Cane toads, like most critters in the Australian wilds, are hungry creatures. But if it were not for their hearty appetite for bugs, the toads — native to the South American jungles — may never have arrived Down Under. In 1935, an entomologist named Reginald Mungomery wanted to solve a problem plaguing Queensland sugarcane farmers: greyback and Frenchi cane beetles were devouring the sugar crops. Another biologist had solved a similar bug problem with an introduction of cane toads just a few years earlier on the Hawaiian island of Oahu, Mungomery knew, so he had a shipment of 102 toads sent from across the Pacific.
Mungomery could not have predicted it then, but the entomologist was about to set off an environmental powder keg. Eighty years later, ecologists are still trying to contain the damage, turning to strange and novel anti-toad techniques.
The first batch of animals — meant to be 52 male and 52 female toads, though a single male died en route — immediately took to northeastern Australia. Mungomery’s critters flourished. Within a few months of their arrival, the cane toads spawned thousands of offspring. What the amphibians did not take to, however, were the pest beetles.
“This experiment failed,” wrote Rob Capon, a professor at the University of Queensland’s Institute for Molecular Bioscience, in an email to The Washington Post. “The toads could not jump high enough to reach the beetles, which live in the upper stalks of cane plants.”
As the toads left the beetles well enough alone, the native Australian predators either ignored or could not stomach the alien frogs. From glands behind their ear holes, cane toads secrete a chemical defense — a milky cardiotoxin harmful to humans, dogs, crocodiles and other animals. The chemical is particularly threatening to quolls, rare marsupials that look a bit like Bambi crossed with a possum. (Australian authorities are now trying to train wild quolls not to bite the toads, including one method involving cane toad sausage spiked with a mild nausea-inducing agent. The sausage is dropped from helicopters by the hundreds, and when quolls nibble on the minced cane toad meat they learn to associate the toads with queasiness.)
Animals that typically eat amphibians are the hardest hit. The toads are “absolutely catastrophic for large predators,” University of Sydney cane toad expert Rick Shine told Nature in November. “We get some figures like 95 percent mortality within the first two months of toad’s arrival.”
Today, the cane toads are described as though the animals are an invading army, with a frog “front” spreading like a stain across the northern Australian coastline. Decades ago, the toads advanced slowly, at a rate of about six miles annually from their Queensland epicenter. But the animals on the front lines are evolving, scientists say, into faster, leggier, more aggressive frogs.
The toads currently move some 25 to 35 miles per year westward. (There are occasional reports of toads having crossed the continent, but those are thought to be vehicular hitchhikers rather than hopping of their own volition.) Estimates put the total toad population at about 1.5 billion, Capon said.
The behavioral difference between a toad from Queensland’s ground zero and a stormtrooper toad at the Australian front is striking. Put a Queensland toad in a cage, and it will lounge until prey is nearby. “But when you get the toads from the invasion front, the poor buggers are bashing their noses against the wall” of the cage, Shine told the New Scientist in a 2014 interview. “They want to get going.”
So how do you stop the tide of killer supertoads? By fighting them with baby-toad-eating ants, perhaps, or by creating a market of toad skins stitched into purses and golf accessories. (Locals are encouraged to freeze the animals rather than smash them with hammers, because, as The Post reported last May, bashing is inhumane and can shoot out the toxin in harmful sprays.) Or, if you are Capon, an expert in the chemical defenses of toads, you turn the animal’s toxin against itself.
Here’s how it works: Capon and his colleagues wring the poison from adult cane toads, coat the chemical onto air stones for “controlled release,” as the biologist wrote, place the stones into plastic boxes and the boxes into breeding areas. The toxin contains a chemical that acts like a dinner bell to the cane toad tadpoles, which happen to be cannibals. After one clutch of eggs hatches, the tadpoles feed on the unhatched eggs from other toad mothers. Smelling food, into the plastic boxes the hungry tadpoles swim.
Using the boxes, which have a narrow, funneled entryway, Australian biologists have successfully lured in and captured cannibal tadpoles in droves. “In 2012, our collaborators tested the bait concept in natural waterbodies and caught more than 42,000 cane toad tadpoles in 48 hours,” Capon told The Post.
On Monday, Capon told the Brisbane Times a more recent two-week trial removed 50,000 larval toads from a botanical garden at Brisbane’s tallest mountain — a significant reduction in the local toad population, he said. (Though a single female toad can lay up to 30,000 eggs, not all will survive to hatch as tadpoles.) The tadpoles are then killed in what sounds like a scene out of froggy “Goodfellas”: dumped in the trash or a “shallow grave,” according to the Times. The chemical attractant does not harm other amphibian species, Capon said, particularly when compared with other methods of fighting cane toad tadpoles — salting waterways or blasting them out of ponds with leaf blowers.
The results are promising, but Capon told The Post victory lies at the end of a “long journey.” The project does not have the funding to mass-produce traps for a continental anti-toad effort, and the biologists are seeking industry donations and citizen scientist support to move forward. Capon is also accepting donated toads to create the tadpole bait.
“Cane toads are extremely unpopular in Queensland, with every resident having experienced encounters with the pests,” Capon said. (Finding a toad in your bag of supermarket lettuce, he noted, is not a pleasant experience.) “We believe we can unite Australians in their shared disdain for these animals and make for a very successful citizen science campaign.”