Effects of ivermectin and moxidectin on the insects of cattle dung

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Abstract
The numbers of insects colonizing the dung of cattle injected with the anthelmintics ivermectin or moxidectin were compared with the colonization of the dung from control animals given no treatment, in a blind field trial. Dung was collected 2, 7, 14, and 21 days after dosing and set out in a randomized grid of standard 2 kg pats. Pats were retrieved from each of the three groups group after 7, 14, 21 and 42 days in the field for analysis of insect fauna. There were no significant differences in the number of adult Aphodius spp. (Coleoptera: Scarabaeidae) found in the dung from the three treatment groups with either time of exposure in the field or time after treatment, showing that adult beetles were attracted to all pats equally. However, dung from animals treated with ivermectin prevented the development of larval Aphodius spp. for at least 7 days after treatment. In contrast, the dung from animals treated with moxidectin supported the development of Aphodius spp. at all stages after dosing, as did dung from control animals. There were no significant differences in the number of cyclorrhaphous Diptera larvae in dung from control or moxidectin-treated animals. In contrast, such larvae were largely absent from the dung collected from ivermectin-treated cattle for up to 14 days after dosing. The results show that moxidectin is less toxic to dung-inhabiting insects than ivermectin following standard injection treatment. The results are discussed in relation to the effects of anthelmintics on non-target pastureland insects.