NISAW: An A to eZ List of Invasive Threats to Forests and Trees

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NISAW Logo - National Invasive Species Awareness Week

An A to eZ List of Invasive Threats to Forests and Trees

Invasive species are among the biggest threats to the health of our forests and trees today. Here we provide a short list of pests and pathogens that you can help us look out for. 

Some, marked with *, are readily identified this time of year. For others, this is a great time to learn so that you are ready to identify these issues when spring arrives. 

* Asian Longhorned Beetle (Hardwoods) *

Not only top of the alphabet, but a number one concern, the Asian longhorned beetle is a woodboring insect whose grub-like larvae tunnel under the bark and into the wood of hardwood trees. Infestations of this pest are most often found by community members who are not tree or insect scientists. Sometimes people who take notice of this pest think trees have been vandalized because of the many perfectly round holes created by adult beetles leaving the attacked trees. Other times, there is fear that the large beetle may pose a danger to people.

The Asian longhorned Beetle has not been detected in Maine. Like many forest pests and diseases, Asian longhorned beetle is spread through the transport of infested firewood. Caught early, this insect can be eradicated from an area through removal of a handful of trees. As we enter the month dedicated to maple sugaring, learn how to recognize this beetle and the damage it causes in maples and other hardwood trees.

asian longhorned beetle adult

Adult Asian longhorned beetle and exit hole. Image: USDA

* Beech Leaf Disease (American Beech) *

The American beech has been ravaged by beech bark disease in Maine and much of the northeast. A second disease, beech leaf disease, is now spreading through beech forests and ornamental trees. In Maine, this disease was first found in the coastal community of Lincolnville in 2021. It has since been found in all but five Maine Counties.

American beech is among our native tree species with marcescent leaves. Many fail to shed all their leaves in the fall. This trait can be used to survey for beech leaf disease throughout winter. To identify potential beech leaf disease, look for dark banding between the veins of beech leaves. Let us know if you think you’ve found beech leaf disease in a new area.

Winter signs of beech leaf disease

Dark interveinal banding is characteristic of beech leaf disease. Image: MFS

Beech Leaf Mining Weevil (American Beech)

The beech leaf mining weevil has not been found in Maine, however it has been in nearby Atlantic Canada for more than a decade and it is killing beech trees in Nova Scotia. The adults overwinter in cracks and crevices and are likely excellent hitchhikers.

The small weevil larvae create characteristic mines between the outer layers of beech leaves. The mine starts at the midrib of beech a beech leaf then expands outwards towards the leaf margin. When you’re in beech woods this summer, keep an eye out for the tell-tale mines of this species.

Beech leaf mining weevil larval damage. Image: NRCan

Beech leaf mining weevil larval and adult damage. Both the expanding mine beginning at the midrib and small holes from adult feeding are characteristic damage from this insect on beech.Image: NRCan

* Browntail Moth (Hardwoods) *

Browntail moth has been in Maine for more than a century. We are currently experiencing an historic outbreak of this pest that began in 2015. The caterpillars cause tree defoliation and their hairs cause rashes and other symptoms in humans. You can reduce your chances of encountering this pest on your property by learning how to recognize and their winter webs and reduce their populations. Here are some tips for:

  1. Identifying and removing winter webs.
  2. Managing browntail moth populations in large trees.
Browntail moth web in February

Winter webs of browntail moth. Image: DACF

* Emerald Ash Borer (Ash Trees) *

Emerald ash borer was detected in Maine in 2018. It has since been found in seven counties, and it continues to spread to new areas.

Bright specks on stems of ash provide a tip that woodpeckers have been searching for food. In some cases, those specks also reveal activity by the invasive emerald ash borer. Dark chips of bark littering the snow are another sign that woodpeckers have been foraging for food just beneath the bark of trees. These signs are often the first clue to this pest's arrival in an area.

If you see evidence of woodpeckers dining on emerald ash borer outside of where the pest is known to be established, let us know.

Ash tree with blonding and emerald ash borer galleries. Image: MFS

Ash tree with blonding and emerald ash borer larval galleries revealed beneath the bark. Image: MFS

* Hemlock Woolly Adelgid (Hemlocks) *

Hemlock woolly adelgid, a piercing-sucking insect native to Asia and the Pacific Northwest, has been on the move in Maine. Use these tips to search for hemlock woolly adelgid:

  1. Look at hemlocks. This insect is not found on other plants in North America.
  2. Inspect the undersides of branches with needles. Evidence of hemlock woolly adelgid will be found on the last couple of years of growth. This will be at the tips of fine branches. 
  3. Look for masses resembling discrete cotton balls attached where the needles join the twig. The insect inserts its straw-like mouthparts into the woody cushion at the base of the needle. 

Pro tip: fresh twigs brought to the ground by animals, weather or harvest are great places to look for hemlock woolly adelgid.

Senior Entomology Technician, Wayne Searles, provides additional pointers in this short video.

Take a look at hemlocks near you and if you think you’ve found this pest in a new area, let us know.

