Bait shyness in possums induced by sublethal doses of cyanide paste bait

Cyanide paste covered in a flour-based lure is commonly used to control populations of the Australian brushtail possum, New Zealand's foremost vertebrate pest. When possums were given sublethal amounts of a standard paste bait, most ate it initially, but 75% refused it 7 days later, having become bait-shy. Most possums were able to discriminate between the toxic paste and the harmless flour-based lure. A less concentrated form of the paste was less effective in killing possums, in contrast to earlier data on the lethality of cyanide. A lower sublethal dose led to a weaker and less persistent bait shyness. Further refinement is therefore required to obtain the optimum concentration that balances reduced shyness and maximum kill. Surviving possums were offered cyanide paste again after 90 and 300 days and only 13 and 17% respectively succumbed, indicating a long-lived shyness, as has previously been demonstrated after sublethal poisoning with 1080 baits. The shyness did not generalize towards 1080 paste or pellet baits, or cholecalciferol gel baits. Sustained control of possums is therefore unlikely to be achieved by repeated use of cyanide paste, but changing to other types of toxic bait should provide more effective long-term control.