They were supposed to save Australia’s sugarcane industry.

But cane toads, originally imported to control cane beetles, have steadily become one of the country’s biggest pest headaches.

An invasive species since 1935, cane toads are causing major environmental problems Down Under, where unhindered population growth has allowed the toads to continually spread, displacing and threatening native species.

Australian scientists and environmentalists have tried and failed with traditional control techniques, and are left asking, “What now?”

Native to Venezuela, cane toads were known to feed on cane beetles that were devastating Australian sugarcane crops in the early 20th century. Using cane toads to control cane beetles seemed to be an easy, natural solution, and in 1935, 102 cane toads were imported from Hawaii. But the beetles easily avoided the toads: the insects prefer the sugarcane plant tops, out of toad reach.

Now, in northeast Australia, the toads are everywhere — except sugarcane fields. Cane toads prefer wet and marshy areas to sugarcane fields and are known to hop fearlessly into human-inhabited areas.

The average adult cane toad is 10 to 15 centimeters long, but can grow up to 23 centimeters — large enough to eat smaller native Australian toads and frogs, even mice.

Toxic Toad

Cane toads have two poison-filled glands behind the ears. When sufficiently squeezed, the glands release the poison bufotoxin. The toads are considered safe for humans to handle because it takes significant pressure to release the poison.

Bufotoxin, however, is lethal to cane toad predators. Snakes and small mammals such as quolls are the most common predators. Cane toad eggs and tadpoles are also extremely poisonous, endangering smaller animals such as toads and frogs that eat them.

Quolls are small mammals, about five to 13 centimeters long and less than two pounds. Their diet includes reptiles, worms, termites, fruit, other small mammals and cane toads.

The northern quoll is facing localized extinction in Kakadu National Park in Australia’s Northern Territory from cane toad poisoning.

Native frogs and toads are also threatened because cane toads out compete them for food and habitat. Cane toads multiply fast: most native Australian frogs and toads lay hundreds or a few thousand eggs per spawn, whereas one cane toad lays about 20,000 eggs. Cane toad eggs also hatch more quickly than the eggs of native species. According to the Commonwealth Scientific and Industrial Research Organization, cane toads are the most common small invertebrate in eastern Queensland, spreading at a rate of 27 kilometers a year.

They Just Won’t Go Away

Australia is no stranger to invasive species. The rabbit-proof fence is one of Australia’s most famous methods of animal control: Australians built fences across the entire island continent to prevent rabbits (an invasive species) from spreading. The fences, however, didn’t prevent people from moving rabbits to new areas. Rabbits now inhabit most of Australia.

Hoofed animals also pose uniquely Australian problems. Because Australia has no native hoofed animals, those that were introduced — such as water buffalos, deer, pigs and goats — trample softer native soils that provide food and habitat for native species. To reduce the impact of hoofed invasives, the government has offered bounties on some hoofed animals and culled them with helicopter snipers. Pigs, however, have been particularly difficult to control in densely forested areas, such as Daintree National Rainforest Park, in northeast Australia.

But cane toads present different challenges. Constantly increasing populations make bounty hunting or mass killing unrealistic, although both have been tried. Researchers found that the bounty hunting was actually compounding the problem; people were also killing native species along with the cane toads, out of mistaken identity or to collect more bounty money. Bounties also grew with toad populations into a costly government expense.

Trapping cane toads has also been attempted, with little success. The problem with traps is they have little impact on overall cane toad populations. The traps are mainly used in localized areas to reduce smaller populations, but these methods cannot solve the growing national cane toad problem.

Island Arks

“[Because of their rapid breeding] even if you were to wipe out 90 percent of the population, the potential for recovery is so high they really can’t all be wiped out,” said Keith Saalfeld, invasive species management officer of the Northern Territory Parks and Wildlife Commission.

