Forest & Shade Tree – Insect & Disease Conditions for Maine

July 25, 2025

In This Edition:

• 2024 Annual Summary Report
• Climate Summary- June 2025
• Insects
• Diseases and Environmental Issues
• Invasive Plant Spotlight
• This Month in Conditions Report History: July 5, 2005
• Calendar

2024 Annual Summary Report

The 2024 Forest and Shade Tree Insect and Disease Conditions for Maine report is available now on the Maine Forest Service, Forest Health and Monitoring’s Conditions Reports page under the Annual Summary Reports drop-down. This is the 35th annual summary in the series that recaps forest health observations and activities.

Climate Summary- June 2025

Image: (left) The percentage of normal precipitation in June 2025 compared to the normal precipitation averages from climate records dated 1991-2020; (right) The average difference in temperature in June 2025 compared to the average normal temperatures for the same month from 1991-2020. Source: Northeast Regional Climate Center.

Maine had a cooler and drier June compared to last year, but saw warmer temperatures overall compared to climate data from 1991-2020. The average temperature throughout the state in June was 61.9°F; about 2.1°F warmer than average. A significant heat wave in the middle of the month set a new high temperature record for Bangor at 98°F, beating the previous record of 93°F set in 1995. Similarly, Augusta reached 100°F, tying the hottest temperature ever recorded for the city. Temperatures are likely to continue to be above average for July and the next three months (National Weather Service).

In June, precipitation was close to normal with 3.57 inches falling statewide. Some areas in northwestern and central Maine received much more rainfall; almost twice the normal amount of rainfall in June. Rain totals are expected to remain normal throughout July and the next three months (National Weather Service).

Most of the state currently has no drought conditions, except for areas in York and Cumberland County; accounting for about 4% of the state with abnormally dry conditions. This is very similar to the drought conditions present in June last year. Soil moisture was elevated throughout most of Maine in June and drought conditions are expected to remain the same (Climate Prediction Center).

Insects

Image: Map of the status of Asian longhorned beetle infestations in various states. Note that several introductions are now considered to be eradicated after years of intensive tree removals and ground survey work.

While several ALB infestations elsewhere have been eradicated over the years, a new population was most-recently detected in South Carolina in 2020. ALB remains of utmost concern for Maine, due to its wide host range and potential for widespread impacts, both environmental and economic. This is especially true for things like the maple syrup industry due to ALBs host preference for maples. ALB also reminds us that there is a very good reason that Maine’s firewood laws pertain to all hardwood species, and that these rules are not only in effect for ash and emerald ash borer. The transportation of illegal firewood into Maine from places with ALB infestations like New York, Massachusetts, and Ohio remains a very real concern, and we ask for your assistance in spreading the word to visitors and reporting any obvious violations of out-of-state firewood entering Maine. The only exception to this rule is the import of certified, heat-treated firewood that is documented to be pest free by regulatory officials.

Given the ability for damaging pests to hide for years, it is important to keep all un-certified firewood local, even that originating within Maine.

Image: Firewood can carry damaging insects and diseases. To protect forests, wildlife habitat, and the forest economy, always use local or heat-treated firewood.

Beech Leaf-Mining Weevil (Orchestes fagi)

In addition to beech bark disease and beech leaf disease, beech trees in Maine may soon have another problem knocking at their door. The beech leaf-mining weevil (Orchestes fagi) is a small, black beetle native to Europe which can highly damage beech foliage, leading to eventual tree mortality in many cases.

First confirmed in North America in 2012 around Halifax, Nova Scotia, this insect has since been found in Prince Edward Island and New Brunswick as of 2020. While it hasn’t yet been spotted in Maine, it is on the move, and it’s worth keeping an eye out for. A long-term study in Nova Scotia documented significant damage and mortality in infested trees, with up to 88% dying after five to seven years of repeated defoliation.

Adult beetles emerge in spring to feed on beech leaves, creating small shot holes. Eggs are laid soon after in the center vein of the leaf, from where the larvae tunnel internally in the leaf tissue, beginning near the midrib and extending outward toward the margins. This results in a diagnostic pattern of thin, linear “mines” which widen as they go, leading to browning tissue which may fall away, giving the leaf a tattered appearance.

Images: (left) Adult feeding holes and leaf mines are highly visible in ornamental beech species; (right) leaf mines begin at the central vein and expand outwards.

