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.
Black rats (Rattus rattus) and house mice
(Mus musculus) from Lord Howe Island were live-trapped,
housed in the laboratory and tested for resistance to the anticoagulant poison
warfarin. All rats fed warfarin (0.025% w/w) in their diet died
within 4–12 days whereas no rats in the untreated group died. Mice fed
warfarin at the same concentration over 21 days all survived. Black rats on
Lord Howe Island remain susceptible to warfarin, but house mice appear
resistant.
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