flies or wasps that emerged when the fly pupae had been buried. These results raise concern about the impact of neonicotinoids on natural enemies, including species that help control pests.