Proceedings of the Vertebrate Pest Conference

Around NZ$76 million per annum is presently spent on controlling and researching the introduced brushtail possum in New Zealand. The national control effort is likely to be further increased over the next decade in an attempt to eradicate bovine tuberculosis and to arrest the degradation of key conservation areas. There is a risk that repeated control operations using toxic baits could result in widespread development of learned “bait-shyness,” behavior that develops when sublethally poisoned animals are able to link the experience of an illness with the memory of the bait eaten. Four studies are reviewed that aimed to determine the likelihood of possums becoming shy after sublethal doses imparted by different types of toxic bait, and to find solutions to the problem. Captive possums were presented with baits containing sublethal doses of the 4 vertebrate pesticides commonly used. Following a recovery period, possums were presented with lethal baits and their response compared with that of naive possums. Between 15% and 90% of possums previously dosed with sublethal doses of cyanide, 1080, or cholecalciferol developed bait-shyness, the proportion being dose and toxicant dependent. Although cholecalciferol induces toxicosis more slowly (i.e., >24 h) than cyanide (5 min) and 1080 (1 h), possums were nevertheless able to associate the effects of this toxicant with consumption of the bait in which it was delivered and avoid eating further baits. However, brodifacoum acts very slowly (1-4 weeks), and possums did not become shy towards baits containing this toxicant after first eating sublethal baits. Shyness induced by either 1080 or cyanide persisted for at least 12 and 24 months respectively, thus jeopardizing the effectiveness of “maintenance” control operations, which can be required every 12 months or less. The persistence of bait-shyness to cholecalciferol remains untested. Use of alternative baits (bait-switching) proved an effective solution for eliminating cyanide- and 1080-induced bait-shyness. Baits containing brodifacoum can also be used very effectively to “mop up” bait-shy populations, but care must be taken to minimize environmental contamination by this persistent compound.