Freeze-dried rat and chemical rat smell are two potential new weapons in the fight again stoats, one of New Zealand’s fiercest and most destructive predators of native birds.

In a project commissioned by New Zealand’s conservation agency, the Department of Conservation, staff at Landcare Research set out to devise new, long-lasting and attractive lures for stoats. At present hens’ eggs are the main lure used, but only last for a few weeks.

Landcare Research’s study showed that stoats are more attracted by the smell of dead prey such as birds and rats. However, the problem with this is that dead prey decay rapidly. Researchers therefore aimed to encapsulate the smell of rats within a stable substance – and to keep the door “smelly” long-term.

Landcare Research ecologist Dr Andrea Byrom says four substances were trialed for use in new baits: cereal, gel, casein, and PVC.

“We incorporated ground up, freeze-dried rats into the substances, and placed them in trap-like tunnels in the open air. We checked them for intactness and odour weekly for 20 weeks.

“The PVC lures remained fully intact, while the others broke down. The PVC lures were formed in test tubes and looked like meat sausages – albeit with rat hair in them!”

Researchers then tried the PVC “sausage” with a “chemical rat” flavor. “We identified key components in the odour of live ship rats, and mimicked them with similar-smelling synthetic chemicals. We then incorporated these chemicals into PVC lures,” Dr Byrom says.

The researchers put tunnels in the pens of 18 captive stoats and monitored the stoats’ interest in tunnels containing either the freeze-dried rat PVC lure, the ‘chemical rat’ PVC lure, fresh dead rat, and no lure at all. “The results of this trial were also encouraging. We found that more stoats entered tunnels containing the freeze-dried rat lure than tunnels with no lure. Also, the first tunnel that stoats entered was equally likely to have the freeze-dried rat lure, the chemical rat lure, or fresh dead rat. In other words, the chemical rat lure was at least as attractive as fresh dead rat,” says Dr Byrom.

The attractiveness of these lures now needs to be tested in field trials in wild stoat habitat. Landcare Research also plans to investigate the potential of incorporating other prey odors such as rabbit into PVC lures to attract other predators such as ferrets, weasels and cats.

Department of Conservation Stoat Control Research programmed manager Dr Elaine Murphy is pleased with the promising developments. “Having a new tool for the toolbox would be great, and the new lures would be a lot less trouble to carry around in the bush than hen eggs,” Dr Murphy says.

Stoats were brought to New Zealand in the 1880s to control rabbits, but have wreaked havoc on many native bird species. Now, cost-effective ongoing control of stoats is crucial for maintaining viable populations of native birds including kiwi.

However, stoats are elusive and difficult to poison or trap. Improving the attractiveness and longevity of lures for traps and bait stations is one key to boosting the success of stoat control.

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