It's Invaders vs. Invaders as Scientists Target Alien Species

John Roach
for National Geographic News
February 22, 2006
Empowered by a lack of natural enemies, invasive species often overwhelm
the regions they infiltrate.

But some scientists are fighting invaders with invaders, importing natural enemies from the problem species' native regions. (Related photos: invasive species in the United States.)

"What happens when we introduce an organism to a new range is that we often leave behind the natural enemies that keep it under control," said Mark Lonsdale in a Pulse of the Planet radio program broadcast today. Lonsdale is an ecologist with the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Canberra, Australia. (Pulse of the Planet and this news story are sponsored by a grant from the National Science Foundation.)

The trick is to select and introduce natural enemies that will not cause additional damage to the ecosystems.

"There is a very long, extensive, and strict process of review" undertaken before any organism is released into a new environment, John Scott said. Scott is an entomologist and colleague of Lonsdale at CSIRO.

Scott added that Australia has a century-long tradition of using so-called biological controls, or biocontrols, in the battle against invasive weeds. Some 200 insects, mites, and fungi have been successfully and safely introduced, he says.

Bridal Creeper

According to Scott, the latest biocontrol success story is the implementation of a fungus and two insects against bridal creeper, an invasive weed introduced to Australia from South Africa in the 19th century.

Bridal creeper was introduced as a garden plant, but is now considered one of the worst weeds in the bushland of southern Australia, where its climbing stems and foliage smother native plants.

Unlike most other invasive plants, the creeper has seeds that can germinate on soil that isn't stirred up. The creeper can therefore spread more quickly, particularly in undisturbed habitats, such as nature reserves. This makes the plant a primary cause of ecological change in some pristine habitats, Scott said.

"And because it's in areas of nature conservation, our only option we have to control it is to use biological control," since chemical methods are banned in these areas, he added.

The first biocontrol approved for release was the bridal creeper leafhopper, a 0.08-inch-long (2-millimeter-long) insect that sucks photosynthetic cells from the creeper's leaves. (Plants rely on photosynthesis to convert light into energy.)

The insects were released between 1999 and 2003 and have slowed bridal creeper's spread.

In 2000 the Australian government approved release of the rust fungus Puccinia myrsiphylli. The fungus infects the leaves and stems of the bridal creeper, diverting resources the plant needs for growth and reproduction.

Later, in 2002, the bridal creeper leaf beetle was released. The beetles' larval young strip the shoots and leaves that enable the plant to climb, which prevents it from reproducing and spreading to new areas.

According to Scott, more than a thousand community groups in Australia are involved in release programs, helping to spread bridal creeper's enemies and to curb the invasive weed's progress.

U.S. Invasion

The U.S. is also serious about controlling invasive weeds, said Cressida Silvers, an entomologist with the U.S. Department of Agriculture's Agricultural Research Service (ARS) in Fort Lauderdale, Florida.

She and her colleagues go to Australia in search of enemies of melaleuca, an evergreen tree native to Australia but considered an invasive weed in the U.S.

The tree was introduced to the U.S about a hundred years ago and now infests over 500,000 acres (200,000 hectares) of the Florida Everglades.

Melaleuca trees, whose seeds are spread by fire, are fast growing and clump together, crowding out native vegetation and sucking wetlands dry.

Melaleucas spread at an average of 15 acres (6 hectares) per day, according to ARS research.

After about a decade of study, scientists introduced the melaleuca weevil to the Everglades in 1997. This short-snouted insect munches the trees' young leaves.

The weevil stunts melaleuca growth, increases the mortality of young trees, and reduces flowering and seed production.

In 2002 the government scientists released the melaleuca psyllid, a sap-sucking insect that has the same effect as the weevil.

"We consider them a tremendous success," Silvers said. In one study area, for example, 65 percent of young trees fed on by the enemy insects died.

But ARS researchers stress that herbicides and physical removal of invasive species are also important weapons in the ongoing battle.

Biocontrols "are not a replacement, but a good tool to go along with the other good tools we have," Silvers said.

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