Control of introduced Rattus rattus L. on Lord Howe Island.: I.: The response of mouse populations to warfarin bait used to control rats

Warfarin bait has been used since 1986 to control introduced black rats (Rattus rattus) in palm-seeding areas on Lord Howe Island, New South Wales. We examined the relationship between bait use and mouse numbers in these areas. In the first experiment, one mouse population was manipulated by removal trapping while baiting for rats was being undertaken. When mouse density was reduced by approximately 193 ha (1), bait consumption fell by 80.0%, suggesting that the mice were not susceptible to warfarin and that the rat bait may have been an important food resource for these mice. During the second experiment, the existing rat-baiting regime was maintained in one area but manipulated in another bait was removed for one year then returned during the second year. Under the existing baiting regime, mouse numbers increased during the two-year period. The mouse population that was denied rat bait declined to near zero after one year, then increased when bait was reintroduced to the area, reaching densities after one year similar to those in the area where bait had been maintained. We conclude that the mice were resistant to warfarin, consumed most of the bait distributed to control rats, were largely dependant on the bait as a food source, and reached high densities in rat-control areas as a direct result of rat-baiting strategies.