Reporting these types of mines is encouraged, particularly where mining appears to begin near the center of the leaf and progresses outward. This damage is distinct from the interveinal banding caused by beech leaf disease and the leaf distortion caused by wooly leaf aphid. All species of beech are potential hosts and trees in eastern Maine are most likely to experience initial introduction due to their proximity to known populations in Nova Scotia and New Brunswick.

Browntail Moth (Euproctis chrysorrhoea)

Browntail moth populations began to sharply decline in 2024, and we are seeing this pattern continue in 2025. Viral and fungal pathogens are likely contributing to decreasing populations in addition to natural population collapse after years of elevated populations and damage. Last year, we recorded roughly 2,119 acres of defoliation from browntail moth caterpillars, and preliminary aerial surveys this July suggest we may see a similar acreage of defoliation for 2025.

In late June, browntail moth caterpillars reached their full size and last developmental growth stage, where they slow down eating and begin to pupate. We observed the first adult browntail moths on July 6, roughly two weeks later than last year. This is consistent with their development this year as their emergence as caterpillars was also two weeks later compared to last year.

Adults are most active in July through early August and are highly attracted to porch lights, flood lights, spotlights, and any other outdoor lights that may be on at night. To avoid attracting browntail moths to your property, turn off any non-essential outdoor lights from now through early August. Fewer browntail adults means fewer browntail eggs laid, and therefore fewer browntail caterpillars on your property.

Adult browntail moths have white wings (sometimes with a single black dot on each forewing) and fuzzy white legs. Browntail moth adults also have a fuzzy reddish-brown abdomen, or “tail” – where they got their common name ‘browntail’! Although adult moths look hairy, the hairs that grow on the moth are not toxic, unlike the hairs present on the caterpillar. Moths may still pick up toxic caterpillar hairs when emerging from the pupal packet they constructed when they were caterpillars, so contact with adult browntail moths should also be avoided to prevent contracting a rash.

Images: (left) An adult browntail moth holding onto a stem with its characteristic underside revealed; (right) An adult browntail moth resting on a door near a porch light. Note the reddish-brown abdomen (“tail”) seen in both positions, indicative of browntail moth.

Maine has many white moths that are active at the same time as browntail moth – check out our update on browntail moth to see our comparison chart to better distinguish browntail from some of the other species you may see during flight season.

Emerald Ash Borer (Agrilus planipennis)

This month, a multi-agency effort between USDA-APHIS, the University of Maine’s APCAW (Ash Protection Collaboration Across Waponahkik) lab, and the Maine Forest Service resulted in over 200 ash trees being protected against emerald ash borer in Maine. In a two-day mobilization of the Northeastern Forest Fire Protection Compact Forest Health Working Team, 14 participants from APCAW, USDA-APHIS, Colby College, the New Hampshire Division of Forests and Lands, and MFS injected brown ash trees with emamectin benzoate as the first treatment of a 10-year integrated pest management study to better understand stand dynamics and survivorship of brown ash during EAB infestation. Biocontrol in the form of parasitoid wasps is also being released at the two sites, and monitoring and injections will continue across the next decade as EAB moves through Maine. This mobilization effort also served as an opportunity for collaborators to share knowledge including injection tools, techniques, and approaches to ash protection. Work was planned in close collaboration with Board of Pesticides Control staff to ensure all requirements were met for those assisting on the project.

Earlier in the month, while headed up Route 201 to work on spruce budworm monitoring in western Maine, a roadside ash tree with woodpecker damage revealed that emerald ash borer (EAB) has now made inroads in Solon, ME and marks the first county detection for Somerset County.

Expect a local expansion of the quarantine boundaries soon in response to this most recent Solon detection. We will continue to survey for EAB at other locations along Rt 201 given that it’s a thoroughfare for outdoor recreation in that part of the state. In the meantime, forest health & monitoring staff are busy checking the rest of Maine’s 201 statewide purple prism traps for adult EAB. These trap locations are focused on areas where EAB has not yet been found, and any new detections using purple prism traps could further shape any regulatory changes to take place during summer 2025. This far into the mid-season trap check, purple prism traps have detected EAB in northern Oxford County in the towns of Newry and Mexico and in southern Somerset County in Norridgewock. We expect all remaining traps to be checked by August 1.

