Acute oral toxicity of zinc phosphide: an assessment for wild house mice in Australia

BACKGROUNDThe efficacy of zinc phosphide (ZnP) for broadacre control of wild house mice in Australia is being reported by growers as increasingly variable. Have mice become less sensitive over time or are they taking a sub-lethal dose and developing aversion? In this laboratory study the sensitivity of groups of wild caught and an outbred laboratory strain of mice was assessed using oral gavage of a range of ZnP concentrations. The willingness of mice to consume ZnP-coated grains was then determined. RESULTSEach mouse group had very similar LD50 values (72 to 79 mg ZnP per kg body weight) which are significantly higher than previously reported. Time to death post-gavage ranged between 2.5 to 48 h. ZnP coated grains (50 g ZnP per kg grain) presented in the absence of alternative food were consumed and 94 percent of wild mice died. Mice provided with alternative food and ZnP coated wheat grains (either 25 or 50 g ZnP per kg grain) consumed toxic and non-toxic grains, and mortality was lower (33 to 55 percent). If a sublethal amount of ZnP coated grain was consumed, aversion occurred mostly in the presence of alternative food. CONCLUSIONSThe sensitivity of wild house mice to ZnP in Australia is significantly lower than previously assumed. Under laboratory conditions ZnP coated grains coated with a new higher dose (50 g ZnP per kg grain) were readily consumed. Consumption of toxic grain occurred when alternative food was available but was decreased. It is important to assess the efficacy of the higher ZnP dose per grain under natural field conditions, especially when background food is low.