Invasive Threats to Maine's Forests and Trees

What is an Invasive Species?

Under Executive Order 13112, a species is considered invasive if it is not native to the ecosystem in question and its introduction causes or is likely to cause economic or environmental harm or harm to human health. Gateway to invasive species information. (USDA)

Report Suspected Findings in Maine

Wood Borers: These insects' worm-like larvae develop beneath the bark or within the wood leaving tunnels as evidence; emerging adults leave holes in the bark.

Adult Asian longhorned beetle.  Resinosis from brown spruce longhorned beetle attack. Photo NRCAN Emerald ash borer adults.  Photo: Maine Department of Agriculture

Piercing-Sucking Insects: These insects feed on fluids of the host plant. Many spend a significant part of their lives attached to the host plant.

Heavy elongate hemlock scale infestation. Photo: Maine Forest Service Hemlock woolly adelgid.  Photo: Maine Department of Agriculture, Plant Industry Red Pine Scale.

Defoliators: These insects consume the foliage of host plants. The two featured here do their damage as larvae.

Late instar (older) browntail moth larvae.  Photo, MFS. winter moth on side of tree

Diseases: The disease featured here had devastating effects in parts of the Western United States and has a broad host range.

Beech Leaf Disease, Credit: Jim Chatfield, OSU Extension Oak Wilt Joe OBrien USFS Bugwood Bleeding cankers on Quercus.  Department of Environment Food and Rural Affairs (DEFRA), United Kingdom

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Firewood: Burn it where you buy it!

Our native trees and forests are being threatened by invasive insects and diseases that live in dead and dying wood. In many cases, these pests are being accidentally spread to new locations by homeowners and recreationists moving firewood from one location to another.

MORE...Buy it where you burn it, don't give bugs a free ride.

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Wood Borers

Asian Longhorned Beetle Asian longhorned beetle (left) beside native whitespotted sawyer.  Photo PA DCNR

Origin: Asia

Hosts: Hardwood trees, especially maples.

**This insect can be spread on firewood. Please leave your firewood at home.**

Nearest Known Occurrences: The Asian longhorned beetle has been detected in Worcester and Boston Massachusetts (More Information). New York City area in New York and adjacent areas in New Jersey.

Description: Glossy black (think patent leather), very smooth beetle with white spots on the wings. Antennae are at least the length of the body and banded with black and white (how to tell Asian longhorned beetle from whitespotted sawyer).

Signs and Symptoms: Oval to round wounds on the bark where the females have chewed out a site to deposit their eggs. Round emergence holes in the trunks and branches of trees. Piles of coarse sawdust at the base of trees.(For images and more detailed descriptions, see DACF, Maine Bug Watch’s Damage Page)

Damage: Tunneling by larvae girdles tree stems and branches. This leads to dieback of the tree crown and eventual death of the tree. (Images of Asian longhorned beetle damage in trees)

MORE. . .


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Brown spruce longhorn beetle adult, Photo CFIABrown Spruce Longhorn Beetle

Origin:  Europe

Hosts: Spruce (usually), Fir, Pine, Larch (secondary), Hardwoods (rare)

Nearest Known Occurrences: Nova Scotia, Canada.

Description:  Flattened brown beetle, very similar to native long-horned beetles.  Reddish-brown antennae ½ length of body.

Signs and Symptoms:  Crown decline. Oval to round holes in bark, resin streams down the stem.  Coarse sawdust at the base of the tree, on the stem and/or packed into the holes.  Flattened, meandering larval feeding tunnels under the bark. L-shaped pupation chamber in the sapwood.

Damage:  This beetle generally attacks unhealthy trees in its native environment.  In Nova Scotia it has been found to attack and kill healthy trees. 



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Adult Emerald Ash Borer, Photo: Howard Russell, MSU

Emerald Ash Borer

Origin:  Asia

**Many new infestations center around campgrounds, implicating camp firewood in this insect’s spread**

Hosts: Ash

Nearest Known Occurrences: Maine; Edmundston, NB; New Hampshire, Northeastern Massachusetts (Where else is EAB?).

Description: Metallic green beetle with wings and body tapered towards the rear. 

