Cholecalciferol Toxicity and Its Enhancement by Calcium Carbonate in the Common Brushtail Possum.

The common brushtail possum (Trichosurus vulpecula), an introduced pest in New Zealand, is susceptible to cholecalciferol (vitamin D3), which is marketed as a rodenticide in many countries. To evaluate cholecalciferol for possum control we offered 242 caged adult possums cereal pellets containing cholecalciferol on its own or with calcium carbonate. Mortality was recorded for two weeks after presentation. For cholecalciferol on its own the LD50 was 16.8 mg kg-1 (95% confidence interval 11.6-21.9 mg kg-1), but the addition of calcium carbonate reduced this to 9.8 mg kg-1 (95% confidence interval 7.0-12.7 mg kg-1). When the sexes were analysed separately, the reduction of the LDS0 by the addition of calcium carbonate was significant only in females. The addition of calcium carbonate appeared to result in a more predictable outcome as shown by the reduced confidence intervals. For possum control, adding calcium carbonate to cholecalciferol bait would improve the bait's efficacy, decrease the amount of toxic material needed, and reduce the cost of bait production.