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

Abstract

Context. Recent studies have shown that the sensitivity of wild house mice to zinc phosphide (ZnP) in Australia is significantly lower than previously assumed, which may account for the reported variability in efficacy of ZnP baits used for broadacre control of house mice in grain-growing regions. Under laboratory conditions ZnP-coated grains with a new higher dose (50 g ZnP/kg grain) were readily consumed but the efficacy of using grains with this higher dose under natural field conditions has not been tested. Aims. To test whether the newly derived ZnP50 (50 g ZnP/kg grain) was more effective under field conditions than the currently registered ZnP25 (25 g ZnP/kg grain) in reducing populations of house mice during a mouse population irruption. Methods. We used a before-after-control-impact (BACI) design to assess changes in mouse population size under different baiting treatments in a replicated field trial. We assessed changes in mouse abundance in recently sown paddocks with either ZnP50 (n = 3) or ZnP25 (n = 3) compared with unbaited control sites (n = 3). Key results. Baiting with ZnP50 led to a median reduction in mouse numbers of >85%. Our modelling showed that under similar circumstances, using the ZnP50 formulation should deliver >80% reduction in population size most (>90%) of the time. In contrast, the current registered bait (ZnP25) achieved approximately 70% reduction in population size, but with more variable results. We would be confident of getting an 80% reduction in population size only 20% of the time by using the currently registered ZnP25 bait under similar field conditions. Conclusions. Consistent with laboratory studies, this study demonstrated the higher probability of achieving a consistently high kill rate under field conditions with the new ZnP50 bait compared with the currently registered formulation (ZnP25). Implications. By using the new ZnP50 bait, farmers are far more likely to get good kill rates, thereby reducing the need for repeated baiting (which is costly and generally ineffective at protecting newly sown crops). Using the new bait should result in lower control costs for farmers and fewer toxic grains being spread to control mice.

Acute oral toxicity of zinc phosphide: an assessment for wild house mice (Mus musculus)

“…Assessment of the efficacy of the higher dose (50 g ZnP/kg grain) compared to the registered dose at a population level under field conditions has now been conducted (Ruscoe et al . 2022 ) and has verified the findings of these laboratory‐based trials.…”

“…In CA systems where available alternative food could be >200 grains/m 2 , this will likely affect ZnP baiting outcomes when the toxic grains are spread at only 2-3 grains/m 2 . Assessment of the efficacy of the higher dose (50 g ZnP/kg grain) compared to the registered dose at a population level under field conditions has now been conducted (Ruscoe et al 2022) and has verified the findings of these laboratory-based trials.…”

“…An assessment of the efficacy of the higher dose (50 g ZnP/kg grain) compared to the currently registered dose (25 g ZnP/kg grain) at a population level under field conditions has recently validated the findings of these laboratory‐based trials (Ruscoe et al . 2022 ). A review of the sensitivity of other rodent species to ZnP may be merited.…”

Effects of harvesting and stubble management on abundance of pest rodents (Mus musculus) in a conservation agriculture system

“…The population reductions we measured following harvest were nowhere near these cull rates, and therefore would not be expected to prevent population increase while breeding was occurring. While population reductions less than 90% may be of benefit to delay peak population onset past the crucial re-sowing season, an average density reduction of up to 41% in this study did not stop significant population increases (reported in a study at the same sites undertaken 3 months later 54 ) and may have resulted in compensatory survival, breeding, and/or emigration from adjacent unaffected higher density populations. These relationships require further investigation.…”

“…Over the longer term, reduced cover might decrease mouse foraging efficiency due to perceived predation risk, which could in turn translate into reduced survival and fecundity, but these outcomes would take longer than our study to manifest. Having said that, the mouse population on these sites remained high 3 months after the present study (see a subsequent study on the same sites 54 ) and the farmers reluctantly burned the stubble in the paddocks prior to sowing the next crop (April–May 2021) in a last‐ditch effort to completely remove vegetative habitat and food (spilled grain and weed seeds) availability, to reduce mouse numbers.…”