Hemlock woolly adelgid in March

Hemlock twig with white woolly masses attached to the twig, characteristic of hemlock woolly adelgid. Image: MFS

Jumping Worms (Forests)

Jumping worms have been in Maine for decades, but awareness of them, and their distribution are both expanding. Few forest infestations of jumping worms are known in Maine. Prevention of their spread into forests is the best thing we can do for forest health when it comes to this pest. How do you slow the spread?

  • Don’t use jumping worms for fishing bait, and if you use any worms, don’t dispose of them in the environment.
  • When moving from a site, remove material that can spread jumping worms. Soil on boots, tires, and other items can carry jumping worm cocoons from one location to another. Clean off soil between sites.
  • Don’t dispose of landscape material such as plants, compost and mulch in the woods.

You can learn more in this brief video from the Cooperative Forest Research Unit.

Hardwood forest soil in Oakland with texture characteristic of jumping worm invasion. Image: MFS

Hardwood forest soil with texture characteristic of jumping worm invasion. Image: MFS

Oak Wilt Disease (Oaks)

Oak wilt disease has not been detected in Maine but is a pathogen of significant concern and therefore a focus of early detection efforts. Oak wilt has been found in trees in several locations in New York State, the closest known locations of this disease. This disease has caused mortality of oaks in the Midwestern United States for decades and is a threat to oaks in forest and residential settings.

* It is important to think about oak wilt disease this time of year. Wounds created during the growing season can attract beetles that carry the spores of oak wilt. This is an added incentive to complete browntail moth web removal and other pruning activities on oak before April! *

Learn about oak wilt disease in this recorded webinar and be on the lookout for signs of attack this growing season.

oak wilt

Oak wilt symptoms on red oak leaves, Pittsburgh, PA. Image: MFS

* Red Pine Scale (Red Pine) *

Red pine scale is a soft scale originally from Japan. Tree death appears to come about fairly rapidly after the insect becomes established. This insect is known to be killing red pine in York, Hancock and Washington Counties. It is likely responsible for red pine death in other parts of Maine as well.

Many stages of this insect are associated with 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. These immatures are often hidden beneath bark flakes. Needles on affected branches and trees first turn olive green, then brick red.

Red pine impacted by red pine scale

Young red pine impacted by red pine scale. Image: MFS

Winter Moth (Hardwoods)

Winter moth can be found in coastal Maine from Kittery to Lubec.

Winter moth has been effectively managed through release of a host-specific parasitoid fly, Cyzenis albicans, in parts of southern New England. Through support of the Elkinton Lab at the University of Massachusetts and the US Forest Service, Maine Forest Service is releasing this parasitoid in Maine. We are in the eleventh year of this effort and have had successes in establishing populations of the fly in small areas from Kittery to Penobscot Bay. You can help efforts to control winter moth:

  • Let us know where you suspect winter moth caterpillars have stripped the leaves from trees.
    • Look for Swiss cheese feeding pattern, developing to whole leaf consumption (feeding often starts as buds swell, and peaks towards the end of May).
      • Trees stripped by browntail moth caterpillars (similar timing) will have visible masses from winter webs towards the tips of the branches (no need to report these, unless they are in new areas)
    • Winter moth caterpillars are green “inchworms.”
  • Reduce the use of insecticides and use an integrated approach to pest management in the outdoor spaces around your homes and businesses. The introduced fly and a multitude of soil-dwelling predators that help it control winter moth are vulnerable to insecticides.
  • Be aware that winter moth pupae are in the soil from June through December. One of the ways winter moth is transported to new areas is by moving plants with soil attached, mulch and similar material from infested areas. This should be top of mind when purchasing or selling plants at local plant sales, or moving plants between properties, such as between your house and camp. If plants are moved, consider removing all soil from their roots (bare root). 
Winter moth damage and caterpillars on oak

Winter moth damage and caterpillars on red oak in early May. Image: MFS

Elm Zigzag Sawfly (Elms)

Like American beech, American elm has a legacy invasive threat, Dutch elm disease. There is a newcomer, the elm zigzag sawfly, an insect that feeds on the leaves of this tree, sometimes consuming all the leaves on trees.

This sawfly has not been confirmed in Maine. We welcome your observations of the characteristic feeding of this pest observed on elm and related species in Maine. If you see a larva along with the damage, please collect it so the identity can be confirmed.

Elm zigzag sawfly feeding damage, Massachusetts. Nicole Keleher MA DCR

Elm zigzag sawfly feeding damage, Massachusetts. Image: N. Keleher, MA DCR

You play an important part in reducing impacts from invasive forest pests. Increase your impact:

Report a Forest Pest or Pathogen!

Let us know if you have spotted any of these or notice other forest health concerns. We have an on-line reporting form, or you can reach us by phone (207) 287-2431 or email.

If you think you’ve found concerning forest or tree damage or an insect invader, please take a picture and accurately record the location. If it is possible to collect a sample without risking the spread of the pest, we encourage you to do so. Be aware that some insects will chew out of plastic bags, hard sided containers are best.

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