Maine forestry officials launched another offensive in an ongoing bug battle to slow the spread of an invasive insect that has been attacking hemlock trees for the past 20 years.

The hemlock woolly adelgid, which is native to Asia, was introduced to southern Maine in 2003 and has been slowly spreading up the coast and farther inland. The insect was recently detected as far north as Acadia National Park, according to Colleen Teerling, an entomologist with the Maine Forest Service.

“There’s millions, possibly billions,” Teerling said.

One way to slow the spread of the woolly adelgid is “biocontrol,” or deploying a predatory insect. Since 2004, the state has planted colonies of Sasajiscymnus tsugae beetles, another Asian insect that feeds on woolly adelgids but doesn’t harm trees.

On Monday, Teerling, with the help of the Maine Coast Heritage Trust, attached 10 colonies with about 100 beetles each to 10 trees infested with woolly adelgids in the Woodward Point Preserve in Brunswick.

“We were able to find plenty of trees that had adelgid on them, unfortunately,” Teerling said.

Beetles are the best way to attack woolly adelgids in forests because the beetles can easily spread from tree to tree, she said. The Maine Coast Heritage Trust purchased the beetles for about $2 per insect from Tree Savers, a Pennsylvania-based business that is the only commercial producer of Sasajiscymnus tsugae beetles.

Woolly adelgids were introduced to Virginia from Japan in the 1950s and have since spread south to Georgia and north to Maine. They have no natural predators in the U.S. and multiply rapidly. All woolly adelgids on hemlock are female and they produce two batches of eggs every year, with as many as 500 eggs per batch.

In the summer of 2021, beachgoers in Wells and Kennebunk reported black stains on their feet that scientists later attributed to dead woolly adelgids.

“It’s on the move,” Teerling said. “It’s moving eastward along the coast and it’s moving inland.”

For homeowners who have a hemlock tree or two with an infestation, Teerling said a pesticide like horticultural oil works best, as beetles would only migrate. She said prevention is key — pruning hemlocks so they don’t touch other trees or objects will likely stop a possible infestation because woolly adelgids, which don’t have wings, won’t be able to crawl on them.

Last year, forestry officials planted about 9,000 beetles in forests across the state, and Teerling said they will plant a few thousand more this year. Officials will soon plant beetles in Freeport, Edgecomb, Waldoboro, Portland and South Berwick, she said.

She credited local land trust groups with taking the situation seriously and purchasing beetles to slow the woolly adelgid’s spread. She said the biocontrol, combined with pruning and pesticides, will be an ongoing necessity to protect hemlock trees, as the woolly adelgid has become well-established in Maine.

“We will never eradicate it,” Teerling said.