Purple Prism Traps

This is also a good time to remind people that since purple prism traps are an early detection tool, it means that EAB is not yet known in that area. We have heard that sometimes people assume seeing these traps means that EAB is already present, which is not the case. If you are unsure whether EAB is found in your area, you can always view the EAB status of your town here.

Image: Location map for purple prism traps throughout Maine to monitor for emerald ash borer in 2025.

Forest Tent Caterpillar (Malacosoma disstria)

MFS was joined by UMaine Cooperative extension recently for a defoliation survey of sugar maples in Big Six Twp. In 2024, aerial surveys mapped approximately 3,385 acres of forest tent caterpillar (FTC) damage in the area, with ground observations revealing significant defoliation of overstory maples. At the time, MFS held a workshop for landowners and sugarbush operators to discuss FTC impacts, management options, and monitoring techniques. Following the workshop, landowners initiated an aerial spray campaign targeting FTC, which was completed earlier this spring.

This summer’s survey focused on evaluating sites both within the spray zones as well as in untreated areas which experienced less prior defoliation. Generally, tree canopy densities were similar among all sites and overall defoliation was low, with very little damage evident in the upper canopy. Caterpillars were observed at eye-level, though not nearly in the numbers currently affecting aspen in parts of Aroostook County.

Image: In addition to caterpillars, surveyors observed many “friendly flies”, which are a known natural enemy of FTC and most often responsible for the decline of periodic FTC outbreaks.

MFS later conducted an aerial survey of the region, which appeared to show some light dieback of maples in the previously defoliated stands. This is likely a result of the heavy caterpillar feeding a year ago, indicating that the FTC outbreak did have a lasting effect on the forest. Absent from the aerial survey observations was evidence of significant current-year defoliation or any significant mortality.

Satellite imagery was also examined to determine any effects from FTC. In June of 2024, distinct patches of defoliated trees were clearly visible via the Sentinel 2 satellite. Close to one year later, imagery shows those pockets to be largely unaffected. The surrounding area does not appear to show signs of defoliation either, indicating there was not substantial movement of adult moths into neighboring stands.

Image: Satellite images of Big Six Twp spread one year apart show the difference in defoliation of sugar maple stands, visible as brown patches in 2024 which are now green in 2025. Source: Sentinel 2.

Taken together, these findings suggest a combination of treatment with Btk insecticide, parasitism by friendly flies, and other natural mortality factors may have helped suppress FTC populations in the area. Conditions in the previously affected stands have improved markedly and may be reassuring to sugarbush operators concerned about long-term impacts. MFS will continue to monitor the region throughout the season. As always, any important developments or observations on the ground are appreciated and may be sent to foresthealth@maine.gov.

Fall Webworm (Hyphantria cunea)

Fall webworm is one of our native tent-building caterpillars found across Maine and much of the United States. The caterpillars have been making their presence known for the past few weeks with the construction of large webs that, when full sized, will encompass an entire branch. Common hosts include alder, apple, ash, birch, box-elder, cherry, elm, mulberry, poplar and willow. Mature caterpillars are a yellowish color, around one inch in length and have many tufts of long hair and black and brown markings. These caterpillars feed within the web as it protects them from predators and harsh weather. Control of this species generally isn’t warranted as the damage is mostly aesthetic and they rarely cause lasting harm to the trees since damage occurs late in the season after trees have already stored up much of their energy reserves from photosynthesis. The adult moths are pure white (sometimes with dark markings) and fly from May to July in the north and are attracted to lights.

Image: The beginning stage of web formation, young fall webworm caterpillars and feeding damage, Liberty, ME (Waldo County).

As a native species, it is part of the complex food web here in the northeast where it is food for many vertebrate predators, insect parasitoids, and insect predators. The parasitoid community around fall webworm is diverse and includes 22 species of flies in the family Tachinidae, which lay microscopic eggs on foliage that are ingested by the caterpillar hosts or eggs are laid directly on the host. This community also includes a wide range of wasps in the families Scelionidae and Pteromalidae. These fly and wasp parasitoids help control fall webworm during almost all stages of its life cycle, including eggs, larvae, and pupae.

Most of the time we hear about invasive species that arrive here from other parts of the world. Fall webworm is one of the species we’ve accidentally shipped overseas where it has become a problem since its natural controls remain here in North America. It was first accidentally transported to Yugoslavia in the 1940s and subsequently spread to the rest of Europe. There have been more recent introductions into northern China and North Korea. One of the parasitoid wasps from fall webworm’s native range, Psychophagus omnivorus, in the family Pteromalidae attacks fall webworm pupae. It has been reared and released for biological control in the affected countries in Europe and Asia.

