Effects of background food on alternative grain uptake and zinc phosphide efficacy in wild house mice

Background: House mice (Mus musculus) cause significant, ongoing losses to grain crops in Australia, particularly during mouse plagues. Zinc phosphide (ZnP) coated grain is used for control, but with variable success. In a laboratory setting, we tested if mice would (i) switch from consumption of one grain type to another when presented with an alternative and (ii) consume ZnP-treated grains when presented as a choice with a different grain.

Results: Mice readily switched from their background grain to an alternative grain, preferring cereals (wheat or barley) over lentils. Mice readily consumed ZnP-coated barley grains. Their mortality rate was significantly higher (86%, n = 30) in the presence of a less-favoured grain (lentils) compared to their mortality rate (47%, n = 29; 53%, n = 30) in the presence of a more-favoured grain (wheat and barley, respectively). Mice died between 4 and 112 h (median = 18 h) after consuming one or more toxic grains. Independent analysis of ZnP-coated grains showed variable toxin loading indicating that consumption of a single grain would not guarantee intake of a lethal dose. There was also a strong and rapid behavioural aversion if mice did not consume a lethal dose on the first night.

Conclusions: The registered dose rate of 25 g of ZnP/kg wheat (~1 mg of ZnP/grain) in Australia needs to be re-evaluated to determine what factors may be contributing to variation in efficacy. Further field research is also required to understand the complex association between ZnP dose, and quantity and quality of background food on efficacy of ZnP baits.