Washington ? It’s no bigger than the head of a pin, but an Asian insect is producing a big headache for many farmers as it stunts and kills soybeans raised for vegetable oil, tofu, soy milk, protein and animal feed.

No one knows how the tiny green aphid immigrated to the United States three years ago, but Agriculture Department scientists theorize it may have clung to a traveler’s pant leg, shirt, shoe or bag. Or perhaps it was hanging from the leaves of an imported plant.

Regardless of how it arrived, the beetle-like insect has spread from Michigan to 15 other states — an area stretching from Maryland to the Dakotas. With 73 million acres of soybeans, the United States is a paradise for the aphid.

Greg Hartman, a scientist for government’s Agricultural Research Service, estimated that farmers were spending at least $20 million spraying pesticides on 2 million acres of soybeans covered in aphids. But the total amount of damage wasn’t clear, he said.

“People just stopped reporting it,” said Hartman. “I think in Minnesota they just gave up because it became so commonplace.”

Bill Wood, agriculture agent with the Douglas County Extension Service, said area farmers may not be reporting the problems either. He said he suspected the aphid was present in some Douglas County fields, but he had not heard of widespread problems.

That may change, though, as the insect continues its westward migration.

“I think it is a potential problem that could erupt in our county, but that day hasn’t happened yet,” Wood said.

The University of Wisconsin is among several institutions helping track the aphid. Its maps this year show that almost two-thirds of Minnesota has the bug, except for the northeastern corner. Other states like Virginia and Maryland have a much smaller problem.

Ken Ostlie, professor and extension entomologist for the University of Minnesota, holds a small soybean plant at left that did not receive pesticide treatment in Rosemount, Minn. The aphid infestation stunted its growth. At right is a normal size plant that was treated by pesticide. Aphids and drought have dampened what had been a promising season for soybean farmers. While lack of rain is an age-old problem, the soybean aphids infesting fields is a relatively new scourge.

John Steele learned about the infestation firsthand when he and his local grain salesman, Larry Larson, found the aphids swarming on 525 acres of his soybeans. Steele, a Sargeant, Minn., farmer, has a total of 675 acres of soybeans.

After noticing thousands of aphids sucking the juice out of his plants in July, Steele reluctantly decided it was time to spray.

“Hopefully, we treated them soon enough,” Steele said.

Spraying is a last resort for most farmers because it costs so much. Chemicals that kill the bugs cost as much as $10 per acre. Steele said he got a bargain, paying about $7 per acre — a total of $3,700 — to control them.

Steele won’t know until this fall when he harvests the soybeans whether he lost any. If he lost the entire infested acreage — roughly 26,000 bushels of soybeans — he’ll be out up to $130,000.

Walter Riedell, an Agricultural Research Service plant physiologist, said farmers in parts of the Midwest are struggling to keep up with the infestation because the weather this year has been ideal for aphids — a dry summer following a cool spring.

“Early on in the season, the insect likes cool weather,” he said. “They in fact can reproduce very very quickly in those conditions. The population can double within two days.”

The soybean aphid is born pregnant. Although adults die within days, they ensure their species’ survival by producing as many as five others per day.

“There’s no sexual reproduction,” said Riedell, who works for the USDA lab in South Dakota. “They’re just kicking out clones.”

Aphids live off the sap inside soybean plants, sucking the nutritious juice like vampires with pins in their mouths that act like hypodermic needles, Riedell said.

The bugs crave nitrogen, which is in the soybean sap. During the winter, the aphids can continue to live and grow because they also live on buckthorn, a common bush that also is a source of nitrogen.

Although the insects multiply at an astounding rate, they aren’t invincible. Riedell said their enemy, the common ladybug, is helping to inhibit their growth.

Hartman, who works in the USDA laboratory in Illinois, said he believes he and a group of scientists may have found a long-term solution to the problem: soybean plants that resist the aphid.

“This is a major discovery for soybean aphid resistance,” Hartman said. “We just need to get it into commercial crops.”

Breeding an aphid-resistant variety could take several years, though. Hartman said if scientists simply find the gene that helps plants fend off the bug, they could genetically engineer current commercial varieties to carry that trait.