At first, Jane Tilka and Bill Dolan noticed an unusual number of blue- and red-dotted caterpillars and their black droppings in their Lowry Hill backyard.

“It was like raining poop,” Tilka said. “It was everywhere. There was a layer every day.”

Then the caterpillars turned into gypsy moths. The invasive species latched onto their five large white oak trees and laid thousands of buff-colored nests, each with thousands of eggs inside.

Their home and others in Lowry Hill have been quarantined since last summer, preventing residents from taking trees and shrubs out of the area.

But an attack plan could kill off the gypsy moths, help slow their statewide advance and save the area’s prized trees.

Later this month, the Minnesota Department of Agriculture plans to eradicate the aggressive leaf-devouring population by misting the area with a biochemical insecticide. The specific date has yet to be determined, but the MDA says it will be between May 15 and 31.

“There are more egg masses in this area than we have ever seen,” said Kimberly Thielen Cremers, MDA gypsy moth program supervisor.

The agency will use a small airplane to fly over the area and surrounding neighborhoods just after sunrise. The first two applications will spray 310 acres with an insecticide called Btk, which will kill the caterpillars. The third will cover the smaller quarantined area.

Btk is a bacterium with no known risk to humans, according to the U.S. Environmental Protection Agency.

The quarantine hasn’t been an inconvenience for the residents, they say, but their main concern is preserving some of the oldest trees in the area.

“They can devastate vegetation,” said neighbor Paul Ragghianti. “My wife was concerned they would chew up the leaves on our trees and there’s a concern for the environment. In a novice way, you think, ‘Wow this can’t be good.’ ”

Gypsy moths, originally from Europe, have caused millions of dollars in damage to trees in the eastern United States.

In the five to six weeks before caterpillars become gypsy moths, they only want to eat. This is the stage that’s most damaging.

Gypsy moths feed on more than 300 different tree and shrub species. The caterpillars can strip trees to bare branches, all the while dropping dark excrement that sounds like the patter of rain. Thielen Cramers said they love willows and oaks, which could be why they gravitated toward Tilka and Dolan’s trees. But they are not particular.

“They love all kinds of trees, but they like oaks, which is akin to liking a lollipop,” Thielen Cremers said.

The pest is common in Wisconsin, and officials have been tracking it in Minnesota since 1973. In 2014, Minnesota declared its first formal quarantine for the gypsy moth in the northeast tip of the state, which is still in effect.

“Once a county goes into full quarantine, then it stays,” she said. “It’s really hard to come out of quarantine.”

Lowry Hill was quarantined under an emergency and temporary status. With successful treatment, its quarantine will be lifted by the end of June.

In a statement, the state said the pests “are now threatening Minnesota.”

Just when the larger wave of gypsy moths will arrive in Minnesota remains uncertain, but Thielen Cremers said the state is part of a national project called “Slow the Spread” that has been instrumental in fighting back.

“We know the population is really close,” she said. “It’s pushing state boundaries, but we don’t know if it’ll be 10 years or 15 years before it’s here in full force.”