VALLEY CITY (AP) - Barnes County officials say flea beetles have helped control leafy spurge, a weed that chokes out grass and ruins pastures.

The tiny bugs, about one-tenth of an inch long, use the weed as their food supply. They don't eat other plants.

Leafy spurge acreage in North Dakota doubled every decade from the 1950s to 1980s, experts say. They estimate the weed infests more than 1 million acres in the state, costing North Dakota an estimated $75 million annually in lost production.

"It was clear something had to be done," said Rodney Lym, a professor of plant sciences at North Dakota State University.

In the early 1980s, researchers identified the flea beetle as a potential, partial solution. Years of trial and error were needed to understand the bugs and how to best use them, Lym said.

Scientists and ranchers have learned the beetles are quite effective on about 40 percent of acres, moderately effective on 30 percent and of limited value on the remaining 30 percent, he said.

The bugs do the least amount of good in sandy soil and in areas prone to regular flooding.

Researchers, ranchers and weed-control workers also are learning the weed can come back in force in areas where it once was controlled.

"We're finding the bugs need to be reintroduced" in those areas, Lym said.

Barnes County Weed Control Officer Jim McAllister said the county has about 21,000 acres infested with leafy spurge, roughly the same as five years ago. Thanks to the flea beetles, the weed is less dense now, he said.

McAllister recently stood in a pasture that a decade ago was filled with tightly bunched leafy spurge. Today, the sloping pasture overlooking the Sheyenne River can be grazed again.

Lym and other experts recommend using herbicide on the fringes of infested areas - "Just like you'd fight a prairie fire by controlling hot spots on the fringe of the fire," he said.

Adult flea beetles eat the weeds' leaves, while the larvae burrow into and eat the lower stems.

The bugs don't eradicate the weed, because they need it to eat, but they do thin it out and limit its spread, experts say.

Infested acreage in North Dakota has held steady for the past decade.

"Leafy spurge can pretty much steal your ranch if you don't control it," said Melvin Leland, a Sidney, Mont., rancher and president of the North Dakota Stockmen's Association.

Overall, about 5 million acres are infested in 35 states and the Canadian prairie provinces, according to the U.S. Department of Agriculture.

The North Dakota State University Extension Service says leafy spurge came to America in 1827. It is native to Europe and Asia.

It was first found in North Dakota in 1909, along a Fargo street, the agency said.

The weed normally grows about 3 feet tall. It also produces seed pods that, when mature, can throw seeds up to 15 feet from the plant.

Spurge can be attacked in fields with chemicals, but they are harder to apply in pastures. Sheep, which also eat spurge, are another weapon against the weed, Lym said.

Oriska farmer Jacob DeVries said flea beetles have worked well in Barnes County.

"There were places that were just solid yellow, covered with spurge. Now you see grass there," DeVries said.

"The bugs have done a tremendous job," he said.