Which toxin is best for eradicating rodents on islands?
Since the 1980s, rats and mice have been eradicated from nearly 600 islands around the world mostly by using anticoagulant toxins. These have delayed symptoms that do not allow rodents to learn to avoid eating sufficient bait to get a lethal dose. The baits used are generally cereal-based pellets or blocks and are either broadcast from the air using a helicopter and sowing bucket or laid by hand as broadcast, trail, bait piles or in covered bait stations.
Brodifacoum, a second-generation anticoagulant toxin, has been most commonly used (72.5% of 546 attempts) while diphacinone, a first-generation anticoagulant toxin, has been used in 9.1% of all attempts. Brodifacoum has an advantage over most other anticoagulant toxins because rodents can acquire a lethal dose at a single feeding. However, it has disadvantages in that it persists in the bodies of rodents and is equally toxic to non-target species such as birds, which are therefore at risk if they eat baits or poisoned rodents. Diphacinone has advantages in that it is not as persistent in the bodies of rodents and is less toxic to birds, but the potential disadvantage that rodents have to eat baits each day over several days to obtain a lethal dose, which might allow some individuals to survive and an eradication attempt to fail.
In 2009, an attempt was made to eradicate Polynesian rats from the 129-ha Lehua Island in Hawai'i, using aerially-sown diphacinone baits. The attempt failed as live rats were found on the island later in 2009. In 2010, John Parkes and Penny Fisher were asked to see if they could diagnose why the attempt failed.
One possibility was due to the problems inherent in the toxin used, so John and Penny looked at whether the failure rates of rodent eradication programmes around the world correlated with the use of one or other of these two toxins. Subsequently more attempts came to light (see Conservation Evidence 8: 100–106) allowing some conclusions to be made:
- Over all attempts against any species of rat or mouse and using any method of sowing bait, brodifacoum had a significantly lower failure rate of 17% (54 of 322 attempts) than diphacinone at 33% (13 of 39 attempts).
- When bait was broadcast from the air, the difference was even greater with only 8% of brodifacoum attempts (12 of 149) failing versus 83% (5 of 6) for diphacinone.
- However, when the baits were applied using various ground-based methods, the failure rates of the two toxins were the same (24% or 42 of 173 for brodifacoum and 24% or 8 of 33 for diphacinone).
John and Penny concluded that for topographically difficult islands where access on foot cannot be gained over all areas, managers are left with aerial baiting as the only option and, on this evidence, with brodifacoum as the toxin of choice. They note, however, that the rather small sample size for the aerial use of diphacinone gives rise to some caution about this conclusion. So where the risk of poisoning non-target species is unacceptable and cannot be mitigated, managers might still use aerially-sown diphacinone baits and accept a higher risk of failure.
The precautionary strategy of over-baiting, with high bait sowing rates or using additional sowings in aerial baiting, to ensure all rodents are placed at risk, increases the risk to non-target species and may not actually be necessary for brodifacoum baits However, a precautionary strategy of over-baiting and additional sowings may be desirable when diphacinone baits are used. Where ground-based methods are used to distribute the baits, either toxin seems to work well in achieving eradication. It is possible that precautionary over-baiting (replacing bait taken from bait piles or bait stations) does account for the equal success of diphacinone (relative to brodifacoum), i.e. over-baiting is an advantage for this toxin as it ensures the last rodent has access to bait at the end of its multiple-feed period.
Overall, John and Penny think some hard data on the spatial pattern of bait removal versus survival of rodents need to be collected, especially for aerial broadcast baiting, to see whether (1) lower bait densities and/or fewer sowing events are efficacious for brodifacoum and (2) more sowing events improve the success of diphacinone.
The review of the Lehua Island project was funded by the University of Hawai’i at Manoa, while the subsequent work to explore the past eradication attempts was funded under the auspices of Invasive Species International.