Signs and Symptoms:  Symptoms and signs include D-shaped adult exit holes, bark splitting, serpentine frass-filled (sawdust-like waste) feeding galleries, wood pecker feeding, crown dieback, and epicormic shoots (whips growing off the trunk and branches). Many of these symptoms and signs are similar to other insects and diseases of ash.  Is it EAB?

Damage:  Larval feeding under the bark girdles and kills ash trees.  Since its discovery in the United States in 2002 emerald ash borer has killed millions of ash trees. 

Biosurveillance: Biosurveillance uses one living organism to monitor for another. A native non-stinging wasp, Cerceris fumipennis, is a efficient survey tool for detecting emerald ash borer.

Purple Traps: Large, purple, sticky traps are hung in ash trees to help look for the emerald ash borer.



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Piercing-Sucking Insects

Elongate Hemlock ScaleFemale (brown) and male (white) elongate hemlock scale.  Photo: Maine Forest Service

Origin: Japan

Hosts: Hemlock, spruce, fir = primary hosts. Secondary coniferous hosts only usually infested in the presence of heavy scale populations on primary hosts.

Nearest Known Occurrences: To date EHS has been found on planted hemlocks in southern Maine and in the forest in Kittery Point, ME.

Description: A member of the armored scale insects, females are covered by a smooth, yellow-brown parallel-sided waxy covering, males by an white, elongate covering. Size ranges from 0.1mm to 2mm.

Signs and Symptoms: Mottled, yellowing needles, thinning foliage. Coverings of females and males (described above) and thread-like floss. Where to look: Planted hemlock, spruce and fir. Planted and natural hemlock in areas affected by hemlock woolly adelgid.

Damage: Needle mortality, crown thinning, tree decline and mortality. Branch dieback, as in hemlock woolly adelgid starts in the bottom branches of the crown. Decline may be more rapid in the presence of hemlock woolly adelgid (described above)



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Hemlock woolly adelgid wool and lady beetle predator.  Photo: Maine forest serviceHemlock Woolly Adelgid

Origin: Japan

Hosts: Hemlocks

Nearest Known Occurrences: ** Hemlock woolly adelgid is known to be established in southern coastal Maine east to Knox County. Additionally, it has been repeatedly found in ornamental hemlocks, both planted less than 20 years ago and and more well established, in coastal Hancock County.

Description: Hemlock woolly adelgid is a small, aphid-like insect covered with white, waxy wool-like material.  This covering makes the insect resemble miniature cotton balls. It is most visible from late-October through July.  Wool masses are located on the undersides of the twigs at the bases of the needles (not on the needle, but on the twig).

Signs and Symptoms: The white waxy cotton ball-like covering of this adelgid is the most obvious sign of this insect.

Damage: Feeding by the adelgid leads to needle loss, crown thinning and dieback and eventual mortality of trees. Decline may be more rapid in the presence of elongate hemlock scale (described below).

Seasons: Due to the life-cycle of hemlock woolly adelgid, work should be conducted in hemlock in late-summer through winter.  During this period, the adelgid is least likely to be in egg and crawler forms. A single egg or crawler can start a new population of hemlock woolly adelgid.

Rules: There is a quarantine on hemlock woolly adelgid which also affects movement of hemlock branches and top material as well as rooted hemlock. 



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Red Pine ScaleRed Pine Scale

Origin: Japan

Hosts: Red pine scale is known to infest red pine (Pinus resinosa), Japanese red pine (P. densiflora), Japanese black pine (P. thunbergii) and Chinese pine (P tabulaeformis).

Nearest Known Occurrences: Red pine scale was detected in Mount Desert, Hancock County, Maine in September 2014.  This is the first known occurrence in Maine. It is found throughout southern New England, New York, New Jersey and eastern Pennsylvania.

Red Pine Scale in red pine standDescription: Red pine scale is one of the Matsucoccus scales, a soft scale.  Most recognizable stages of the insect are covered in white, waxy, wool like material.  The sub-adult males spin capsule-shaped, loosely woven cocoons which can be found on the branches of affected trees.  The females create wispy, woolly ovisacs.  When the immature insect is settled on the tree it resembles a smooth, waxy pod, lacking antennae and legs, but adorned with sparse tufts of wool.

