MOST Australian immigrants are warmly welcomed in the United States. But there is one stubborn Australian visitor that Florida has been trying to get rid of for years, and it has enlisted other Australians in the fight.

Florida's Everglades, which is a World Heritage-listed wetland area, has suffered for a century from the spreading infestation of the Australian melaleuca, or paperbark tree.

Introduced first as an ornamental tree, the species spread over 250,000 hectares, taking over grass wetlands and crowding out native species such as the pond apple tree.

Back in the early part of the 20th century, as people began to settle southern Florida, there were attempts to drain the Everglades to provide more land for housing and to get rid of mosquitoes.

John Gifford, the first American to hold a doctorate in forestry, suggested using melaleucas around the edges of the wetland to dry it out in a more environmentally sensitive manner. But it was the wrong tree.

Without any natural enemies, it thrived. For a while it was a popular tree. One nursery owner flew over the Everglades, conducting aerial seeding.

He would then come back when the trees were saplings, uproot them and sell them.

The tree has since proven difficult to remove. As a weed alert from the Florida Environmental Protection Department says: "Melaleuca can flower five times per year. Any damage to the tree that cuts water flow to the stems containing seed capsules, such as fires, freezes and control techniques, will result in seed release."

The seeds can remain viable for 10 years and a single tree can store up to 20 million of them.

Attempting to kill the trees with herbicide had only limited success. In the past decade, authorities have started to bring the melaleuca under control by introducing Australian insects that feed on the tree.

With the help of the Sydney Botanic Gardens, Allen Dray, an ecologist with the Agricultural Research Service of the US Agriculture Department, studied the genetic origins of the tree in order to find exactly the right biological control agent.

The melaleuca snout weevil was released in 1997 and plant lice were tried in 2002. A small fly nicknamed a ferg is scheduled to be the next weapon released against the melaleuca.

To ensure these insects didn't turn into Florida's version of the cane toad — introduced into Queensland from Hawaii in an unsuccessful attempt to stop a beetle destroying sugar cane crops — the first insect was studied for 11 years before being released into the wild.

The insects have helped Florida reclaim 40,000 hectares, mostly of public land.

"It's still a problem, but we are getting ahead of it," Mr Dray says.

Despite common belief, biological agents have proved very successful at controlling exotic weeds.

One of the famous examples is the introduction of the cactoblastis caterpillar to Australia, which wiped out massive infestations of prickly pear.

Mr Dray points out that melaleuca is not the only Australian pest.

Australian pine, as casuarina is called there, is a problem along the Florida coast.

"Don't feel too guilty," he says. "You're now having a problem with a tree we call pond apple, which is a Florida species, which I understand is taking over areas in North Queensland where melaleuca was."