Using poisoned hen eggs for stoat control

Forest and Bird, Issue 281, 1 August 1996, Page 22

Using poisoned hen eggs for stoat control

Landcare Research, Lincoln

Eric Spurr

HE USE OF poisoned hen eggs is a new concept in stoat control. Poisoned eggs placed in specially designed bait stations killed more than 85 percent of the stoats attracted to the bait stations in three trials carried out by Landcare Research and the Department of Conservation in 1994 and 1995. Until recently, the department used hen eggs only as non-toxic baits to attract stoats to Fenn traps set in tunnels. Because both traps and tunnels are bulky and heavy to carry in the field, DoC contracted Landcare Research to develop a toxic bait for stoat control. Initially, I tested a variety of potential long-life baits, such as those used for controlling rats, cats, pigs and possums, but captive stoats wouldn't eat them. I then decided to test poisoned hen eggs as bait. In 1994, I set up a trial in Craigieburn Valley near Arthur’s Pass, using hen eggs injected with 1080. This quick-acting poison is used for possum and rabbit control, and kills stoats within a few hours. Bait stations were designed with entrances large enough to allow stoats to

enter but too small to allow them to remove the poisoned eggs. This forced stoats to eat the eggs inside the bait stations and prevented them leaving uneaten eggs available to non-target species outside the bait stations. Video recordings showed stoats, possums and kea approaching the bait stations, but only stoats were able to enter and eat the eggs. The numbers of stoats eating eggs were reduced by about 86 percent within three weeks of the poisoned eggs being placed in the bait stations. Because the use of 1080 is restricted, I tested the anticoagulant diphacinone the following year. Diphacinone is currently not used for pest control in New Zealand, but a similar anticoagulant, brodifacoum, is widely used under the trade name Talon for control of rats, mice and possums. Two trials were carried out with DoC’s Stephen Phillipson and Graeme Loh — one in the Hawdon Valley near Arthur’s Pass and the other in the Caples Valley west of Lake Wakatipu. Stoat numbers were reduced by 87 percent in the Hawdon and 85 percent

in the Caples within two to three weeks of baiting with diphacinone-poisoned eggs. Last summer, DoC used 1080 and diphacinone in research-by-management trials in the Hurunui, Hawdon, Landsborough, Dart and Catlins areas for the protection of mohua. Final results are not yet available, but the number of eggs eaten by stoats declined in all trials. In some trials, rats and mice also ate the poisoned eggs. The latter result is a potential advantage of the technique, because if both stoat and rodent populations are reduced at the same time this avoids the problem of one predator replacing another. Radio-tagged stoats have proved essential for checking the effectiveness of the trials. Stoats are caught in live traps and a miniature radio transmitter attached to each animal before release. Each stoat transmits its own distinct signal so their movements and fate within the poison area can be monitored. When animals die they can be found quickly and autopsied to determine their cause of death. While monitoring stoats this summer, DoC scientists Peter Dilks and Elaine Murphy found that the dose of poison that killed stoats during pen trials did not always kill wild stoats. They suspected something was amiss when radio-tracked stoats living in the poison grid did not die when large numbers of poisoned eggs were being eaten. One animal showed no effect after

eating an egg that had been injected with 1080 a week earlier, another became ill but quickly recovered after eating a freshly injected egg. Further trials with higher doses of 1080 found that all stoats died within a few hours of eating poisoned eggs. A disadvantage of diphacinone and other anticoagulants is that stoats do not die until seven to ten days after eating a lethal dose. During this time they may continue to prey on birds. Consequently, I am now testing an alternative toxin, cholecalciferol, that is quickeracting than the anticoagulants and doesn’t have the use restrictions of 1080. The concept of using poisoned hen eggs for stoat control is promising. Hen eggs are much easier to carry in the field than traps, and bait stations for poisoned hen eggs can be made much smaller and lighter than tunnels for traps. If large numbers of poisoned eggs (say five to ten) were placed in bait stations, they could be checked and replaced at one to two week intervals whereas traps must by law be checked every day. If poisoned eggs were used for longer than in the initial trials (two to three months rather than two to three weeks) even better results may be obtained. However, research still needs to continue to find an alternative bait that will attract stoats and will be readily eaten, can carry a poison and will remain toxic to stoats over a long period of time — a tall order. Stoats are intelligent and cunning, and selective about what they eat. They are also well adapted to survive in New Zealand forests and are able to quickly respond to increased food supplies with a rapid increase in numbers. A long-life bait would be a distinct advantage in the battle to reduce their population.