Japanese Beetle (Popillia japonica)

The invasive Japanese beetle is usually thought of as a horticultural pest; the adults feed on well over 300 species of yard and garden plants, and the larvae can be a major turf pest. However, adults also feed on many species of trees, including linden, birch, Norway maple, and several fruit trees. The adults rarely kill trees, but cause unsightly damage to ornamental plants as well as feeding on flowers and fruits.

Control is generally not needed to protect trees, but hand-picking adults and knocking them into a container of soapy water can reduce populations in small yard areas. Although there is a pheromone trap for this insect, it is of value only as a monitoring tool, and research has shown that it draws more adult beetles into an area than it kills. Dry soil conditions lead to increased egg and larval mortality, so refrain from watering your lawn when possible, to reduce damage to turf. There is a parasitoid called the Winsome fly that lays its eggs on the thorax of the beetle and will kill it within 5-6 days. Although not available for purchase like some other biological control agents, it is well established in many areas of Maine. If you are hand-picking beetles look for a white dot (or multiple dots) on the thorax of the beetle and let those live. They will die within 5-6 days and more flies will emerge. Selective protection of parasitized beetles may lead to a reduced population of Japanese beetles over time.

This is also a good time to remind all that the University of Maine Cooperative Extension office is a good resource for ID of yard and garden pests and information about them. They can be reached at (207) 581-3880 and their website can be found at: https://extension.umaine.edu/ipm/

Spongy Moth (Lymantria dispar)

Spongy moth is an invasive hardwood defoliator that was introduced to North America over 150 years ago. It has been in Maine since the early 1900s, but only recently spread into the far reaches of Aroostook County. Our Northern Maine Senior Entomology Technician fielded an early-July report of hairy caterpillars in a stand in Woodland (Aroostook County) that had experienced significant feeding by forest tent caterpillar. He knew feeding was completed for forest tent caterpillar. Pictures showed that the culprit was the non-native spongy moth, with diagnostic markings at this stage of paired blue and red spots along the back of the caterpillar. Spongy moth feeds later in the year than forest tent and likes a lot of the same hardwood hosts (aspens, white birch and oak, for example).

A field visit revealed few live spongy moth larvae still feeding and a nice surprise of many cadavers of disease-killed spongy moth. Based on positions of the cadavers, it is believed that both the Lymantria dispar multicapsid nuclear polyhdrosis virus(LdMNPV) and fungal pathogen Entomophaga maimaiga were present in the population. Entomaphaga is one of the key natural controls of spongy moth and its detection in far northern Aroostook county, prior to any outbreak in the area is both promising and somewhat expected (See Hajek et. all in Environmental Entomology, 2021 vol. 50 no. 5).

Image: (left) Late instar spongy moth caterpillar (arrow) above a disease-killed caterpillar; (right) Disease-killed caterpillars. V-shaped position is typical of virus-killed caterpillars. Dried out caterpillars hanging straight down is typical of fungus-killed caterpillars. Dissection in the lab is necessary for confirmation.

Elsewhere in the state, we are in the second year of mapping mortality related to the western Maine spongy moth outbreak. This occurred primarily in oak and hemlock, but other species were also impacted. The areas mapped this year are adjacent to the almost 8,000 acres captured during last year’s aerial survey (Map page 83 of the 2024 annual summary report). Elsewhere we have seen scattered spongy moth larvae associated with defoliated stands that also had other caterpillar activity, such as in the Penobscot River Valley in southern Penobscot County. This may be an early sign of building populations in those areas.

We welcome your reports of observations of large numbers of spongy moth larvae (many will have completed feeding by now), especially associated with noticeable leaf consumption or re-foliation; high incidence of spongy moth adult activity (beginning around now in central Maine, and easiest to observe in the erratic flights of day-flying males) and, later in the summer, high densities of egg masses.

Spruce Budworm (Choristoneura fumiferana)

Although the true measure of the success of the 2025 spruce budworm (SBW) aerial spray program will be the overwintering larval data available at the end of the year, anecdotal evidence from recent aerial surveys shows favorable results on the landscape. Compared to 2024, areas along the western border with Quebec are not showing any visible discoloration or defoliation this year. We only noticed a few discolored areas immediately across the border in Quebec, which might also owe to substantial spraying efforts on the Canadian side in 2025 as well.