The inability to control cane toad populations is forcing experts such as Saalfeld to pursue alternative strategies. Instead of trying to eradicate the invasive species, a program called Island Arks is relocating endangered native species to offshore islands free of cane toads. The focus of the program is the northern quoll.

Voracious eaters, quolls unwittingly turn cane toads into their last meal because quolls have not learned to avoid the poisonous prey.

“Quolls are pretty dumb… they’ll go for it again,” Saalfeld said. “If a quoll grabs a cane toad and tries to feed on it, it’s dead.”

The Island Arks relocation program transports quolls to islands off the coast of the Northern Territory. Because it is becoming nearly impossible for quolls to survive in areas inhabited by cane toads, the goal is to establish a stable quoll island population that will remain safe from the threat of extinction.

The program is two years old and Saalfeld calls it a success. Since Island Arks began, the relocated quoll island population has increased and almost all of the females have given birth.

Officials are considering expanding the program to help other endangered species. The oenpelli python, a rare snake only found in the Northern Territory and threatened by cane toads, may be the next Island Arks resident. But first it must be established that the pythons would not harm their new environment.

Critics question whether introducing quolls to islands could create another potential invasive disaster, because the quolls could displace native island inhabitants. Saalfeld dismisses the idea, noting a theory that quolls once existed on the islands before burning regimes killed them off. “It’s highly unlikely that the quolls will have any effect on the overall island.”

It is Saalfeld’s job to assure island biosecurity by preventing harmful species, especially cane toads, from invading the islands. Through community education, he encourages people to take great caution not to accidentally transfer cane toads when traveling to the islands. Saalfeld talks with groups such as students to tell them about the cane toad problem, and how to prevent it from increasing.

People traveling to the islands are only one concern, however. Barges carrying goods to the islands can transport toads accidentally because toads live where barges are loaded. The Northern Territory Parks and Wildlife Commission proposes amending wildlife laws to designate the Island Arks as “feral animal exclusion areas.” This would make it illegal to transport any non-native species to the islands, and allow officials to prosecute offending barge owners.

Biological Short Circuit

While Saalfeld calls Island Arks successful, he said he believes biological control is the only way to solve the cane toad problem.

That is precisely what the Commonwealth Scientific and Industrial Research Organization is researching. CSIRO scientists are working to find a gene that, when turned off, will stop cane toads from developing beyond the tadpole stage. If metamorphosis is stopped before cane tadpoles become toads, they will not be able to reproduce.

It sounds beautifully simple, but there are dangers. To alter or turn off the specific gene, scientists would release a virus into cane toad habitats. But first the virus and the gene it targets must be proven specific only to cane toads. If it isn’t, native species could also be infected and wiped out.

Although biological control is touted as the only permanent solution to the toad problem, it is still in a research stage and years from having any possible impact.

A Changing Ecosystem

For nearly 70 years, the cane toad invasion has been expanding, and the Australian environment is beginning to show signs of adaptation.

Crows have figured out how to safely prey on the toads by first killing, then flipping the toads over to avoid the poison glands. Most pythons — with the exception of oenpelli — are also less vulnerable to cane toads. Exactly how this works is unknown, but one theory is that when pythons constrict the toads, bufotoxin is released into the air where it loses toxicity, making the toads safe to eat.

But species that haven’t adapted to the cane toads may not survive without human management. Some native toads and the northern quoll could pay the ultimate price — extinction — if the Australian ecosystem is left alone to balance a problem that started with human intervention.

Cane toads do make a few welcome public appearances Down Under. Johnos, a pub in Cairns, Queensland holds weekly cane toad races. Souvenir toads — some stuffed to pose with toad-sized bottles of whiskey, others shaped into wallets and purses with glued on plastic eyes — fetch tourist money.

But while pesticides now control cane beetles and sugarcane once again flourishes throughout Queensland, Keith Saalfeld said he believes the cane toad is likely in Australia to stay.

“The bottom line is that cane toads will become a part of the natural environment.”