Poisoning mammalian pests can have unintended consequences for future control: Two case studies

Vertebrate pest control operations using toxic baits can have unintended consequences for nontarget species. some of which may themselves be pests. Learned avoidance behaviour (termed 'aversion') can be induced by sublethal dosing, which can arise when species with high and low susceptibilities to a toxin co- exist in the same area. In such cases the less-susceptible species (e.g., possums Trichosurus vulpecula) may be sublethally poisoned by control work targeting the more- susceptible species (e.g., rabbits Oryctolagus cuniculus). A case study of rabbit control on North Canterbury farmland is presented to demonstrate this effect. When control is being rePeated at frequent intervals, it is prudent to vary the control methods used. Nevertheless, aversion induced by the use of one toxic bait (e.g., cyanide paste) can in some situations 'generalise' so that the efficacy of control using other toxins (e.g., 1080 and cholecalciferol in cereal baits) is also compromised. A case study of initial and follow-up possum control in four discrete areas of Canterbury forest provides an example of this problem. The implications of these findings for future pest management in New Zealand are discussed.