When the enemy reached Australia's largest state last year, the Kimberley Toad Busters knew the battle was on. But they didn't expect that officialdom might strip them of their most effective weapon.

The enemy? The cane toad. The weapon? Plastic bags full of carbon dioxide _ long considered the animal-friendly alternative to whacking the creatures with golf clubs or cricket bats.

But Western Australia's Department of Environment and Conservation isn't so sure that euthanizing Bufo marinus with carbon dioxide is the kindest way to go, and says further tests are needed.

Should the tests prove the toads are suffering, the carbon dioxide option could be banned across Western Australia. And that, the Toad Busters fear, would make the war against cane toads virtually unwinnable.

Keep on whacking them instead, says the government. But to many, that makes no sense.

"Oh my lord, what are they saying?" cried Lisa Ahrens, a veteran toad fighter. "That's going right back to giving people a golf stick and telling them to go forth and conquer!"

This all may sound like a simple matter of bureaucracy and humane pest control, but cane toads are a 75-year-old Australian nightmare, and they amount to a cautionary tale about the difficulties that can crop up when humans try to reverse their environmental blunders.

The toads, native to Central and South America, were deliberately introduced to Queensland, on the other side of the continent from Western Australia, in 1935 in an unsuccessful attempt to control beetles on sugarcane plantations.

The toads bred rapidly, and their millions-strong population now threatens many species across Australia. They spread diseases, such as salmonella, and their skin exudes a venom that can kill would-be predators. They are also voracious eaters, gorging on insects, frogs, small reptiles and mammals, and birds. Cane toads are only harmful to humans if their poison is swallowed.

In recent years, Australians have held festive mass killings of the creatures, complete with sausage sizzles and prizes. Ahrens, of Cairns in Queensland, organizes the state's annual "Toad Day Out," when people gather to collect the creatures and either freeze them or expose them to carbon dioxide.

But the toads are constantly on the hop, and by early 2009 had migrated more than 2,400 kilometers (1,500 miles) from their original landing point in Queensland to the Western Australian border.

Lee Scott-Virtue, an archaeologist in the Kimberley region of Western Australia, saw it coming. Five years before the toads reached her state, she founded the Kimberley Toad Busters to mount a pre-emptive offensive across the border into the Northern Territory.

"We were confronted literally with walls of toads _ tens of thousands of them. It was like watching a moving carpet," she said.

Since then, the group's thousands of volunteers have killed more than 500,000 toads, largely with carbon dioxide, which she says is fast and painless. By the time toads finally crossed into Western Australia, their numbers had been reduced to the point "where we're only picking up handfuls."

But the state Department of Environment and Conservation says it ran tests in 2008 that showed the toads regained consciousness after initially passing out. That, the department says, might violate the state's Animal Welfare Act, which requires all killing of vertebrates to be humane.

Pending further tests scheduled for next month, the department advises people to go back to the freezing and clubbing options. "It's quick, it's effective," said a spokeswoman who spoke on condition of anonymity, in line with department policy.

That suggestion has outraged the cane-toad-killing community, which believes clubbing is a far more painful way to end a toad's life.

"For it to suddenly be dropped on us as the toad reaches Western Australia has been quite shattering," said Scott-Virtue. "If you hammer a toad, you've got to be very clever and very quick to be able to kill it instantly."

The Royal Society for the Prevention of Cruelty to Animals agrees that a strike to the head is the best method _ provided the toads are first chilled into unconsciousness.

But Shane Knuth, a Queensland state legislator who has suggested placing a 40 Australian cent ($0.37) bounty on cane toads, says freezing them takes too long. Besides, he said: "Mums and dads don't want toads in their freezers."

"We can go on and spend the next 50 years debating on how to dispose the toads _ but in reality, they're one of the greatest environmental catastrophes Australia has ever seen," he said.

"The do-gooders need to see the painful death our native animals go through after coming in contact with a cane toad."

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On the Net:

Kimberley Toad Busters: http://www.canetoads.com.au/