If you live in Queensland, you know the scene: a warm, wet summer night, the porch light on, and a fat, warty cane toad squatting on the path like it owns the place. They’re so much a part of life up here that it’s easy to forget they don’t belong. Australia’s cane toads all descend from a single shipment of 102 toads brought over in 1935.
So how did cane toads get to Australia, and how did a plan to save the sugar industry become what Australian Associated Press (AAP) calls one of the worst examples of biological control gone wrong?
The beetle problem
In the early 1900s, Queensland’s sugar cane growers had a serious pest. The grubs of native cane beetles, especially the greyback cane beetle, were eating the roots of the cane. Entomologists at the state’s Bureau of Sugar Experiment Stations had been searching for a solution for years, as the national pest management program PestSmart explains. At the time, there were few chemical options, and biological control, using one living thing to control another, was the cutting edge of pest management.
The cane toad looked like the perfect candidate. It’s native to Central and South America, and in the decades before 1935, it had been carried to Puerto Rico and then Hawaii to eat pests in sugar cane fields there, with reports of great success. According to a team of historians writing in The Conversation, government documents show the idea of bringing it to Australia had broad support from scientists and officials alike. On paper, it looked like a safe bet.
102 toads in two suitcases
In June 1935, entomologist Reginald Mungomery of the Bureau of Sugar Experiment Stations travelled to Hawaii and caught 102 toads, 51 males and 51 females. He packed them into two small cases with moist wood shavings and brought them back by ship, according to AAP’s short history, published in The Canberra Times.
When he arrived at the bureau’s Meringa experimental farm near Gordonvale, south of Cairns, on 22 June 1935, all but one toad had survived the journey, PestSmart records. They were put into a purpose-built enclosure and began breeding almost straight away.
In August 1935, about 2,400 young toads bred from that small group were released into the Little Mulgrave River and other sites around Gordonvale. Not everyone was convinced. Some scientists warned that the toads hadn’t been tested and could harm native wildlife, and in December 1935, the federal government temporarily banned further releases. But the ban didn’t last, and toads were soon released throughout Queensland’s sugar-growing districts.
Why it didn’t work
The toads didn’t control the beetles. In fact, there’s no evidence that anyone tested whether they would even eat Australia’s cane beetles before they were released. The beetles in Queensland were different species from those causing problems in Hawaii and Puerto Rico, and their habits kept them out of the toads’ way.
Adult greyback beetles spent their time up on the cane stalks, rarely touching the ground where the toads hunted, and French’s cane beetle was active at times when the toads weren’t, as EBSCO’s Research Starters summary explains. EBSCO stands for the Elton B. Stephens Company, a research database provider. The toads simply ate whatever else they could find, and there was a lot of it.
Why cane toads were so hard to stop
Cane toads had almost everything going for them. A female can lay two clutches of up to 35,000 eggs a year, AAP reports, and the toads are toxic at every stage of life, from egg and tadpole to adult, so few Australian animals can eat them safely. They’ll eat almost anything that fits in their mouth, they can live in a huge range of habitats, and in Australia they had no natural predators adapted to their poison. Adults can grow surprisingly large, weighing up to around 2 kg.
The march across the north
From those first releases around Gordonvale, cane toads spread along the Queensland coast, then inland and across the north. They also pushed south into northern New South Wales. At first they moved slowly. A 2006 study in Nature by Ben Phillips, Richard Shine and colleagues at the University of Sydney found they expanded their range by about 10 km a year between the 1940s and 1960s, but were by then invading new areas at more than 50 km a year.
By 2006, the toads already occupied more than a million square kilometres of tropical and subtropical Australia. They crossed into Western Australia in the late 2000s, and have since pushed deep into the Kimberley.
The toads evolved to go faster
Why did they speed up so much? The answer turned out to be evolution in action, happening fast enough for scientists to measure. In their 2006 study, Phillips and Shine radio-tracked toads at the invasion front, 60 km east of Darwin, and found some moved up to 1.8 km in a single night during the wet season.