Signs and Symptoms: Off-colored needles progressing from olive-green, to yellow, to red—first on individual branches, then whole trees.  White wool-covered insects (as described above).

Damage: needle, branch and whole tree mortality.



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Browntail Moth Winter web of browntail moth.  Photo: Maine Forest Service

Origin: Europe

Hosts: Hardwoods (oak, shadbush, apple, cherry, beach plum, and rugosa rose)

Nearest Known Occurrences: Browntail moth is known to be established in Southern and Coastal Maine and Cape Cod Massachusetts.

Description: Large larvae, about 1 1/2 inches long, are dark brown and have a broken white stripe on each side of the body and conspicuous, unpaired, reddish spots on the posterior end of the back. is dark brown and hairy, with parallel white markings running down the back. Reddish spots will be apparent in early instars as well.

Signs and Symptoms: Feeding by the browntail moth will vary with the season. Larvae overwinter, so feeding begins early in the spring, damage is visible as the leaves unfurl. As the larvae grow the consume larger and larger chunks of the leaves, and can completely defoliate host trees. Affected trees usually refoliate within the same season. After the overwintering larvae hatch in the summer, they will skeletonize leaves, and tie them with silk to the host branches. Winter webs on the outside edges of host crowns are conspicuous, tightly woven clusters of skeletonized leaves silk and sometimes fruit, filled with tiny larvae and frass. Winter web abundance can be a good barometer of summer suffering.

Damage: Although browntail moth is a forest pest, and can cause mortality of host trees, the biggest impacts are on human health and economics. Exposure to irritating hairs can cause a wide range of symptoms, from mild to very severe. See our Fact Sheet for more information.

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Winter Moth Male winter moth. Photo: Bo Zamba

Origin: Europe

Hosts: Primarily hardwoods (Oaks, Maples, Ashes, Cherries, Apple, Blueberries)

Known Occurrences: Winter moth is known to be established in Coastal Maine From Kittery to Bar Harbor. Defoliation by this pest was first noted in May 2012.

Winter moth adult female.  Photo P. Johnson.Description: Larva is a pale green inchworm caterpillar. Adult males (photo right) are delicate tan moths, part of the "hunters moth" group so named because species in this group are typically seen in late fall and early winter. Adult females (photo left) have vestigial wings, but cannot fly, they are most easily found by investigating the focus of male moth activity (other than your lights). Winter moth is very closely related to a native moth, the Bruce spanworm (Operophtera bruceata), dissection or DNA analysis are usually required to tell adult males apart.

Winter moth cterpillars in a handSigns and Symptoms: Feeding by the winter moth caterpillars begins before buds burst in the spring. Early feeding damage to expanding leaves can be damaged to the point that they look like swiss cheese. Later in the season trees in areas with heavy infestations can be completely stripped of foliage. Caterpillars are present from before the leaves emerge to mid June. (Photo guide to damage, PDF | 3.3 MB)

Damage: Defoliation leads to loss of capacity to produce energy. Severe defoliation over several years can cause tree mortality, as has been the case in Massachusetts in recent years.

Human Aided Spread: Winter moth caterpillars pupate in the ground and can be moved in soil from late May through December. During this time period moving plant material associated with soil is not recommended.  Caterpillars can also be accidentally spread on and in cars, boats and other conveyances.

Biocontrol program: A parasitoid fly called Cyzenis albicans has proven to be an effective biocontrol agent against winter moth in other parts of New England. The fly is very host specific with little to no non target effects. We have been releasing C. albicans in Maine since 2013 with a list of release sites including Harpswell, Cape Elizabeth, Kittery, South Portland, Vinalhaven, Portland, Bath, Boothbay harbor and East Boothbay Harbor.In it’s native range in Europe winter moth caterpillars are an important food source for migrants and birds gearing up for nesting season since it is one of the first caterpillars available in the spring. Fortunately here in the northeast, birds have begun to key into this food resource and give some supplemental control.


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Beech Leaf Disease (BLD)

Dark banding between beech leaf veins is a symptom of BLD.
Dark banding between beech leaf veins is a symptom of BLD.

Origin: Unconfirmed

Hosts: All beech trees are susceptible to BLD, including European and Asian species (this includes beech cultivars available in the nursery trade).

