Bait Substrate

Zinc phosphide (ZnP) coated grain bait is the only mouse bait registered in Australia for broadacre application within crops. ZnP coated grain bait has been used continuously since the mid 1990’s to control house mice numbers in crops and reduce damage. The shift from conventional to conservation farming over this period has likely changed how and where mice find food as well as the amount of residual food in the system. This means it’s crucial to establish the efficacy of ZnP coated grain bait for controlling mouse numbers and crop damage, especially during mouse plagues.

Our laboratory study (Henry et al. 2022) indicated that wild mice can readily switch from a background food to an alternative food and preferred cereals, i.e., wheat or barley, over lentils. Mice consumed toxic ZnP barley grains regardless of the background grain type. However, mice consumed more toxic bait when the less-favoured background grain (lentils) was available than the more-favoured background grain (wheat or barley) was available. The mortality rate was also higher (86%) when lentils as the background grain than when wheat (47%) or barley (53%) was available. Most importantly, we found that consumption of single ZnP-coated grain did not always kill the mouse, and some mice can even survive the consumption of up to four ZnP-coated grains. These mice then rapidly switched back to eating the background food (see figure below). The result suggests that the lethal dose rate is likely much higher than 25 g ZnP/kg and the strong taste aversion in mice when a sub-lethal dose was taken.

One possible explanation to the results is that regular and constant bait applications may have selected mice that are more tolerant to ZnP. To test our hypothesis, we designed a laboratory study (Hinds et al. 2022) to re-examine the acute toxicity of ZnP in wild house mice compared to mice that have never been exposed to the toxin. We found the estimated LD50 for each mouse group ranged between 72 – 79 mg ZnP/kg body weight and there was no difference among all groups (see figure below). The result suggests that wild house mice in Australia need to receive a higher dose rate (LD50 of 72 – 75 mg ZnP/kg) to be lethal than previously reported for wild mice in the USA (LD50 of 32.68 mg ZnP/kg). The later value was used by the Australian Pesticides and Veterinary Medicines Authority (APVMA) to define the dose rate of 25 g ZnP/kg grain (ZnP25) for mouse control. Given the similar LD50 values across three groups of mice, 20 years of regular field application of ZnP has not led to selection either for non-responders or for tolerant individuals.

We then tested whether mice would consume the toxic bait when the grain was coated with a higher dose rate of 50 g ZnP/kg grain (ZnP50). Our results show that most mice consumed the ZnP50-coated grains on Day 1 no matter whether background food was provided. When no additional food was provided, 94% mice died after their first consumption of ZnP50 grain. Therefore, increasing the ZnP dose rate does not decrease its palatability. In addition, aversion is more likely to occur with the ZnP25 treatment due to the sub-lethal adverse effects of the toxin.

To further examine the performance of ZnP50 bait in the wild, we used a randomised field experiment (Ruscoe et al. 2022) comparing the population reduction of house mice between the new bait (ZnP50) and the currently registered bait (ZnP25) treated sites. Three treatments (control, ZnP50, ZnP25) were involved in the experiment and each treatment had three sites. In general, the number of mice present on the control sites decreased between the pre-treatment survey and the post-treatment survey. However, there were larger population reductions between treatments on sites treated with ZnP50 (-95.1%, -85.2%, -95.5%; see figure below).

The results of before-after-control-impact analysis show that the ZnP50 is expected to reduce ~90% of the population with a 50% probability of a population reduction between 84% and 92%. While the ZnP25 had the median expected reduction in population size of ~70% with a 50% probability of a population reduction of between 58% and 78%. This study clearly indicates that the ZnP50 bait was highly effective in reducing mouse population sizes.

Check out our Publications page to find out more about this research.