Review of the Lehua Island rat eradication project 2009

Abstract

The eradication of introduced mammals is a prerequisite for a larger program to restore biodiversity on 126-ha Lehua Island, Hawaii. Rabbits were eradicated by 2006 and an attempt was made to eradicate the remaining species, the Polynesian rat (Rattus exulans), in January 2009. Planning for the eradication of rats began in 2005 and covered legislative, regulatory, environmental risk assessment, operational and contingency response aspects of the project. Aerial application of rodenticide baits, the most commonly used (and sometimes only practical) method to eradicate rodents on large or topographically challenging islands, was chosen as the method for Lehua Island. Two rodenticide baits containing brodifacoum and one containing diphacinone rodenticides are registered for aerial application on islands in the USA. The particular history of rodent control on the larger islands, public sentiment, and the policy climate in Hawaii meant the use of diphacinone was favored for the Lehua project. The use of diphacinone has some major advantages over brodifacoum in eradicating island rodents (and for sustained control when eradication is not feasible) largely because its toxicity and environmental persistence confers lower hazard to non-target wildlife. However, diphacinone does not have such an established operational history as brodifacoum, particularly when baits are broadcast from the air. An analysis of 206 previous eradication attempts against five species of rodents on islands using brodifacoum or diphacinone is presented in an appendix to this report. For all methods, 19.6% of 184 attempts using brodifacoum failed, while 31.8% of 22 attempts using diphacinone failed. This difference is not statistically significant (Fisher’s Exact Test P = 0.26). The two toxins have similar failure rates for ground-based operations (29% for brodifacoum and 23% for diphacinone; Fisher’s Exact Test P = 0.77). The limited evidence suggests aerial baiting using brodifacoum has a lower failure rate (11% of 93 attempts) than for diphacinone (75% but of only 5 attempts) (Fisher’s Exact test P = 0.010). However, we caution against drawing firm conclusions about these differences because of the small sample size for the diphacinone attempts. The attempt to eradicate rats from Lehua Island was therefore a logical step in expanding the ‘track record’ for effective use of diphacinone in eradicating rodents. However, 7 months after two aerial applications of diphacinone bait, Polynesian rats were found on Lehua Island, indicating that the operation had failed – or that rats had reinvaded the island, or both. In early April 2010, the Research Corporation of the University of Hawaii on behalf of the Hawaii Department of Land and Natural Resources contracted Landcare Research to review the Lehua Island rat project. The senior author visited Hawaii between 17 and 21 April 2010 to discuss the project, and followed this with email and telephone dialogue with the aim of providing a draft report for comment by 13 May 2010. This draft report was revised following comments by internal referees and Hawaiian stakeholders.