Trees beware.

A huge infestation of leaf-loving gypsy moth caterpillars has been discovered in Grant -- and if they launch into a serious feast on the city's foliage this spring, look out.

"It's the largest infestation that I've ever seen," said Lucy Hunt, who oversees the gypsy moth unit at the Minnesota Department of Agriculture. "It's definitely one of the biggest in Minnesota."

The Grant infestation, which covers 844 acres of private land, is the first confirmed in Washington County since 1984. Metrowide, smaller infestations have been discovered this year in Coon Rapids and Minnetonka, Hunt said.

The gypsy moth, a true pest in the tradition of emerald ash borers and Japanese beetles, is an invasive species that hasn't yet hit Minnesota full strength. The wave, moving west from Massachusetts, now has invaded portions of Wisconsin.

Damage comes from leaf-chomping caterpillars that haven't yet morphed into moths. The caterpillars can strip deciduous trees to bare branches, all the while dropping excrement that sounds like the patter of rain. They love oaks and willows but even have an appetite for coniferous trees.

Gypsy moths can multiply with alarming speed if left unchecked, consuming leaves again and again until trees die from fatigue. The moths have few natural enemies.

In Grant, crews from the Agriculture Department found hundreds of egg masses, each bearing 500 to 1,000 eggs. "If it's a bad infestation, we're all concerned," said Tom Carr, mayor of the 4,000-resident city just west of Stillwater. "We love our woodlands, our country life."

Hunt plans to attack the caterpillars in May when they hatch from egg masses clinging to trees. A biopesticide known as BTK will be sprayed from low-flying aircraft to kill the caterpillars before they can do much damage.

Treatment will cost about $30 an acre, paid for with a federal grant, Hunt said. Residents will be asked to comment in open houses before spraying begins, she said.

In the five to six weeks before caterpillars become gypsy moths, they want only to eat. The adult moths are far less harmful to vegetation because they're preoccupied with mating.

The Agriculture Department has set about 20,000 gypsy moth traps in Minnesota. The traps bring the first clue of an infestation, and that's what happened in Grant, Hunt said.

"We have a high degree of confidence that when we see these counts going up, we're honing in on the actual infestation," she said.

Hunt said she's found no evidence of other infestations in Washington County.

Gypsy moths tend to travel by attaching eggs to firewood, lawn furniture, barbeque grills and other items that are moved around from camping areas and other destinations.

"It's impossible for us to know exactly what happened here," Hunt said of how the Grant infestation started. "I want to be careful not to assign blame on anybody."

The gypsy moth caterpillar is a cousin to the forest tent caterpillar, Hunt said. The difference is that the forest tent caterpillar, being native to Minnesota, has natural enemies that feed on it.

Residents can help in the war against gypsy moths by reporting new infestations to the Agriculture Department and inspecting furniture and other items they transport, she said.

Just when the larger wave of gypsy moths will arrive in Minnesota remains uncertain, said Mike Schommer, an Agriculture Department spokesman, but nobody doubts it will happen.

"What we're seeing is another example of how invasive species can have a tremendous impact on our environment," he said.

Kevin Giles • 651-735-3342