The Cane Toad Catastrophe: How 101 Toads Became Millions

In 1935, a handful of Queensland scientists had a pest problem, a theory, and a crate of 101 toads fresh off a boat from Hawaii. The problem was beetles eating the sugar cane. The theory was that a big, warty South American toad might eat the beetles instead. Nobody actually checked whether the toad could reach the beetle before letting all 101 of them loose. Spoiler: it couldn't.

Ninety years on, we're still living with that one unchecked assumption. A crate of just over a hundred toads is now a population so enormous that nobody can even agree on the number — estimates run from the low hundreds of millions into the billions. This is the story of Australia's favourite ecological own-goal: how it happened, why it went so spectacularly wrong, and where the toads are hopping to next.

The Beetle That Started It All

Before cane toads became public enemy number one, that honour belonged to the greyback cane beetle. As grubs, they chewed through sugar cane roots underground; as adults, they stripped the leaves. For a Queensland sugar industry that entire towns depended on, from Cairns to Bundaberg, a bad beetle year meant genuinely frightening losses.

By the early 1930s, the industry's own research body was under pressure to find a fix that wasn't an expensive chemical spray. "Biological control" — using one species to wipe out another — was the hot new idea of the era, and word had filtered through from Puerto Rico and Hawaii that a certain toad seemed to be doing the trick there.

Enter the Cane Toad

A cane toad (Rhinella marina) — harmless-looking, genuinely toxic, and now numbering in the hundreds of millions across Australia. Image: Wikimedia Commons (public domain).

The toad in question, Rhinella marina, is a large, warty customer native to Central and South America, already introduced to Puerto Rico in the 1920s and, from there, Hawaii, where a glowing 1932 report credited it with knocking down beetle numbers in the cane fields. Convinced, Bureau entomologist Reginald Mungomery sailed to Hawaii and came home with a crate of live toads — widely recorded as 101, though one or two sources say 102, with one dying en route. A strikingly small cast for what was about to become a continent-sized disaster.

Bred up at the Meringa Sugar Experiment Station near Gordonvale, just south of Cairns, their offspring were released into cane fields around Gordonvale, Edmonton and Innisfail by August 1935. Within a couple of years, tens of thousands more had followed. The toads had officially arrived, and they had absolutely no intention of leaving.

The Bit Nobody Checked

Here's the part that makes ecologists wince nearly a century later: nobody seems to have confirmed that a ground-dwelling toad could actually catch a beetle that spends its life either underground as a grub or up on the cane stalk as an adult. Cane toads simply can't jump that high, and they can't dig. As pest control, they were useless from day one — the beetle problem eventually got solved by the chemical sprays everyone was trying to avoid in the first place.

What the toads turned out to be extremely good at was everything else. They carry bufotoxin, a potent poison in glands behind their eyes, at every life stage from egg to adult. Predators in their native range had millions of years to learn to avoid it. Australian quolls, goannas, freshwater crocodiles and snakes had no such memory, and many died within minutes of trying to eat one the way they'd always eaten frogs. Populations of animals like the yellow-spotted monitor have reportedly dropped by up to 90% in areas the toads move through. Add a female laying up to 30,000 eggs in a single clutch, zero natural predators, and a warm, wet climate that suited them down to the ground, and you have a population explosion that nobody could put the brakes on.

Where They've Got To Since

From that one release point near Cairns, cane toads have marched across virtually the entire east coast of Queensland, into the Northern Territory (Kakadu included), and over the border into Western Australia and the Kimberley. Nobody agrees on exactly how many there are now — estimates swing from roughly 200 million to more than two billion, depending who's counting — but everyone agrees the number only moves one direction. Stray toads have even turned up as far south as Dural on Sydney's outskirts, almost certainly hitching a ride in a car, a plant pot or a load of building supplies rather than hopping the whole way themselves.

They're getting faster, too. Research led by biologist Rick Shine found toads at the very front of the invasion have evolved noticeably longer legs than the ones further back, letting the leading edge travel about five times faster than the original wave did — evolution quietly building its own ultramarathon team, whether anyone asked for it or not.

The fightback is a genuinely charming mix of backyard grit and actual science. Community group Watergum runs an annual "Great Cane Toad Bust," with volunteers across four states heading out at night to humanely remove toads before they can breed. Researchers have trialled pheromone tadpole traps and "taste aversion" training, using small, non-lethal toads to teach native predators to leave the real thing alone. On the more sci-fi end, the same company chasing thylacine de-extinction announced a 2024 project to gene-edit endangered northern quolls so they can survive eating a toad. Meanwhile some animals are figuring it out on their own: certain crows and water rats have learned to flip toads over and eat only the non-toxic bits, and some snakes have evolved smaller heads, making it physically harder to swallow one large enough to kill them.

It's one of the most Australian stories going, honestly: a genuinely good intention, zero testing, and a problem now measured in the billions. The cane toad never did save the sugar industry. What it did do was become a permanent, warty reminder that "it worked somewhere else" is not the same thing as "it'll work here." Next time someone pitches you a fix with total confidence and absolutely no evidence, you now know exactly which toad to bring up.

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