Even if initial results of the 2025 spray program prove favorable, we must keep in mind the regional nature of SBW and remember that this will not be a “one and done” management program. Regional dispersal models show that like seasons in our recent past, Maine likely received an influx of moths dispersing from core outbreak areas in Canada again in 2025, which can bolster Maine’s SBW populations depending on condition of dispersing moths.

Image: Canadian flight models show an active dispersal night on July 8 where moths from the Gaspe Peninsula in Quebec dispersed to the south and were deposited into northern Maine. Credit: 2025 Spruce Budworm Atmospheric Transport Model Team.

Unsprayed areas in northern Aroostook County are a different matter, however, and the degree of damage to appear in the greater Fort Kent area from one year to the next is impressive. Damage is not concentrated, but apparent on mature conifers scattered throughout mixed forest stands and on isolated conifers growing throughout town and surrounding residential areas.

Image: Drone-captured aerial photo of spruce budworm damage observed in 2025 in the areas south of Fort Kent in Aroostook County, ME.

In addition to our aerial survey efforts, we are very interested in learning about locations with SBW damage in the areas surrounding Fort Kent that we may not have flown directly over. If you live in Aroostook County and believe you have SBW damage on your trees, please reach out and submit photos of damage to foresthealth@maine.gov

Diseases and Environmental Issues

Severe Weather Events in 2025

While performing aerial survey in mid-July, we came across an unexpected patch of what appeared to be severe defoliation in southern Aroostook County. Since we typically keep tabs on the locations of defoliators from one year to the next, we were left scratching our heads over this seemingly random location, especially seeing as all tree species were affected. Fortunately, we were able to find a spot to land and quickly collect some branch samples. We found no evidence of our initial guesses of forest tent caterpillar or spongy moth, rather a forest full of shredded leaves and branches covered in small lesions with splitting bark, all located on the same side of the affected trees. This made a severe hail storm the most likely cause of this type of damage.

Fortunately, a quick message to our friends at the National Weather Service in Caribou confirmed our suspicions. They report that on July 3, 2025 at around 10:40 AM EDT, a severe thunderstorm tracked across this area with a large hail signature. Another tool indicates the Maximum Estimated Size of Hail (MESH) values were estimated up to 3″ in size. Though hailstones did not necessarily reach this huge size, they were certainly large enough to do a significant amount of damage and prove that you never know what you’ll happen upon in the woods.

Images: (left) Radar displaying a strong hail signature in purple and (right) the damage produced by hail, mimicking the appearance of defoliation by caterpillars.

Beech Leaf Disease (Litylenchus crenatae mccannii)

A new potassium phosphite-based treatment was highlighted in an update on beech leaf disease biology and management by The Connecticut Agricultural Experiment Station dated May 14, 2025. Basal bark drenches of potassium phosphite, are thought to work in the same way as the potassium phosphite soil drenches. It is presumed that more active ingredient is absorbed by the bark compared to the earlier-promoted potassium phosphite soil drench applications. There are several basal bark drench products available for purchase by the general public. Some basal bark drench products are listed in the linked bulletin from The Connecticut Agricultural Experiment Station found at the beginning of this section. Maine Forest Service began trialing the basal bark application method this month at several sites including close to 40 trees at eight locations in Kennebec, Lincoln, and Penobscot counties. Basal bark drenching can be done at a fraction of the cost of macroinjections of Arbotect 20-S (Thiabenzadole) and may be practical for larger-scale BLD management.

IMPORTANT: When considering beech leaf disease treatment/management options, it is important to keep in mind that when a chemical is used in pest management, it is considered a pesticide and for-hire applications or applications in public areas need to be done by properly licensed commercial pesticide applicators.

