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

Abstract

BACKGROUND: The 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. RESULTS: Each 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 mg 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 mg 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. CONCLUSIONS: The 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 mg 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.

Improved house mouse control in the field with a higher dose zinc phosphide bait.

“…Without background food availability and wet weather affecting bait take, we conclude that, as found in the laboratory trials of Henry et al (2021) and Hinds et al (2021) the ZnP25 formulated bait containing on average 1 mg ZnP was not sufficient to kill mice unless they found and consumed multiple baits before starting to feel sick. Any animal that only found and ate one grain of ZnP25 probably consumed a sublethal dose and become averse to the baits likely refusing to take more (Henry et al 2021).…”

“…This led to a reexamination of the median lethal dose rate (LD50) using oral gavage trials on both Australian wild caught and a laboratory strain of mice. These laboratory trials found that the LD50 was 72-79 mg ZnP/kg body weight (BW) (Hinds et al 2021), a value significantly higher than the 32.7 mg ZnP/kg BW previously reported by Li and Marsh (1988), which was the value on which the currently registered bait is formulated. Given the results of Hinds et al (2021) a new higher strength bait was developed based on a lethal dose rate (LD90: 110 mg ZnP/kg BW) which equates to an application rate of 50 g ZnP/kg wheat bait (hereafter referred to as ZnP50), whereby each wheat grain is coated with ~2 mg ZnP (LD90 dose for 15 g mouse).…”

“…These laboratory trials found that the LD50 was 72-79 mg ZnP/kg body weight (BW) (Hinds et al 2021), a value significantly higher than the 32.7 mg ZnP/kg BW previously reported by Li and Marsh (1988), which was the value on which the currently registered bait is formulated. Given the results of Hinds et al (2021) a new higher strength bait was developed based on a lethal dose rate (LD90: 110 mg ZnP/kg BW) which equates to an application rate of 50 g ZnP/kg wheat bait (hereafter referred to as ZnP50), whereby each wheat grain is coated with ~2 mg ZnP (LD90 dose for 15 g mouse). Laboratory feeding trials showed that mice readily consumed this new ZnP50 bait and mortality was high (94%) in the absence of alternative food (Hinds et al 2021).…”

“…When ingested, ZnP reacts immediately with stomach acids to release highly toxic phosphine gas that is quickly absorbed into the bloodstream to affect the lungs, liver, kidneys, heart and central nervous system (Guale et al 1994). Death usually results from heart and kidney failure within 24 hours but can be up to 112 hours (Henry et al 2021;Hinds et al 2021). One benefit of using acute poisons is that secondary poisoning risks to predators and scavengers from ZnP exposure are low especially when compared to other rodenticides (anticoagulants).…”

“…Given the results of Hinds et al (2021) a new higher strength bait was developed based on a lethal dose rate (LD90: 110 mg ZnP/kg BW) which equates to an application rate of 50 g ZnP/kg wheat bait (hereafter referred to as ZnP50), whereby each wheat grain is coated with ~2 mg ZnP (LD90 dose for 15 g mouse). Laboratory feeding trials showed that mice readily consumed this new ZnP50 bait and mortality was high (94%) in the absence of alternative food (Hinds et al 2021). This ZnP50 bait was granted an emergency permit for use during the 2021 eastern Australia mouse plague (Australian Pesticide and Veterinary Medicines Authority (APVMA) permit -PER90799, May 2021).…”