Six major forest pests and diseases pose varying threats to Maine’s trees — from the near-certain death of ash trees to the rapid spread of diseases that weaken hardwoods. The outlook reflects a broader challenge facing forests across the Northeast as climate change alters conditions that once naturally constrained pest populations.
Maine’s forests face an expanding pest threat as climate change weakens natural defenses against infestations. While recent cold snaps and coordinated pest-control efforts have slowed some prominent insect and disease populations over the past two years, warming winters are likely to allow pests to move inland and establish in forests that had been protected by cold, state forestry officials warned at the Maine Agricultural Trades Show on January 14.
“Everything we do is impacted by these climate conditions, so drought doesn’t help anything in the forest health world,” said Mike Parisio, an entomologist for the Maine Forest Service, addressing forest landowners.
The challenge confronting Maine reflects a broader problem across the Northeast: six major forest pests and diseases pose varying threats to the region’s trees, from the near-certain death of ash trees to the rapid spread of diseases that weaken hardwoods and rob wildlife of food sources. The outlook shows how climate change is dismantling the natural pest controls that forests have relied on for centuries.
Six Pests in Maine’s Forests
Hemlock adelgid poised to move inland
The hemlock woolly adelgid — a small insect from outside North America — has concentrated along Maine’s Midcoast for over two decades. Recent deep freezes slowed its expansion, with the pest appearing in only a couple of new towns in 2025, down from years of steady migration. That decline also reflected coordinated efforts from the Maine Forest Service, local landowners, and land trusts to release predator beetles that feed on the adelgid.
But Parisio warned the state cannot rely on those winter kill-offs moving forward. “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,” he said.
Browntail moth success — amid uncertainty
Maine has achieved notable success curtailing the invasive browntail moth, whose caterpillars can cause respiratory issues and painful rashes in humans. The state’s campaign to cut down the moth’s winter webs before spring emergence has produced steep results: browntail moth coverage declined from a peak of 150,000 acres in 2022 to just under 5,000 acres in 2025.
Yet the success story also serves as a cautionary tale. The decline depended on sustained effort and favorable conditions, making it vulnerable to resource constraints or further climate shifts.
Red pine scale: Rapid and difficult to detect
The opposite trend is unfolding with red pine scale, a small, pinhead-sized insect from Japan that is killing Maine’s red pines in quick succession. The scale has concentrated in Downeast Maine, Mount Desert Island, and Acadia National Park, but its tiny size makes new infestations difficult to detect until damage is severe.
“This is a very fast acting disease complex,” Parisio said. Red pine scale can kill an entire stand of trees “from the time of infestation, which you may not even be aware of visually, in just two years.”
Emerald ash borer’s northward march
The invasive emerald ash borer, a metallic green beetle roughly half an inch long, continued its northward expansion in 2025, moving into three new Maine counties: Somerset, Waldo, and Hancock. The beetle burrows into ash trees and lays eggs, remaining hidden until the damage is severe. An infestation typically results in the tree’s death within six years.
Ash trees hold particular significance in Maine. Wabanaki Nations have used brown ash bark for basket-making for centuries, while white ash is a valuable timber species. The Maine Forest Service runs a monitoring program and releases parasitic wasps that prey on the borer’s eggs.
“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 and budworm threats
Beech leaf disease has become one of Maine’s fastest-spreading forest maladies, engulfing and defoliating large areas of beech forest in just five years. When paired with beech bark disease, which creates cankers on tree exteriors, the combined pressure can severely weaken beech trees and eliminate the nutritious nuts that forest wildlife depend on.
In northern Maine, a less prevalent but rising threat looms: spruce budworm larvae have appeared in Aroostook County after running rampant in the Adirondacks and Quebec. The Maine Forest Service has had success containing early outbreaks with targeted pesticide applications, an approach Parisio called an “early intervention strategy” that aims to detect and treat pest hot spots before they expand beyond management capacity.
“This is the concept of EIS, 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,” he said.
The Climate Multiplier
The underlying driver linking all six pest and disease challenges is the same: a warming climate destabilizing the natural controls Maine’s forests have relied on. Warmer winters allow cold-sensitive pests to survive. Drought stresses trees’ ability to defend themselves. Together, these shifts are reshaping Maine’s forest ecosystem in ways that could take decades to fully understand.
State forestry officials are banking on a combination of early detection, targeted biocontrol measures, and adaptive management. But they acknowledge that success ultimately depends on whether warming trends can be slowed — a challenge that extends far beyond Maine’s borders.