Organic farms conserve a dung beetle species capable of disrupting fly vectors of foodborne pathogens

Abstract

Produce can be contaminated with enteric bacteria when livestock or wildlife feces are deposited in vegetable fields. Coprophagous beetles and flies might mitigate this threat as they feed but could also transmit pathogens if they contact plants. Improved food safety will result only from farming practices that enhance coprophage benefits and limit harms. On 49 mixed-vegetable farm fields across the western US states of Oregon and California, we found differences in coprophagous fly community composition under organic versus conventional management practices. While dung beetle (Coleoptera: Scarabaeidae) community assemblages did not differ significantly based on farm management system, organic farms fostered populations ofOnthophagus taurus, a dung beetle species that is a known antagonist of human-pathogenicEscherichia coli. We examined the possible implications for food safety of interactions between O. taurus and a common fly species on the farms, Calliphora vomitora, in microcosms containing pig (Sus scrofa) feces inoculated with human pathogenicE. coliO157:H7 and placed near broccoli (Brassica oleracea) plants. In the absence of dung beetles, Calliphora vomitorareadily acquired the bacteria and transmitted them to broccoli foliage. In the presence of the dung beetleO. taurus, however,E. coliin the soil and fly survivorship were reduced, and the pathogen was rarely recovered from foliage. Altogether, our results suggest the potential for O. taurus to both directly suppress enteric pathogens in vertebrate feces and to indirectly reduce the spread of these bacteria by co-occurring flies. The beneficial beetle O. tauruswas common only on organic farms, suggesting these benefits of beetle-fly interference for food safety could be more likely under this farming regime. Future research that investigates interactions between the many other common dung beetle and fly species on these farms would help fully delineate any net benefit of these species-rich coprophage communities, and the farming systems that shape them, for food safety.