They also found that the toads arriving first in new areas had longer legs than those in long-established populations, and that longer-legged toads travelled further. The logic is simple: at the front of an invasion, the fastest travellers arrive first and breed with each other, so each generation at the front tends to be a little faster than the last.
Later research found more changes. A 2014 study by Gregory Brown, Phillips and Shine found that the offspring of invasion-front toads travel in straighter lines than those from older populations. And 3D scans by Shine’s team and Smithsonian researchers found that invasion-front toads have narrower pelvises, slimmer skulls and longer, stronger forearms, which may help them make many short hops and cushion their landings. In less than a century, Australia has effectively bred a faster, longer-distance cane toad.
The damage to native wildlife
The biggest damage cane toads do comes from the animals that try to eat them. When threatened, a toad releases a toxin from large glands behind its head, and Australian predators, which never evolved alongside toads, have no idea it’s dangerous.
It took until 1975, 40 years after the release, for the first survey of the toads’ impact on Australian wildlife to be published, by Mike Archer and Jeanette Covacevich of the Queensland Museum, The Conversation notes. Since then, the damage has become clear. Northern quolls, goannas, some snakes and freshwater crocodiles have all been hit hard as the toads arrived. The northern quoll, a small, spotted carnivorous marsupial, is now endangered, with toads a major cause.
There are glimmers of adaptation. Research by Phillips and Shine found that some Australian snakes in toad-infested areas have evolved smaller heads, making them physically unable to swallow a toad big enough to kill them. And while toads have devastated local populations, there are no documented extinctions of a whole species attributed to them so far.
Teaching animals to say no
There’s no known way to stop cane toads spreading, so much of the effort has gone into helping native animals survive their arrival. One approach uses conditioned taste aversion: if an animal eats something that makes it sick but doesn’t kill it, it learns to avoid that food in future.
In the Kimberley, scientists have made sausages from minced toad meat, with the most toxic parts removed and a nausea-inducing chemical added, and dropped them in quoll habitat ahead of the toad invasion. In trials, quolls that ate a single sausage showed much less interest in attacking real toads, according to research published on bioRxiv.
Results in the wild have been mixed. The Australian Wildlife Conservancy found the sausages induced an aversion in an estimated 24% to 42% of quolls, but failed to protect the quoll population at its Mornington sanctuary when the toads arrived. Researchers are still refining the approach.
Goannas wouldn’t eat the sausages, so researchers tried a different trick. In the East Kimberley, University of Sydney researcher Georgia Ward-Fear and Balanggarra Indigenous rangers released small “teacher toads” ahead of the main invasion. These were young toads with enough toxin to make a goanna sick but not enough to kill it. The idea is that a goanna made sick by a small toad will leave the far more toxic adults alone when they arrive. The rangers’ knowledge of goannas, which are an important part of Aboriginal culture in the region, was vital to the study’s success.
Cane toads and your pets
This part of the story got very real for me recently. We have a new puppy, and he’s in the middle of toilet training, which means a trip outside for a wee last thing every night, exactly when the toads come out. All he wants to do is chase them. One hop across the lawn and he’s off, nose down, tail up, convinced it’s the best game in the world. So these days, the bedtime toilet run comes with a torch and a lead.
That worry is well founded. The Royal Society for the Prevention of Cruelty to Animals (RSPCA) Queensland warns that an adult cane toad carries enough toxin to kill an average-sized dog in about 15 minutes, according to its guide to cane toads. A puppy is smaller than an average dog, so the same amount of toxin goes further, and puppies are curious about everything and tend to grab first and think later.
The toxin sticks to the gums and tongue when a dog licks or bites a toad. Signs include heavy drooling or frothing at the mouth, very red and slimy gums, and pawing at the mouth. More serious cases can involve vomiting, wobbliness, a racing heart or seizures, as Greencross Vets explains.
If your dog or puppy has mouthed a cane toad, take a damp cloth and wipe the gums, tongue and roof of the mouth, rinsing the cloth after each wipe, for 10 to 15 minutes, then call your vet. Don’t point a hose into your dog’s mouth, because it can force water into the lungs, the Animal Emergency Service warns.
