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Great invader’s poison could also be its downfall

The rapacious invader has defied all efforts to stop its spread across tropical Australia. Flickr/blundershot

The cane toad’s highly toxic poison could ultimately prove the most effective weapon against the invasive species itself, according to a team of researchers who say the poison is an effective bait for luring and trapping its young.

In a series of experiments, scientists from the University of Sydney and University of Queensland planted the poison in traps set in water bodies to catch tens of thousands of toad tadpoles. “A chemical ‘bait’ created from the toads’ poison is a real magnet for toad tadpoles,” said Rick Shine, Professor of Evolutionary Biology at the University of Sydney’s School of Biological Sciences, and the lead author of the study, published in the Proceedings of Royal Society B today.

“This is the first powerful tool we’ve created to control cane toads.”

Cane toads (Rhinella marina) are spreading through tropical Australia with a devastating impact on native species. Predators that try to eat them are killed by the invader’s powerful poison; in some local populations of crocodiles, goannas and quolls, more than 95% of the animals are killed within a few months of its arrival, Professor Shine said.

The cane toad, native to Central and South America, arrived in Australia from Hawaii in June 1935, brought by the Bureau of Sugar Experiment Stations in a bid to control the native cane beetle. The species spread rapidly throughout Queensland, the Northern Territory and eventually into northern New South Wales.

Attempts to halt the march of the ugly amphibian have been thwarted by the rate at which it reproduces. A cane toad lays more than 30,000 eggs in a clutch - ten times as many eggs as native frogs.

“This means that even if you catch and kill 99% of the adult toads in an area, the few that are left can produce so many offspring that before you know it you are back to where you started – just as many cane toads as ever,” Professor Shine said.

The only way around the problem is to stop the toads from reproducing, Professor Shine explains.

“If we can do this, then removing adult toads can make a big difference because there are no new toads being born to replace the ones we are eliminating.”

The study has discovered that the secretion from the shoulder glands of dead toads can be used as bait. It is cheap and easy to obtain, and highly attractive to toad tadpoles - but repels the tadpoles of native frogs.

“This is perfect to use in funnel-traps in ponds to catch toad tadpoles. Other native fauna such as fish and insects aren’t attracted to this chemical but toad tadpoles are incredibly good at detecting it, and they search for its source as soon as they encounter it,” Professor Shine said.

“When we use this chemical as bait in a funnel-trap we catch thousands of toad tadpoles and almost nothing else. In one natural pond, we collected more than 40,000 toad tadpoles in less than a week. And I think we got them all - over the next few weeks, not a single toad emerged from that pond.”

The researchers hope to train people from community groups in the proper methods of collection. The toad’s poison is very dangerous to humans as well as many native species and pet dogs, so collecting it must be done carefully, by people who have received suitable training and protective equipment.

Ian Musgrave, a Senior Lecturer in Pharmacology at the University of Adelaide, explained that the technique developed by Professor Shine and his team was successful because cane toad tadpoles are cannibals.

“If all the cane toad eggs in a pond hatched, the tadpoles would be on short rations, as any pond will have a finite amount of food,” he said. “So it pays the early hatching tadpoles to hunt down and eat the cane toad eggs that have not yet hatched. Of course, cane toad eggs are resistant to the toxins that they secrete, so the toxin is not a deterrent to them.”

The cane toad tadpoles would hunt the eggs by following the chemical cues unique to the cane toad eggs - the very toxin that protects them from other predators.

“The hungry cane toad tadpoles seeking to consume their as-yet unhatched brethren swarm up into the traps and are caught,” he said. “Very clever. The current methods which are used to catch adults are very labour intensive and difficult to apply in wild terrains, but this method has the potential to be very efficient and easily portable for use in a variety of landscapes.”

But Associate Professor Mike Tyler, a cane toad expert from the University of Adelaide’s Department of Ecology and Evolutionary Biology, was sceptical: “The cane toad occupies 1 million square kilometres. So when people say, ‘Look we’ve got a new way of controlling it,’ I say, ‘Is it practical?’ And the answer to that is no.

"It’s an interesting paper that adds more knowledge to the 200 or so papers that have been written in this area - they just produce so many, you’ve no idea.

"But 1 million square kilometres - how are you going to knock off a frog by putting a few funnels around the edges?”

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