Palatability and efficacy of rodent baits for eradicating house mice (Mus musculus) from Gough Island, Tristan da Cunha

Context Introduced house mice (Mus musculus) are a serious predator of seabird chicks at Gough Island, resulting in declining populations of several threatened species. This has prompted the preparation of plans to eradicate mice from Gough and other islands. However, relatively little is known about the palatability and efficacy of rodent baits for house mice, as most studies have focussed on rats (Rattus spp.). Aims The study's aim was to test the palatability of non-toxic forms of two commercially available rodent pellet bait formulations (Pestoff® 20R and Final®) and a new pellet bait formulated specifically for mouse control (Pestoff® 20M) to house mice on Gough Island. We also tested the efficacy of toxic Pestoff 20R and Final pellets containing 20 and 25ppm of the anticoagulant toxin brodifacoum. Methods Five trials with 50 mice housed in individual cages and kept at ambient temperature and light cycles, were undertaken during the year. Palatability of bait pellets was measured over five days and compared with a non-toxic control food (standard 'rodent pellets' sourced from a pet food supplier). Toxic bait trials were run for 25 days with bait administered at 1, 2 and 10g for a 24-h period and at 20g per day for 72h. Key results All three baits were highly palatable; however, mice showed the greatest preference for Pestoff 20M and 20R, consuming a larger mass of bait. Estimated oral LD50 values of brodifacoum for Gough mice were 0.44mgkg-1 and the average time to death following exposure was 5.5 days (range 0-16 days). Two mice (~1% of those tested) survived after apparently ingesting doses of brodifacoum estimated to be 5 and 10 times the oral LD50 values, potentially indicating a lower susceptibility to brodifacoum in some individuals, although subsequent exposure at higher doses resulted in mortality. Conclusions The results of this study confirmed that house mice on Gough Island find bait pellets highly palatable and that brodifacoum is an effective toxin with LD50 values and time to death within the same range (0.4 to 0.52mgkg-1 and 5.2 days) as other studies, indicating no major difference in the susceptibility of Gough mice to this poison. Implications We recommend that bait manufacturers produce formulations designed to be attractive to mice and consider the use of higher concentrations of brodifacoum to increase the likelihood of all mice obtaining a toxic dose when small quantities of bait are consumed, although higher toxin concentrations must be balanced against the increased risks to non-target species. Eradication operations targeting mice should undertake more than one bait drop to ensure any individuals surviving the initial drop have access to sufficient toxic bait to cause mortality upon second or subsequent exposure.