Scientists use toxin to lure cane toad tadpoles to surprising effect without harming other species.
The march of the cane toad (Rhinella marina) across tropical Australia started as the result of a misplaced attempt at biological control. These large American amphibians were introduced in 1935 as a means of controlling beetles in the sugar cane fields. Unfortunately, they found conditions very much to their liking, and started preying on Australia’s unique wildlife as well.
Scale of the Problem
Protected by their powerful bufotoxin, cane toads face no serious predators, apart from people. Those Australian native species that do attempt to prey on them, including reptiles such as goannas and crocodiles, can end up being killed instead. The situation is compounded by the reproductive potential of these toads, as a single female can lay over 30,000 eggs at a single spawning.
"This means that even if you catch and kill 99 percent 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," explains Professor Shine of the University of Sydney’s School of Biological Sciences. He has been spearheading the fight to find a way to control their population.
Working in conjunction with colleagues from the University of Queensland, Shine believes they have made a breakthrough – by utilizing the toad’s own toxin to control its numbers. "This is the first powerful tool we have created to control cane toads," he says.
The key revolves around preventing the toads from breeding successfully. "A chemical 'bait' created from the toads' own poison is a real magnet for toad tadpoles," explains Professor Shine. This is extracted from the parotid glands on each side of the head. It is cheap, as well as being freely available – and has the major advantage that it actually repels the tadpoles of native frogs, so they will not be harmed.
Developing the Technique
"This solution is perfect to use in funnel-traps in ponds to catch toad tadpoles. Other native fauna such as fishes 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 explains.
"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 cane toad’s toxin is potentially harmful to people too, so collections need to be carried out using appropriate clothing. "In continuing work with our collaborators at the University of Queensland we are developing an even stronger, safer, and easier-to-use bait," Professor Shine said. "To do this, we will isolate the active agent in the toads' secretion, and use it in pure form without all of the associated poisons."