Proceedings of the Vertebrate Pest Conference

Strategic management of brushtail possum populations in New Zealand is presently dependent on the use of a standardized trap-catch procedure for monitoring population trends. Where this has been used in the first few months after control, calculated rates of increase often far exceed known reproductive and dispersal rates, suggesting that trapping-based population indices immediately following control are biased low. We are investigating the problem by genotyping DNA extracted from possum fecal pellets and using matching genotypes in the ear tissue of trapped possums as a measure of trappability. We have used quantitative polymerase chain reaction (PCR) to determine the threshold of possum DNA required from fecal samples in order to obtain an accurate genotype (>99%) from field samples. This has enabled the removal of “allelic dropout” as a source of error in obtaining accurate genotypes from such noninvasive DNA samples. Validation tests were conducted on fecal samples collected from caged (i.e., identifiable) possums, and fecal pellets and ear tissue from trapped possums in the field. The tests confirmed that the sample collection and preservation procedures used resulted in accurate identification of possums from both types of sample material, although fecal pellets older than about 7 days were unlikely to yield sufficient DNA for amplification and genotyping. Genotyping of a large quantity of sample material collected before control, and at 1, 4, and 9 months after control, in 2 replicated field trials is proceeding and revealing information on the trappability of possums that survive control, and the contribution of immigration to the populations after control.