DULUTH, Minn. — Agricultural officials are preparing to launch their largest air war ever to protect the forests of northeastern Minnesota and northwestern Wisconsin against gypsy moths.

In the wake of infestations found in the region last year, including in Duluth, experts in both states are expanding aerial spraying to keep the leaf-eating, tree-killing gypsy moth caterpillars in check.

In June, planes will spray a natural soil bacterium called Bacillus thuringiensis kurstaki (Btk) that kills the caterpillars. In July, they’ll drop synthetic hormone flakes to confuse male moths so they can’t mate and help the population expand. State officials say neither Btk nor the mating disruptant are toxic to people, animals, fish or plants.

The invaders, which originated in Europe, have been on a century-long march westward across North America, and the front lines have reached northeastern Minnesota and northwestern Wisconsin.

Lucia Hunt, gypsy moth expert for the Minnesota Department of Agriculture, told the Duluth News Tribune for a story published Monday there’s little hope of ultimate victory, but any delay may help as scientists look for a more permanent solution. The introduction of a new fungus in Pennsylvania, for example, seems to have reduced moth numbers there to much smaller levels.

But the pockets of gypsy moths in this region are growing larger and closer together and eventually will “coalesce to become established,” Hunt said, just as they have in every other state to the east.

“You’re in the action area, that 100 kilometer zone where the goal is to reduce the numbers and stall reproducing populations,” Hunt said.

The discovery of a live gypsy moth caterpillar in Duluth last summer was the first ever in Minnesota. Only flying male moths had been found in the state before. The caterpillar may be an omen of a major outbreak sometime soon.

Like the native forest tent caterpillar, gypsy moths spend several weeks as caterpillars munching leaves. The caterpillars then transform into moths that then spread out to mate and lay eggs and start the cycle again. It’s believed they make their biggest advances by laying eggs on railroad cars, trucks, boats and camper trailers and other vehicles that then spread the population into new areas.

“I have memories of the hills being all brown as the caterpillars chomped their way through summer . everything turned brown as they fed,” said Duluth resident Judy Gibbs, who grew up in Pennsylvania. “The leaves that would grow back later in the summer never seemed to be as dark green as the first leaves. And trees just can’t sustain year after year after year defoliating.”

The caterpillars can defoliate vast areas quickly, killing already stressed trees and interfering with tree growth. In some eastern states, more than 20 percent of affected trees have died.

“If you had an extensive (forest tent caterpillar) outbreak, then some drought and then add on gypsy moth defoliation, that might be too much stress too fast for some of our trees to handle,” said Mike Albers, a forest health specialist for the Minnesota Department of Natural Resources.

Last year, the Minnesota Department of Agriculture found infestations of the adult moths covering a record 114,000 acres in St. Louis, Carlton, Cook and Lake counties.

In Wisconsin, the Department of Agriculture plans to spray 254,166 acres across 22 counties this year, including Ashland, Bayfield, Burnett, Sawyer and Douglas counties.

“The gypsy moth is a serious pest that threatens our forests and urban trees, and it can have a negative impact on Wisconsin’s timber, paper, nursery and tourism industries,” said Chris Lettau, Wisconsin’s gypsy moth program coordinator.