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Weed-eating insects take the starthistle challenge.

A team of tiny but tenacious insects might someday stop the spread of a poisonous weed, yellow starthistle.

Imported from Greece, the diminutive weedeaters are three kinds of hardworking weevils and one species of delicately colored fly. Each would easily fit on the tip of your little finger.

The insects' offspring - grub or maggotlike larvae when young - are seed-destroyers that feast in starthisthe's flowerhead, where seeds are formed. They vary in when and how they attack. Some larvae eat developing seeds. Others destroy tissue that otherwise would nurture young seeds.

"Without seeds, starthistle can't reproduce," says ARS researcher Charles E. Turner at Albany, California. "With this team of flowerhead feeders, we hope to slow or stop the spread of starthistle."

The insects offer an environmentally friendly alternative to using herbicides. This type of biological control has proven successful with other weedy pests in the United States and worldwide. A classic example: About 40 years ago, an imported beetle took only 5 years to knock down the northern California infestation of another poisonous plant, klamathweed.

At its worst, however, klamathweed probably infested no more than 2 million acres of California rangeland. Yellow starthistle is a more formidable foe. Introduced here in the 1800's, probably from southern Eurasia, starthistle now rows in 23 states. For four of them - California, Oregon, Washington, and Idaho - the weed has become a major pest. In California alone, starthistle now infests some 8 million acres.

In summer, a fully grown starthistle can measure from 6 inches to 6 feet high. Beneath its bright yellow flowers, needle-like thorns form a star-shaped circlet that gives the weed its name.

A starthistle toxin causes a chewing disease, nigropallidal encephalomalacia, in horses. Unable to chew or swallow, poisoned animals may die of thirst or starvation. Too, the nasty weed crowds out native plants and other greenery that could be safely razed by horses, cattle, or sheep. And its painfully sharp thoms stab unwary hikers.

Since 1984, ARS scientists in the United States and colleagues Rouhollah Sobhian, Luca Fornasari, and others at the ARS European.biological Control Laboratory, Montpellier, France, have systematically recruited the five insect species that make up today's American team of weed warriors.

Each species has undergone intensive study and testing: The scientists had to find out as much as they could about each potential new recruit. That included checking to make sure each candidate species attacks only starthistle - not native plants or crops like safflower, sunflower, or artichoke - three starthistle relatives. The scientists used these tests results, and others, in seeking federal and state approvals to set the recruits free in the West.

Right now, the most numerous of the immigrants is Bangastemus orientalis, a dark-brown, quarter-inch-long weevil with a funny-looking snout.

In 1985, ARS scientists in Europe hand-collected B. orientalis to turn loose in three thistle-infested sites in California. Surrounded by a sea of food and a mild climate reminiscent of their Mediterranean home, the weevils flourished. Counties that were the insect's first California home soon became nurseries, providing new generations of healthy, hungry weevils. Today, more than 45 California counties have their own colonies of the little weevil.

Turner credits B. orientalis and other insects overseas with keeping the weed in check in its homelands. "In Europe, you can easily find many different kinds of insects living on a single starthistle plant," he says. "You'll see individual plants here and there, and some occasional thistle patches. But there's nothing like the vast infestations of starthistle that we have here.

"These teams of insects are apparently holding the weed back. And that's what we want them to do for us.
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Title Annotation:control of starthistle weeds
Author:Wood, Marcia
Publication:Agricultural Research
Date:Jul 1, 1993
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