Nearest Known Occurrences: BLD was first detected in Maine in Lincolnville (Waldo County) in 2021. Symptoms of the disease have also been seen in other Maine counties. The exact extent of the disease is unknown, and may be widespread. BLD was first recorded in Ohio in 2012.

Description: A microscopic roundworm (the nematode, Litylenchus crenatae) has been consistently associated with symptoms, but the role of the nematode and other microorganisms in the proposed disease complex is not fully understood. Mature beech can be significantly stressed by BLD, and in some cases killed. Impacts to understory beech can be severe, leading to tree death in 3 to 5 years. The interaction of BLD and beech bark disease may hasten beech tree decline.

Signs and Symptoms: BLD symptoms include dark-green striped bands between lateral leaf veins, reduced leaf size, leaf curling and premature leaf drop. The banded areas of leaves may be leathery in more advanced infections. As the disease progresses, buds are aborted in greater numbers leading to reduced canopy and tree death. Larger trees take longer to succumb to BLD.

Damage: This disease has caused extensive mortality of understory beech trees wherever it has become established. Beech is a common tree species in Maine with high ecological importance, already compromised by the beech bark disease complex.



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Oak Wilt Joe OBrien USFS Bugwood

Oak Wilt Disease

Origin: Unconfirmed

Hosts: Oaks; symptoms are severe and end in mortality on species in the red oak group (oaks with pointed leaf tips). Species in the white oak group (oaks with rounded leaf tips) are also susceptible but decline more slowly.

Nearest Known Occurrences: Tree damage and mortality by the causal fungus, Bretziella fagacearum, has been confirmed in three locations on Long Island, New York, one location in eastern and one in western New York State. Oak wilt has not been detected in Maine.

Description: The disease causes wilting symptoms leading to mortality and is vectored (spread) by sap-feeding beetles that are attracted to wound areas on trees. The beetles can carry spores from infected oaks to recently wounded oak trees. Once oak wilt is established in a stand of oak trees, it can spread below ground through tree root grafts (junctions where adjacent trees join their root systems) creating pockets of oak mortality.

Signs and Symptoms: The oak wilt fungus kills the xylem tissues in the inner bark leading to wilting and killing of branches (flagging). As the disease progresses in the tree, wilting and branch mortality expands with trees dying in the same year or the next year, depending on timing and mode of transmission. When the disease is spread through the roots, the entire tree will die rapidly. The disease persists for several years in white oak species, causing noticeable but less-severe symptoms and an overall slower tree decline. White oaks can serve as a reservoir of oak wilt disease.

Oak Wilt Ronald F. Billings_bugwood

Damage: This disease has caused mortality of oaks in the Midwestern United States for decades and is a threat to oaks in natural and residential areas. Damage to oak trees via pruning or otherwise from April to October should be strictly avoided in areas where the oak wilt pathogen is present. This is because the insect vectors of the fungus spread the disease from infected trees to wounds on otherwise healthy trees. If oak trees are wounded during this time or must be pruned, a wound dressing or latex paint should be thoroughly applied to avoid attracting the beetle vectors.


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Sudden Oak Death Oozing canker of Phytopthora ramorum infected oak.  USFS photo

Origin: Unknown

Hosts: Numerous (see list on APHIS site)

Nearest Known Occurrences: Canker damage occurs in California and Oregon. The disease causing organism, Phytophthora ramorum, was found on one potted lilac plant in Maine, but it is not believed to be established in Maine. Surveys of four forested watersheds in central and southern Maine were conducted during 2007, with no P. ramorum found at any location.

Description: The causal agent of sudden oak death is a fungus-like micro-organism called Phytophthora ramorum. Laboratory tests are necessary to confirm this species as the cause of disease.

Signs and Symptoms: P. ramorum causes two types of symptoms. In "bark canker" hosts, such as oaks, large cankers can be found on the trunk or main stem. Crown browning will also occur. In "foliar hosts", leaf blight (gray to brown lesions) and twig dieback may be found.

Damage: This disease has caused mortality of oaks and tanoaks in parts of the western United States (bark canker hosts). Foliar hosts may serve as a reservoir for disease inoculum.


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