THE sound of the rifle shot rings outthroughtherainforest, scattering a flock of snow-white cockatoos. The .22 calibre bullet hits the wild pig between the eyes. It keels over into the mud, legs thrashing.

"It's not something we get a kick out of," said Simon Kaukiainen, a trapper with Boar Busters, a professional pig-hunting business set up two years ago. "But it needs to be done."

The former farm worker is on the front line of a fight against Australia's burgeoning wild pig population.

It was recently estimated by a federal government agency that there are now 23 million pigs living in Australia - outnumbering the continent's human population of 21 million.

They are the descendants of domestic pigswhichEuropeanexplorers, including Captain James Cook, released as part of a "living larder" for future expeditions. The pigs found Australia to be hog heaven, with plentiful food, a balmy climate and no natural predators aside from the occasional crocodile.

They have grown bigger and brawnier than their British ancestors, with some bristle-backed males weighingmore than 150kg and capable of goring a human with their formidable tusks.

In the tropical state of Queensland they are causing millions of pounds worth of damage to sugar cane and bananaplantations,andthreatening endangered rainforest animals.

"There's no question that they are on theincrease,"saidNormanKippin, from the farming lobby group AgForce. "They are the biggest single problem up here in the wet tropics region and the government won't do a bloody thing about it."

Feralpigsinhabitabout40%of Australia, colonising habitats ranging from forests and mountains to semi-arid savannah plains. Populations have risen during decades of inaction and blame-shiftingbetweenfarmers, national parks and government.

IntheQueenslandsugarcane growing regions of Innisfail and Tully, the pig problem has worsened since a cyclone hit the area in March last year.

Cyclone Larry flattened large swathes of rainforest, depriving the pigs of the wild fruit they had previously depended on and driving them on to farmland.

"Over the last 12 months there have beenextraordinarynumbers,"said WayneThomas,offarminggroup Queensland Canegrowers. "The cyclone destroyed their habitat and displaced them."

Where farmers once shot a dozen pigs a year, they are now bagging hundreds. Pig hunting has been popular with rural Australians for decades. Amateur pig huntersusehigh-poweredrifles, hunting dogs and "pig rigs" - specially equipped trucks. They call themselves "grunter hunters".

Thesportevenhasitsown magazines.BaconBustersis particularlypopularforits regular "babes and boars" page, featuring photographs of bikini-clad young women sitting astride dead pigs.

But amateur hunters are struggling to control pig numbers and the battle against the boar is increasingly turning professional. Boar Busters was set up by a former soldier, Paul Smith, a veteran of Iraq, East Timor and Somalia.

Since establishing the business two years ago, he and his trappers have caught1200pigsinthedense rainforestsandopenfarmland surroundingInnisfail,Tullyandthe resort town of Mission Beach.

"We can't shoot them because the forest is too dense and we can't bait them because that would kill native wildlife too. So we trap them," said Smith, checking his traps along a track where tangled rainforest sweeps down to farmland. "Big or small, we catch 'em all, that's our motto."

One of the metal cages he checked was crowded with nine frantic pigs, grunting and squealing as they charged at the metal bars.

One by one they were dispatched with a .22 rifle, to be buried in a pit. Their meat cannot be eaten because of worm infestation and disease.

Farmers are being driven close to financial ruin by the depredations of the pigs, which congregate in large herds and lay waste to sugar cane fields and fruit plantations - some banana farmers have lost up to 40% of their crop. Nationwide, the animals are estimated to be costing agriculture more than £40 million a year.

"They're a terrible pest," said sugar canefarmerRobertCollins,who employed Boar Busters to eradicate the pigs on his land. "In more open country you can shoot them from helicopters, but here in the scrub rainforest you can't even see them."

The pigs also have a serious impact on the rainforest by fouling streams and competing for wild fruit with animals such as the endangered cassowary, a giant flightless bird with a kick powerful enough to disembowel a human.

"Only1200 remain in the wild in Australia, which makes them rarer than the giant panda," said Roger Phillips, head of the Australian Rainforest Foundation. The birds are key to the health of the rainforest because they disperse the seeds of more than 100 species of tree which will only grow if they have passed through the cassowary's gut.

"Trapping won't solve the feral pig problem. But at least we can remove pigs from critical cassowary habitat and givetheforestafightingchance," Phillips added.

Eradicating pigs from Australia would be impossible. "They're entrenched in very wild country that is difficult to access on foot, let alone by vehicle," said Thomas.

"It'sallthankstoCaptainCook. Letting them loose was not the smartest thing he ever did."