The potassium phosphite-based bark drench treatment is applied as follows:

• Make sure to always follow the label closely for best results and applicator safety.
• A potassium phosphite product labelled for use on beech trees is typically mixed with the appropriate amount of water and other materials as directed by the product label.
• The bark is wetted with the mixed potassium phosphite product via a hand sprayer applied directly onto beech bark from the root flares at the base of the tree to a height calculated based on the diameter of the tree.
  • For each inch of tree diameter measured at breast height (4.5 feet above the base of the tree ), the bark is drenched 1 foot in height. For example, a tree 5 inches in diameter will be thoroughly wetted with the mixed product from the tree’s base up to a height of 5 feet.
  • For trees more than 10 inches in diameter, wet the bark to a height that is practical. For example, a 20-inch diameter tree could be thoroughly wetted to 10-12 feet.
  • When applying the bark drench via a sprayer, use a moderate pressure to ensure good coverage but larger droplet size. Higher pressure discharges the product as more of a mist that does not provide good coverage and can lead to drift of the product to off-target areas.
• Apply the bark drench once in summer and follow the same procedure one month following the first application. The recommended window for treatments is between May and August, so now is not too late. Also, one application in late summer is better than no application at all for managing BLD symptoms in high-value beech.
• Repeat annually. It may be several years before symptoms improve, although some have reported positive results in the year following treatment.
• Potassium phosphite products may harm nearby vegetation if exposed, so be sure to protect nearby vegetation and follow all environmental warnings associated with the applied product. Also, to ensure applicator safety, personal protection equipment must be used in accordance with the product label.

Image: Basal bark drenching on the lower, middle and upper bole of a smooth-barked beech tree using a potassium phosphite product to manage BLD symptoms.

Dutch Elm Disease (Ophiostoma ulmi, Ophiostoma novo-ulmi)

Observations and reports of Dutch elm disease (DED) symptoms have been increasing since the beginning of July. The timing and number of reports is typical for this time of year in Maine. New infections result from elm bark beetles emerging from other infected trees and feeding on the branches of disease-free elm trees. Symptom expression and progression depend on the part of the tree infected and the mode of infection (which can depend on which species of bark beetle is responsible for disease spread, based on their feeding preferences). The fungus that causes DED is a vascular wilt pathogen, meaning that the fungus parasitizes vascular tissues which affects the ability of the tree to transport water and nutrients to areas of the tree distal to the point of infection. So, when a branch is infected with the DED fungus, the foliage on the infected branch turns yellow and dries out, causing a visible symptom referred to as ‘flagging’. As the infection progresses, defoliation and wilting symptoms become more prevalent. Trees that contracted the disease in the upper fine branches of elm trees this year will show mild discoloration (yellowing) and (flagging) and wilting that will expand and eventually lead to tree death, likely the next year. In situations where the DED fungus travels to a neighboring tree via a grafted (joined) root system, these newly infected trees can die rapidly due to infection of the root system. Once an American elm tree is infected there is little that can be done. The tree will likely die in the short term – there is no curative treatment, and pruning out flagging branches has not been found to be effective. There are several elm cultivars available that are resistant (not immune) to DED that may provide some of the excellent tree services of the American elm.

Images: (left) Elm trees dying from DED infection outside a Waterville area school; (right) A twig from the infected trees showing the brown streaked inner bark, a field diagnostic characteristic of DED infection.

Flagging Fir Branches

MFS staff, the public and others have observed and reported randomly dispersed dead balsam fir branches on trees in various settings throughout Maine. The needles on these affected branches are an eye-catching orange color. Stress-related canker fungi in the genera Phomopsis, Valsa (Cytospora, more frequently associated with spruce) and Diplodia (more frequently associated with pine tip dieback) have been recorded in other states parasitizing fir trees’ vascular tissues at the base of symptoms. Of course, other agents can be responsible for the damage, like insects, animals or storm damage. In any instance, these dead and dying branches can be pruned all the way back to the main stem, or at least 12 inches behind the beginning of symptoms, which may be indicated by a canker. This will improve the appearance of impacted trees and remove potential cankers that could eventually expand and form spore-producing structures near their margins leading to more future infections in the same tree and neighboring trees. Pruning equipment should be sterilized between cuts with rubbing alcohol or a 1:5 bleach water solution to make sure fungi are not spread via pruning. If practical, cultural management practices to address any source of stress are recommended.

Images: (left) A fir tree with several reddish flagging branches; (right) A close-up of a small branch with dieback, with the bark removed, showing the junction where dead tissue killed by the canker fungus is expanding into uninfected tissue.

Invasive Plant Spotlight

Many invasive species of thistle have become ubiquitous across the United States. Two species of invasive thistle, bull thistle (Cirsium vulgare) and Canada thistle (Cirsium arvense), have invaded all 50 states and much of Canada. These invasive species are extremely damaging to their surrounding environment. Thistles are notorious for outcompeting native plants, degrading wildlife habitat, and altering soil composition and hydrology. Additionally, Canada thistle is allelopathic, meaning it releases chemicals into the soil that are toxic to neighboring plants. The aggressive nature of these invasive plants highlights the importance of managing their spread and protecting Maine’s natural areas. However, several key differences between bull thistle and Canada thistle call for very different management techniques. This means that distinguishing between these two invasive thistles is imperative when planning to effectively manage their populations.

