ABSTRACT Broad‐scale anthelmintic use against gastrointestinal nematodes in livestock can lead to genetic resistance in nematodes and detrimentally affect non‐target organisms. The targeted selective treatment (TST) of vulnerable or infected livestock allows susceptible nematodes to persist in anthelmintic‐free dung refuges, slowing genetic resistance development. However, few studies have examined TST effects on non‐target dung fauna, especially when these species are also pests. We measured the survival and reproductive output of the Australian bush fly, Musca vetustissima , under controlled conditions at five TST levels (increasing proportion of untreated dung pats: 0.00, 0.25, 0.50, 0.75 and 1.00) and an ivermectin concentration of 125 ppb in treated pats. We used generalised linear mixed models to test for lethal effects of ivermectin exposure on F0 generation adults, the number of F1 pupae produced, F1 adult emergence and offspring sex ratio. Mortality of F0 adults did not differ significantly between TST levels. However, at intermediate and low TST levels, there was a reduction of 77%–78% in F1 pupae produced. When all dung pats contained ivermectin (TST = 0.00), no pupae developed. The proportion of F1 adults emerging from pupae was the same at all TST levels (ca. 80%–90% emerging), suggesting lethal effects of ivermectin occurred prior to pupal development. Anthelmintic‐free TST refuges could benefit bush fly populations. However, similar to the case for gastrointestinal nematodes, maintaining a refuge of susceptible individuals could also reduce the risk of genetic resistance evolving in bush fly populations. Investigating TST effects on other non‐target species is essential to ensure a successful integrated approach to gastrointestinal nematode management.
Lewis et al. (Mon,) studied this question.