The Efficacy of Baits Containing 1080 for Control of Brushtail Possums

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A significant proportion of brushtail possums (Trichosurus vulpecula) in New Zealand survive pest control operations using sodium monofluoroacetate (1080) baits. To be effective, bait needs to contain an appropriate concentration of 1080 and lie eaten in amounts lethal to all possums. In our trials, the acute toxicity of 1080 to captive possums was estimated when the toxicant was included in different bait types, and results compared with published data for 1080 administered in aqueous solutions by oral gavage. The effectiveness of baits containing different 1080 concentrations, and of different palatabilities, was also assessed. Captive possum S that fed on baits in this trial were less susceptible to 1080 (LD 50 = 1.5 mg/kg) than in previously reported trials where the toxicant was administered to caged possums by oral gavage (LD 50 = 0.8 mg/kg; P < 0.001). The differences in the acute toxicity of 1080 in water and in baits are attributed mainly to the reduced bioavailability of 1080 in baits. Also in our trial, captive animals feeding voluntarily on baits more closely approximated the status of animals in the wild than when acute toxicity was estimated by anaesthetizing possums and intubating solutions by oral gavage. Genetic polymorphisms caused some animals to be much more susceptible to 1080 toxicosis than others. Body mass differences cause females and young possums to ingest a higher toxic dose (mg/kg) than adult males, and the concentrations of toxicant affected the amounts of bait eaten and the resultant mortality For the range of baits tested, the palatability of bait was a more important determinant of amounts eaten, and the percentage of possums sublethally poisoned, than was toxicant concentration. Possums that ate baits of low palatability and inappropriate 1080 concentration ingested small doses of 1080 and either endured a protracted time to death or survived. The size and quality of bait have implications for the management of wild populations of possums in different climatic areas where aerial and land-based methods of control are used.