Images: Bull thistle (left); Canada thistle (right)

The most important difference between bull thistle and Canada thistle has to do with how they spread. While both species reproduce via seed production, Canada thistle (often called creeping thistle) also spreads through rhizomes. A modified portion of the stem, rhizomes run horizontally underground and periodically sprout new above ground shoots. Rhizomes help to make Canada thistle a fast and aggressive spreader and greatly increase above-ground and below-ground competition. While this is an important diagnostic feature of Canada thistle, it is also an essential aspect when considering the management strategy for this species. During removal, damaging the rhizome or leaving portions of the rhizome in the soil can encourage new growth. This means that mechanical or manual removal of Canada thistle is rarely effective on its own due to its tenacious rhizome structures. However, since bull thistle lacks a rhizome, manual and mechanical removal of bull thistle has proved to be very effective, demonstrating the importance of proper identification of these two species. Another way to distinguish between Canada and bull thistle is their leaves. Bull thistles have stiff white hairs on the upper side of their leaves, while the upper side of Canada thistle leaves are generally hairless or have very small, inconspicuous hairs.

Finally, while it is important to recognize invasive plants, it is equally important to identify and protect Maine’s native plants. There are a couple key traits that can be used to ensure that you are dealing with an invasive species, rather than a thistle that is native to Maine. Canada thistle is the only species of thistle found in Maine that reproduces via rhizomes. If you can locate a rhizome, then you are assuredly dealing with Canada thistle. Bull thistle is the only species of thistle in the state of Maine with decurrent leaf bases. This means that the leaves extend down the stem at their bases, giving them a winged appearance.

The USFS provides a review of Canada thistle on forested National Forest Inventory (NFI) plots in the Northeast. Maine is one of the 14 states in the 24-state region with this species detected on the forested plots within the NFI assessed for invasive plant presence.

For additional resources on invasive plant identification and management, refer to the GoBotany website or the MNAP Web Gallery and the Advisory List of Invasive Plants provided by the Maine Natural Areas Program (MNAP) website at www.maine.gov/dacf/mnap/. Please also visit our website (under the Invasive Species tab) to sign-up for a free iMapInvasives account and to view iMapInvasives help resources. iMapInvasives is an online mapping tool and the central repository for invasive plant data in Maine. iMapInvasives allows you to map the location of invasive species with photos, where you can also see other mapped locations of invasive plants and set up email alerts for your area or species of concern.

This Month in Conditions Report History: July 5, 2005

Ash Defoliator (Palpita magniferalis) – This is a little-known moth that has been defoliating ash trees in the Owls Head/Islesboro area for the past two years. Moth numbers are high again so defoliation from the larvae is expected this year as well. Larvae can be found feeding on the undersides of ash leaves under a thin film of silk. The damage becomes pronounced in late July to early August.

Calendar

August 20, 2025, 4:30 PM-6:30 PM, Sly Brook Rd in Fort Kent: Spruce Budworm Tour

Maine Forest Service Entomologists and District Foresters will host a free Spruce Budworm Woodland Tour with Maine Woodland Owners to hear from experts on spruce budworm and forestry, see spruce budworm damage firsthand in both treated and untreated areas, learn more about the upcoming cost share program for 2026 treatments. This event will also include Forester and Pesticide Credits, door prizes, and offer an opportunity to chat directly with experts about spruce budworm.

No registration required. Tour will start only 0.5 miles from the paved road. An information table will be staffed for those who just want to drop in to learn the basics. Rain date is 8/25/25 and updates will be posted on www.sprucebudwormmaine.org.

Conditions Report No. 3, 2025

On-line

Department of Agriculture Conservation & Forestry, Maine Forest Service – Forest Health and Monitoring

Contributors: Zoe Albion, Aaron Bergdahl, Amy Emery, Samuel Foye, Chad Hammer, Gabe LeMay, Allison Kanoti, Mike Parisio, Brittany Schappach, Thomas Schmeelk, Colleen Teerling, and Andy Whitman

Report a Pest