BEHOUST, FRANCE -- When the southern green stinkbug appeared for the first time in California tomato fields last fall, an urgent call went out to a team of American agents with headquarters in this tiny French village. Their mission was to comb Europe -- the stinkbug's native land -- for a simple biological weapon that could turn back the pest invasion and save the valuable crop.

The agents searched in roadside weed beds and fields in Spain, France and Italy. Their quarry was a natural enemy of the stinkbug, a parasite no bigger than a speck, but they knew where to look for it.

"In Italy, six of us were on our hands and knees under green beans and tomatoes, turning leaves," said Walker A. Jones, leader of the mission.

Later this month, scientists from the University of California at Davis will release hundreds of thousands of minuscule stinkbug parasites onto the tomato fields of the Sacramento Valley and hope that nature does its work -- saving the tomatoes for human consumption and preventing the need to use chemical pesticides.

The bugs were collected and bred by Jones and other entomologists at the U.S. Department of Agriculture's European Parasite Laboratory here. It is a permanent foreign outpost for the collection and shipment of insect parasites -- bugs that live on other bugs or feed off their eggs.

Back home in the United States, farmers have been waging a chronic multimillion-dollar struggle against the bugs that would eat the nation's vital crops. Aside from native agricultural pests, American farmers are constantly fighting imported foreign bugs. The pests arrive from other countries in food or grain shipments and take hold in their new environment.

"A good number of the insect pests that we are trying to control in the United States came from Europe," said Raymond F. Moore, director of the parasite lab. "They may not be a pest in Europe," he said, "because they grew up over the centuries with natural enemies -- parasites and predators -- that keep them under control."

With an annual budget of about $1 million, the European Parasite Lab has successfully reunited several imported pests with the natural enemies they left behind. The alfalfa weevil and the alfalfa blotch leafminer have been conquered, saving farmers more than $65 million dollars annually, according to the USDA. The linden aphid rules no more, and the fig scale has bitten the dust.

Moore said there is no danger of parasites multiplying out of hand when sent

back to the United States. The parasites are "married" to their specific hosts and do not feed on anything else. "I don't think there's been a single instance where a parasite has become a pest itself," he said.

Not all the lab's endeavors, however, are success stories. The outpost was established in 1919 in Auch, France, with the original mission of finding a parasite for the European corn borer. An effective parasite was never discovered.

The gypsy moth and the Hessian fly, two notorious visitors from the Old World, are still a bane in the New World.

The lab's current projects include a search for natural enemies of the potato leafhopper, the asparagus beetle and the African filth flies -- the horn fly, the face fly, the stable fly and the false stable fly.The southern green stinkbug that invaded California is thought to have originated in Africa or Asia, but it has been endemic to Europe for hundreds if not thousands of years. It is a flat green bug about the size of a fingernail that secretes an offensive-smelling fluid when attacked.

The stinkbug's parasite lays its eggs in the egg masses of its host, killing them. This interferes with the stinkbug's reproductive cycle, holding down the population.

Using a parasite to control insect pests -- as opposed to using chemical insecticides -- is known as "classical biological control." Heightened public concern over the environmental dangers of farm chemicals has brought renewed government interest in using bugs to kill other bugs. California has been a leader in this approach.

"The obvious thing to do was import a natural enemy of the stinkbug," said Lester E. Ehler, professor of entomology at the University of California's agricultural experiment station. "The state has basically decided the bug is here to stay. We want to get this thing before it spreads. It's just in the Sacramento area now, but it will spread over the state."

When Ehler contacted the European Parasite Lab, a hunt began for the stinkbug's tiny parasite. It looks no bigger than a speck to the naked eye but resembles a wasp under the microscope.

The entomologists actually were searching for the egg masses of the stinkbug, hoping that some of them were injected with the eggs of the parasite and could be hatched in the lab.

Walker set out in June for a farming area in Antibes in the south of France whose climate matched that of the hot, dry Sacramento Valley. This would increase the chances of finding a variety or "race" of the stinkbug parasite that could adapt to the new environment. In Antibes, he was met by a French government scientist who was also interested in stinkbug parasites and could serve as an entree with the local farmers.

Before getting down to business, however, the Frenchman brought Jones to his home for lunch, which turned out to be a lucky break. "He has a large vegetable garden," Jones explained, "and I went crawling around out in his tomatoes, and I found what I was looking for -- underneath a leaf."

Other varieties of the parasite from Andalusia were provided by a Spanish government entomologist. "He wants no money in return," said Moore, "just some parasites of the Colorado potato beetle."

American bugs like to travel too, he explained. In its 68-year history, the European Parasite Lab has relocated several times in France, but recently the USDA committed itself to the bug-eats-bug campaign by estab- lishing a permanent lab on five acres of prop- erty it has purchased in Behoust, a village about an hour outside of Paris.

The lab is a modest affair, two large stone farm buildings behind an ivy-covered stone wall. A few rooms contain long tables, microscopes and climate-controlled cases that look like refrigerators. Inside, bugs are happily munching French green beans and multiplying. The lab employs just 17 people -- two Americans and the rest European nationals, mainly French. Another project this year -- one that city dwellers might appreciate -- was to try, yet again, to find an effective parasite for the cockroach. Herfried Hoyer, a German who has worked at the lab for 13 years, set out directly for the places he was sure he would find lots of cockroaches -- the bakeries of Paris. But, he reported, "Whenever you go in a baker's shop, people throw you out. They say, 'This is considered a clean house.' "

Instead, he wound up in the Paris sewers and Metro tunnels, where he encountered a cosmopolitan mix of cockroaches -- the German cockroach, the Oriental cockroach and the American cockroach. He found, however, only one kind of parasite tagging along; that one was already known back in the U.S. and obviously had not made a dent in the cockroach population there.

But there have been several cockroach parasite sightings reported in Africa, he said, still holding out hope.

Robin Herman is a free-lance writer in France.