The USDA affirmed Monday that a leaf-eating insect from Asia can be turned loose on knotweed, an invasive plant that’s cost more than $30 million over the past 15 years to control in Washington.

The psyllid Aphalara itadori will be the first biological control used against Japanese knotweed, as well as the related Bohemian and giant knotweeds.

The deep-rooted knotweed thrives along rivers and streams, crowding out shade trees and plants that provide better nourishment to bugs at the low end of the food chain, leading to less food for fish. The large herbaceous perennials are a problem for livestock forage, too.

Animal and Plant Health Inspection Service announced in the Federal Register that it has concluded introducing the insects won’t harm wildlife or spread to native plants, confirming preliminary findings.

Oregon State University scientist Fritzi Grevstad of the College of Agricultural Sciences petitioned the USDA to use the insects and led the research that resulted in the approval.

She said Monday that the insects are proven killers of potted knotweed plants in greenhouses. To be effective, they will have to survive Northwest winters and whatever predators are out there, she said.

“I’m eager to see how the insects will perform in the field after working with them for 13 years in the lab,” Grevstad said.

The three types of knotweed were introduced to North America in the late 19th century. The plants have been found in 42 states. Oregon, Washington and northeastern states have the worst infestations.

Knotweed has been confirmed in every Washington county. The state agriculture department reports spending $7.5 million since 2004 on controlling it. The USDA estimates federal, state and local spending has totaled $30.4 million between 2004 and 2016.

Herbicides containing glyphosate or imazapyr are used against knotweed. Because roots are deep, knotweed must be sprayed annually. Small patches can be covered with tarps or dug out by hand.

“Without a biological control program, chemical and mechanical inputs are likely to be needed on a permanent basis with variable to limited success,” according to USDA’s environmental assessment.

Working with APHIS, Grevstad said she already has issued permits to government agencies in eight states, including Oregon and Washington, to release the insects. The other six states are in the Northeast.

“One of the big tests will be if they survive the winter,” she said. “I think they have a better chance here (the Northwest), but it’s hard to say.”

The U.S. and United Kingdom began working on developing a biological control for knotweed in 2000. Aphalara itadori has been used in the UK since 2010. It was the first biological control of a weed allowed by the European Union. Canada approved using the insects in 2014.

Research showed the insects would not spread to non-knotweed plants, according to the USDA. The agency was particularly concerned about buckwheat, a plant in the same family.

Buckwheat is grown in Japan close to knotweed, but is not bothered by Aphalara itador, according to the USDA.

Knotweed provides late-summer nectar to pollinators, raising some concern among beekeepers over targeting the plants. In comments to the USDA, the Washington State Noxious Weed Control Board said the benefits “far outweigh any minor negative implications.”