Context-dependent effects of ivermectin residues on dung insects: Interactions with environmental stressors, size, and sex in a sepsid fly (Sepsis neocynipsea)

Abstract

Insects associated with livestock dung frequently encounter veterinary medication residues. These residues often have negative effects on insect survival, reproduction, and ecosystem functioning and may contribute to the rapid decline in temperate insect populations. Ivermectin is an antiparasitic drug widely used to treat parasites in livestock. While it has long been recognized that ivermectin effects insect survival, the potential interactive effects between ivermectin exposure and other ecologically relevant abiotic stressors remain poorly understood. Here, we study these effects in the black scavenger fly Sepsis neocynipsea, which depends on cow dung for reproduction. Using a fully factorial design, we test whether the effects of ivermectin exposure on adult survival interact with heat and desiccation stress, and whether these effects depend on size and sex. Ivermectin exposure had strong negative impacts on adult survival, but its effects were stronger in females and large individuals. While heat stress also had a strong effect on adult survival, the combined effects of heat and ivermectin exposure were less severe than the expected additive effects of both stressors applied independently, suggesting some cross-resistance. We did not find an interaction between ivermectin and desiccation stress. Taken together, our findings highlight how the complex interactions between insecticides and abiotic stressors could drive changes in coprophagous insect populations and their ecological functions across different ecosystems and climates.