Injectable ivermectin for cattle: effects on some dung-inhabiting insects

Dung from cattle injected with a single dose of either 20 micrograms ivermectin/kg of body weight or the recommended therapeutic dose of 200 micrograms/kg was bioassayed in the laboratory weekly for 10 wk with the horn fly, Haematobia irritans (L.), and two species of dung-burying Scarabaeidae, Euoniticellus intermedius (Reiche) and Onthophagus gazella (F.). Dung from the same cattle was also bioassayed for 6 wk with two species of predaceous Staphylinidae, Philonthus flavolimbatus Erichson and P. longicornis Clark. Emergence of adult horn flies was reduced 42.3-100% for 6 wk when reared on dung from cattle that received the 20 micrograms/kg dose and compared with emergence of horn flies reared on dung from untreated cattle. When reared on dung from cattle that received a dose of 200 micrograms/kg, horn fly emergence was reduced 78.7-100% for 8 wk. The two treatment levels had no apparent effect on the production of brood balls by either species of dung beetle. However, emergence of adult E. intermedius and O. gazella from brood balls made with dung from cattle that received the 200 micrograms/kg dose was reduced for 1 and 2 wk, respectively. There was no reduction in the emergence of adults of either beetle species at the 20 micrograms/kg dose. The number of P. flavolimbatus progeny reared on dung from cattle that received the 200 micrograms/kg dose was reduced for 1 wk compared with the number of progeny reared on untreated dung. There was no apparent effect on the number of P. longicornis progeny when reared on dung from cattle that received the 200 micrograms/kg dose of ivermectin. Dung from cattle that received the 20 micrograms/kg dose had no apparent effect on the number of progeny of either predator species