Feed-through insecticides for pest fly management on beef cattle pastures: impacts on dung-inhabiting Coleoptera

Dung-breeding flies of pastured cattle have substantial economic impacts on livestock production. To prevent losses, many producers use insecticidal fly treatments which have off-target impacts on beneficial pasture insects such as coprophagous and predatory dung-inhabiting beetles. These insects suppress pasture pest fly populations either through resource competition or direct predation of fly eggs and larvae in the dung. Feed-through insecticides such as insect growth regulators (IGRs) are thought to have fewer harmful off-target impacts because they target immature fly stages in the dung. However, impacts on the larval development of dung beetles are not well understood. The aim of this study was to analyze dung beetle and pest fly populations on grazing beef farms using feed-through insecticides for pest fly management. Between May and September in 2022 and 2024, populations of horn flies, face flies, and stable flies were counted on cattle, and dung-inhabiting Coleoptera populations were surveyed on pastures. The abundance and diversity of dung beetles (Scarabaeidae: Onthophagus and Aphodius spp.) and the abundance of predatory dung insects (Hydrophilidae and Staphylinidae) were significantly lower on farms using feed-through insecticides compared to farms not treating. Horn fly populations rarely exceeded economic thresholds without the use of feed-through insecticides, implying treatment was unnecessary. Face fly populations mostly exceeded economic thresholds even on farms using feed-through insecticides-indicating treatments were not adequately controlling their populations. The use of feed-through insecticides over the whole season is therefore not an integrated approach to controlling flies on pastures and puts other beneficial organisms in the pasture agroecosystem at risk.