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Weevils, mites may thwart gorse's spread.

Thick, tough, and thorny, a hardy shrub known as gorse is nature's own barbed wire fencing. Hedges and thickets of the rugged yellow-flowered plant are nearly impenetrable.

Native to western Europe, the weed flourishes in the coastal climates of northern California, southern Oregon and Washington, and on some islands in Hawaii.

Though sheep and goats can graze gorse's tender new foliage, it quickly crowds out other plants that are better forage. And in parklands, spines can spear unwary hikers and campers,says research entomologist B. David Perkins. He recently retired from ARS but still collaborates with the agency at the ARS Western Regional Research Center, Albany, California.

In the United States and abroad, Perkins has sought natural enemies of gorse - insects and mites, for example. Perkins and colleague Luca Fornasari have scrutinized gorse plants along roadsides and in fields and pastures in Italy, France, Spain, and Portugal. Fornasari is the ARS European Biological Control Laboratory in Montpellier, France,

The best find from the expeditions may be seed weevil the researchers brought back from western Spain. The weevil eats seeds growing inside gorse's beanlike pods. Scientist at ARS' Systematic Entomology Laboratory at the Smithsonian Institution, Washington, D.C., are now trying to identify the insect.

Oregon funded part of the search. The State ranks gorse as its number one worst weed.

A tiny spider mite known as Tetranychus lintearius may prove another potential recruit. It has already been put to work fighting gorse in New Zealand, where farming and forestry industries spend about $9 million a year to control the weed. "Anyone who doubts the potential for gorse to get out of control in the United States should look at New Zealand," comments Perkins.

The beneficial spider mites are so tiny that about a thousand of them could fit on a single, inch-long gorse thorn. Mites damage the plant by sucking juices from the weed's leaves.

While in New Zealand, Perkins let the mites sample some specimens of American gorse and other plants from the western United States. These tests indicated the mites will feed voraciously on our gorse. Fortunately, they won't harm other greenery Perkins offered, including several native California lupines - distant gorse relatives.

With further testing and government approvals, the weevil and mite - and perhaps additional candidates - may eventually be turned loose in the United States. They would augment the efforts of other insects that feed on gorse. Those include a weevil, Apion ulicis, and a moth, Agonopterix nervosa.

The A. ulicis weevil is "about the size of a pen tip," says ARS botanist Charles E. Turner, who has examined the insect in California gorse thickets. It lays its eggs in gorse's hairy seed pods. Eggs hatch into wormlike larvae that feed on the developing seeds.

Colonies of A. ulicis have managed to destroy more than 30 percent of the aboveground gorse seeds on some sites in California and Hawaii and 90 percent of the seeds checked in a Washington State survey.

But the weevil hasn't stopped the spread of gorse thickets in America. Explains Perkins, "The major problem is that the weevil doesn't attack seeds hidden in soil, which may remain alive for as long as 30 years."

The insect may offer another avenue of attack, however. Perkins has found a mysterious apricot- or rust-colored fungus on some gorse seed pods he inspected in California. The fungus, after entering holes chewed by the female weevil before she inserts her eggs, causes seeds to shrivel and die.

No one quite knows how another gorse foe, the A. nervosa moth, ended up on the West Coast. About one-third-inch in size, the moth has blotchy beige and grey wings. Its larvae eat gorse's young green shoots. It left alone, those shoots would form hard spines.

Some of the gorse Perkins examined in California had 25 to 50 percent of new shoots infested by the tiny larvae. That's still not enough to hold back the weed, though. Perkins says it will likely take an assortment of enemies - perhaps each with its own unique way of causing damage - to stop the stubborn shrub.
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Author:Wood, Marcia
Publication:Agricultural Research
Date:Aug 1, 1992
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