If your dog has eaten a toad, has more than mild drooling, is unconscious or is having a seizure, don’t wait: take it straight to an emergency vet and call on the way. With a puppy, I wouldn’t wait to see how bad it gets. The vet is the safe call.
Night-time toilet trips: what has worked for us
Toads are mostly active at night, especially on warm, humid evenings after rain, so the late-night toilet trip is the riskiest moment of the day for a dog in Queensland. A few habits make a big difference:
- Use a lead, even in your own yard. It’s the only reliable way to stop a puppy lunging at a toad before you’ve seen it.
- Take a torch and check first. Do a quick sweep of the lawn and paths before the puppy goes out, because toads are easy to spot in torchlight.
- Think about your outside lights. Porch and garden lights attract insects, and insects attract toads, so the spot right under the light is often where they gather.
- Pick a toilet spot and stick to it. Taking the puppy to the same small patch every time means you only have one area to check, and it helps with toilet training too.
- Bring water and food bowls inside overnight. Toads like to sit in them and can leave toxin behind.
- Don’t let them practise the habit. Every successful chase makes the game more exciting, so call the puppy away and reward it with a treat for coming back.
In south-east Queensland, toads are busiest in the warmer months, but in tropical north Queensland they’re around all year, according to Greencross Vets, so the night-time routine is worth keeping up whatever the season.
What to do with a cane toad in your yard
First, make sure it really is a cane toad, because many native frogs are mistaken for them, and the Queensland Museum has a helpful identification guide. Look for the large, swollen glands behind the head, the bony ridges over the eyes, and dry, warty skin.
If you do have toads, RSPCA guidance is to kill them humanely, not to hit them, run them over or use household chemicals. The RSPCA recommends a spray containing the chemical eugenol, sold as products such as Croaked, which sedates the toad and then renders it unconscious within minutes, leading to death. Wear gloves, wash your hands afterwards, and avoid touching your eyes.
Are cane toads dangerous to people?
For adults, cane toads are mostly an unpleasant nuisance rather than a serious danger, as long as you don’t handle them carelessly. The toxin from their glands can irritate the skin and is painful if it gets in your eyes or mouth, which is why gloves and hand-washing matter. The bigger risk is to young children, who might pick up or mouth a toad, and to anyone who swallows the toxin.
If a child has put a toad or its toxin in their mouth, or anyone becomes unwell after contact, call the Poisons Information Centre on 13 11 26 straight away, or 000 in an emergency.
A cautionary tale that still matters
The cane toad story isn’t really a cautionary tale about a villainous scientist. Reginald Mungomery was doing what experts of the time thought was sensible, with the backing of his government and much of the scientific community. What went wrong was the lack of testing, and the confidence that a solution from one part of the world would work in another.
Today, proposals to release a new biological control agent in Australia go through extensive testing and approval before anything is let go, and PestSmart notes that the lessons of the cane toad likely shaped those rules. Meanwhile, more than 90 years after that first release at Gordonvale, the toads are still pushing west through the Kimberley.
If your pet has had contact with a cane toad, contact your vet or nearest emergency vet straight away.
If you’re a pet owner, my post on why chocolate is bad for dogs covers another common household poison, and pizzly bears is another story of what happens when species meet where they shouldn’t.
Sources
- PestSmart: How did the cane toad arrive in Australia?
- AAP, via The Canberra Times: Cane toads in Australia, a short history
- The Conversation: Everyone agreed cane toads would be a winner for Australia
- EBSCO Research Starters: Cane toads in Australia
- Phillips et al: Invasion and the evolution of speed in toads, Nature (2006)
- bioRxiv: Toad sausage trials with northern quolls
- Australian Wildlife Conservancy: Innovative approach required as cane toads arrive in the Kimberley
- RSPCA Queensland: Cane toads, an introduced menace
- Greencross Vets: Cane toad poisoning in dogs
- Animal Emergency Service: What to do if your pet licks a toad