Cholecalciferol-induced bait shyness in possums (Trichosurus vulpecula)

Cholecalciferol (vitamin D₃) is used in a variety of bait formulations to control populations of the brushtail possum (Trichosurus vulpecula), an introduced pest that seriously damages and threatens primary production and native biota in New Zealand. In cage trials, possums readily ate sublethal baits containing either an estimated LD₁₅ or LD₄₀ dose of cholecalciferol, leading to a depression of appetite that lasted for 7-15 days. When lethal quantities of baits were presented 21-30 days after the initial LD₁₅ or LD₄₀ baits, 40 and 88% of possums survived, respectively, compared with a 21% survival rate among naive possums, and bait consumption was also reduced among survivors of the initial baits. Subsequent switching of the bait base to a gel was partially successful in overcoming bait shyness, killing 40% of one group (formed by pooling the original groups), while switching both the bait type (to gel) and the toxicant (to 1080) resulted in a 63% mortality rate in a second group. By comparison, 95 and 100% of naive groups were killed by cholecalciferol and 1080 gel respectively. Practical measures are identified for both avoiding and overcoming bait shyness based on the use of alternative bait types and toxicants.