A closeup of spruce budworm larvae and the damage it can do when it is feeding on the needles of a spruce or fir tree. Courtesy of Maine Forest Service

A spruce budworm outbreak has plagued Maine’s northern borders for nearly two decades, with the tree-killing moths making sporadic incursions from Quebec but never reaching numbers that suggest a repeat of the outbreak that ravaged the state a half-century ago.

This summer, though, state entomologist Michael Parisio’s heart sank as he surveyed the northwestern woods of Aroostook County by plane. A 3,000-acre hot spot of partially denuded spruce-fir forest suggested the once-a-generation outbreak everyone had feared might have begun.

The patch of defoliated trees near Little East Lake just east of the Canadian border was a sign the native budworm population had grown so large that its consumption of fir and spruce needles was visible from a thousand feet above the forest.

“We’ve had a few scares here and there, but 3,000 acres, that’s significant damage,” Parisio said. “That is a hot spot that won’t go away in a year. All evidence suggests it will persist and expand. We knew it would get here eventually, but knowing doesn’t make what’s going to happen any easier.”

University of Maine modeling shows that more than 178,000 acres are on the verge of defoliation when budworm larvae emerge from their already-hung cocoons next spring hungry for the buds that grow at the end of spruce and fir trees in Maine, the most heavily forested state in the country.

The last outbreak lasted from 1967 to 1993, covering 136 million acres across eastern Canada and Maine. It stripped the needles from fir and spruce trees across most of northern Maine, killing 7 million acres of trees and costing the state’s forest economy hundreds of millions of dollars.

Maine hopes to avoid a repeat of that ruin through an aggressive whack-a-mole management approach.

The state has expanded its ability to look for cocooning budworms before they emerge in the spring and begin to feed, giving landowners time to spray the infested stands with just enough pesticide to return a hot spot to non-threatening levels that can be kept in check by weather and natural predators, like birds.

New Brunswick has used this early intervention strategy to stop Quebec-based budworms at its borders.

Spruce budworm larvae leave behind partially eaten needles that dry out and turn a reddish-brown color in July and August, a telltale sign of an outbreak. State entomologist Michael Parisio identified 3,000 acres of damaged spruce-fir forest, seen here, during an aerial survey of northwest Aroostook County in July. Courtesy of Maine Forest Service

“Think of it like whack-a-mole,” said Angela Mech, a University of Maine associate professor of forest entomology. “We try to hit it as soon as you see an uptick in the population, not after it explodes. That way we can contain it before it causes significant defoliation and creates a landscape of dead trees.”

To do this, Maine opened the country’s only spruce budworm lab, where limbs from hundreds of areas across Maine are taken each winter and inspected for signs of overwintering budworms: cocoons. Now dormant, those cocooned larvae emerge in the spring with a hankering for fir-spruce buds.

Seven or more cocoons per branch mean the limb collection area is at risk of defoliation in the spring. Mech will share the counts with landowners, who may decide to spray the area with pesticides that coat the needles and will kill enough of the emerging larvae to knock a hot spot back down to normal levels.

Using this strategy, Maine hopes to keep budworm hot spots from erupting into an outbreak, Mech said.

The eastern spruce budworm, or Choristoneura fumiferana, is a native species but can be hard to find outside of an outbreak. The dark brown caterpillar has light-colored spots on its back and a dark, shiny head. It grows into a small, half-inch moth with mottled brown and gray wings.

Budworm caterpillars like to eat the buds off mature trees. They prefer balsam fir and white spruce, but when pressed they will eat other fir and spruce species, and older needles, too. Over time, trees don’t have enough needles to convert sunlight into food and they die.

Trees that are damaged but not killed by budworms become more vulnerable to other insects such as bark beetles and wood borers and diseases like root rot, which can cause a decline in the commercial value of the trees and indirectly lead to their death.

