An effective new weapon in the fight against the spread of cane toads has been developed by researchers at the University of Sydney, in collaboration with colleagues at the University of Queensland.
Cane toads – Rhinella marina – were introduced into cane farms in Queensland in 1935 in an attempt to kill off native cane beetles, which attack the cane plants. The first toads were brought from Hawaii by the Bureau of Sugar Experiments but appear to have had no impact on the beetles.
Instead, the toads rapidly multiplied and now number more than 200 million while spreading south and west across the continent at an average of 40 kilometres a year, with devastating impacts on native species.
Predators that try to eat the toads are killed by the invaders' powerful poisons: in some places, more than 95% of the populations of crocodiles, goannas and quolls have been wiped out.
Despite years of efforts to prevent the spread and to attack the toad, nothing has proved successful. The biggest obstacle is that a single clutch – the number of eggs laid at a time by one female – can contain more than 30,000 eggs. So, even if 99% of the adult toads in an area are killed, the few left can produce so many offspring that the same number of toads, or more, reappear.
Scientists realised that the only way around the problem was to stop the toads from reproducing.
The research at Sydney found that the same powerful poisons cane toads use to kill native species could be developed as a weapon against the toads themselves – by using the poison as ‘bait’ in traps set in pools of water to catch their tadpoles.
Secretion from the shoulder glands of dead toads was cheap, easy to obtain and highly attractive to toad tadpoles while repelling the tadpoles of native frogs.
“This is the first powerful tool we have created to control cane toads,” says Professor Rick Shine, from Sydney’s school of biological sciences, and the lead author of the study. “A chemical ‘bait’ created from the toads’ poison is a real magnet for toad tadpoles. This is perfect to use in funnel-traps in ponds to catch toad tadpoles.”
Shine says other native fauna such as fish and insects aren’t attracted to the chemical but toad tadpoles are incredibly good at detecting it and they search for its source as soon as they encounter it.
“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 following few weeks, not a single toad emerged from that pond.”
The researchers hope to train people from local ‘toad-busting’ community groups in the proper methods of collection. Until then, even a dead toad inside a funnel-trap can serve as an effective ‘bait’ for toad tadpoles, without requiring any risky squeezing of poison glands.
“In continuing work with our collaborators at the University of Queensland we are developing an even stronger, safer, and easier-to-use bait,” Shine says. “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.”
A report on the research was published last month in the Proceedings of the Royal Society B: Biological sciences.
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