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

Abstract

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 morefavoured 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.

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

“…However, there was variation between grains in their ZnP loading which could have affected mortality rates and aversion particularly when consumption of these ZnP coated grains occurred in the presence of other food. Similar variations were observed for a commercially prepared batch of grains used in another study 23 and may be due to varying amounts of ZnP powder being trapped in the crease of grains of different sizes and shapes during coating.…”

“…Secondly, a third group of animals was provided with a choice of alternative food and ZnP-coated grains (25 g ZnP/kg grain) (hereafter F25). We did this to confirm results obtained for animals receiving a choice of ZnP-coated grains (25 g ZnP/kg grain) and alternative food as described in 23 and to compare this F25 group with the above-mentioned F50 group. Two sequential independent trials (Trial 1 and Trial 2), each using ten animals per treatment group, were undertaken over a two-week period.…”

“…23,30 This reflects the expected effects of phosphine gas production in the gut and/or direct uptake of ZnP across the gut and subsequent failure of major internal organs such as the liver, kidney, brain and heart. 8 When wild mice were presented with ZnP-coated wheat grains (either ZnP50 or ZnP25), there was also considerable variation in responses after consumption including a rapid development of aversion as previously reported for several species including wild mice 23 , common voles, Microtus arvalis 31 and prairie voles (Microtus ochragaster Wagner). 32 Any wild mouse taking a sub-lethal dose on first exposure, rarely consumed ZnP-coated grains subsequently and this aversive response was higher when there was a choice of alternative food (42 -47% mice became averse after consuming ZnP25-or ZnP50-coated grains).…”

“…This time-to-death range and the development of various signs of acute toxicity are similar to that observed in other studies in mice and rats. 23,30 This reflects the expected effects of phosphine gas production in the gut and/or direct uptake of ZnP across the gut and subsequent failure of major internal organs such as the liver, kidney, brain and heart. 8 When wild mice were presented with ZnP-coated wheat grains (either ZnP50 or ZnP25), there was also considerable variation in responses after consumption including a rapid development of aversion as previously reported for several species including wild mice 23 , common voles, Microtus arvalis 31 and prairie voles (Microtus ochragaster Wagner).…”

“…The effectiveness of ZnP-coated grains has previously been assessed in the laboratory in the presence of alternative food. 23 In that study, wild mice showed a strong preference for consumption of cereal grains over lentils, and this influenced their uptake of toxic ZnP-coated barley grains. The mortality rate for mice presented with a choice of lentils and toxic ZnP-coated cereal grains was 86%, whereas for mice presented with a choice of cereal grains and toxic ZnP-coated cereal grains the mortality rate was only 50%.…”