Maine forests battle six pests as warming climate weakens defenses

Maine’s forests face a mounting threat from six invasive pests and diseases that are expanding despite recent control successes, state forestry officials warned at the Maine Agricultural Trades Show on January 14.

The situation underscores a broader challenge for forest management: while deep freezes and targeted control measures have knocked back some pests over the past couple of years, warming winters driven by climate change and drought conditions are making forests more vulnerable to future outbreaks. “Everything we do is impacted by these climate conditions, so drought doesn’t help anything in the forest health world,” said Mike Parisio, entomologist for the Maine Forest Service.

Parisio outlined the state of each pest threat, describing a landscape where some victories mask deepening vulnerabilities.

Adelgid slowing, but danger remains

The hemlock woolly adelgid, a tiny insect that has ravaged Maine’s hemlock forests, shows signs of slowing. The pest originates from outside Maine and has concentrated in the Midcoast for over two decades, feasting on its namesake tree. It was found in only a couple of new towns last year—a result of both unusually cold winters and a coordinated effort by the Maine Forest Service, local landowners, and land trusts to release predator beetles that prey on the insect.

But this temporary reprieve is fragile. “We’re having more and more survivorship here in Maine, which, again, would increase its ability to move inland into other areas that it’s historically been kept out of by weather,” Parisio said. As winters warm, the adelgid could migrate further inland into regions that have so far remained protected by coastal climate patterns.

Browntail moth turned back

Maine has achieved more dramatic success against the browntail moth, whose caterpillars trigger respiratory problems and painful rashes on contact with human skin. The state has pursued a widespread campaign to exterminate the moth by cutting down its winter webs from host trees before the insects can spread.

The effort has paid off dramatically. Browntail moth coverage declined from 150,000 acres in 2022 to just under 5,000 acres in 2025—a steep collapse that offers a rare piece of good news for Maine’s forest managers.

Red pine scale spreading

While browntail moth populations have crumbled, red pine scale—a tiny, pinhead-sized insect from Japan—is rapidly killing Maine’s red pines. The insect concentrates in Downeast Maine, across Mount Desert Island, and throughout Acadia National Park, but its small size makes new infestations difficult to detect until damage is severe.

“This is a very fast acting disease complex,” Parisio said. Once established, red pine scale can kill an entire stand in as little as two years from the time of infestation—often before visual damage is apparent.

Ash borer moving north

Invasive emerald ash borer populations expanded in 2025, migrating north into three new Maine counties: Somerset, Waldo, and Hancock. The metallic green, half-inch-long beetles burrow into ash trees and lay eggs that later feed on the tree’s vascular system.

The threat carries cultural and economic weight. Wabanaki Nations have used brown ash bark to make baskets for centuries, while white ash is a valuable commercial timber species. An emerald ash borer infestation is nearly fatal; the beetle can fell an ash tree in fewer than six years.

The Maine Forest Service runs a monitoring program and deploys biocontrol measures, releasing tiny wasp-like parasites that prey on the beetle’s life cycle. “Wherever there’s emerald ash borer in Maine, and we have a suitable release site, we’ll continue to pursue these efforts,” Parisio said. “We certainly do rely on private landowners if they’re willing.”

Beech disease spreading rapidly

Beech leaf disease has emerged as one of Maine’s fastest-spreading parasites, engulfing and defoliating vast numbers of beech trees in just five years. Combined with beech bark disease, which causes unsightly cankers on tree bark, the two diseases weaken beech trees and deprive forest wildlife of the nutritious nuts the trees produce.

Spruce budworm crosses the border

A less prevalent outbreak of endemic spruce budworm has begun appearing in Aroostook County, defoliating some of the region’s evergreen trees. Spruce budworm larvae have run rampant in the Adirondacks of northern New York and in Quebec; this represents one of the first confirmed incursions across the Canadian border into Maine.

The Maine Forest Service has curtailed the outbreak using targeted pesticide applications as part of an “early intervention strategy” that aims to detect and treat outbreaks while they remain manageable. “This is the concept of EIS,” Parisio said, “to detect these hot spots early and treat them when they’re at a manageable size, knowing that it’s not a one and done.”

The six pests represent a compounding challenge for Maine’s forests. While some control efforts have succeeded, state officials warn that the landscape is shifting. Warmer winters, changing precipitation patterns, and the arrival of new pests mean Maine’s forest managers cannot rely on past strategies alone. The work of holding back invasive pests, they say, will intensify in the years ahead.