In Maine, where a third of all forestland contains spruce and fir, the budworm is big news. During the last outbreak, defoliation caused 84%-97% mortality in impacted balsam fir and 30%-66% mortality in impacted red spruce.

ECONOMIC IMPACT

Economists focus on the financial impact of a budworm outbreak, which in the past sparked a rush to harvest spruce and fir before the trees died, a devaluation of the pulp market, and the loss of hundreds of logging jobs once the market collapsed. Foresters from that time refer to it as the Battle of the Budworm.

“It was the most significant economic event in the history of Maine’s forest industry,” Parisio said. “While it was concentrated in the north, the outbreak touched every corner in Maine. In some areas, everywhere you looked, all you could see were rust-colored trees.”

An outbreak could hurt Maine’s wildlife, tourism, and its efforts to achieve carbon neutrality.

The lifecycle of an eastern spruce budworm Courtesy of Maine Department of Agriculture, Conservation and Forestry

For example, the spruce-fir forests in Maine’s colder, higher-elevation regions create unique habitats for Bicknell’s thrush, which Maine added to its endangered species list last year. Also, fir and spruce forests provide critical deer wintering areas.

A budworm outbreak also could reduce the amount of heat-trapping carbon dioxide emissions removed from the atmosphere by Maine’s spruce-fir forests by 5%-16%, depending on the severity, according to Professor Adam Daigneault, director of the University of Maine’s School of Forest Resources.

Such defoliation wouldn’t affect Maine’s efforts to hit its 2030 and 2050 carbon reduction goals, but it would make it harder for Maine to achieve carbon neutrality by 2045. Carbon neutrality is when the amount of carbon emissions is offset by the amount absorbed or removed by carbon sinks.

The budworm hotspots popping up in Maine now are connected to a massive outbreak in Quebec that began in 2006 on the north shore of the St. Lawrence River. A decade into the outbreak, the defoliated area in southern Quebec was the size of Maine. By 2020, over 33.5 million acres had been damaged.

A decade of early intervention work kept the Quebec outbreak out of Maine. But in 2019, a large group rode high winds into Aroostook County. In 2021, aerial spotters found 850 discolored acres in northern Aroostook County. And in July, Parisio spotted 3,000 denuded acres near Little East Lake.

The Maine Forest Service can’t afford to survey the whole state by plane. Some years, the state can’t do them at all. Bad weather can ground planes during the critical larval feeding time. In 2023, smoke from Canadian wildfires obscured their view.

Dead adult spruce budworm moths. The dark brown caterpillar with light-colored spots on its back and a dark, shiny head grows into a small, half-inch moth with mottled gray and brown wings. Courtesy of Maine Forest Service

Models suggest that the actual amount of fir and spruce acreage damaged by budworm feeding in 2024 is closer to 10 times the 3,000 acres identified in the Maine Forest Service’s aerial surveying, said Neil Thompson, associate professor of forestry at the University of Maine at Fort Kent.

PERSISTENT AND EXPANDING

It is too early to estimate the size of Maine’s spruce budworm population in 2025 from the overwintering sampling because the state lab has only processed a third of the branches to be tested, Thompson said. But, he noted, local budworm hotspots appear to be persistent and expanding.

“We have a massive population across the border that is inflating our numbers, with probably half of our moths coming over from Canada and half expanding out from local hot spots,” Thompson said. “But it kind of doesn’t matter. The budworm doesn’t stop at the border. Their problem is our problem.”

A budworm moth can fly 20 to 30 miles on its own wing power, Thompson said, but a few good gusts of wind can expand its flight range to several hundred miles, putting Aroostook County within easy reach. Quebec City is less than 100 miles from the international border.

One grim upside of a potential invasion is that it may be the last budworm outbreak Maine will face. Maine is at the southern edge of the budworm’s range. Thirty to 60 years after the coming outbreak ends, temperatures here will likely be too warm for the spruce budworm, Parisio said.

“Unfortunately, that will also be too warm for fir and spruce,” he said. “I wouldn’t call that a win.”