Forest & Shade Tree – Insect & Disease Conditions for Maine

June 18, 2025

In This Edition:

• Climate Summary – May 2025
• Insects
• Diseases and Environmental Issues
• Personnel Update
• This Month in Conditions Report History: June 19, 1978
• Calendar

Climate Summary – May 2025

During May 2025, statewide temperatures were normal or only slightly above normal (1.5°F) with an average temperature of 52.5°F. Temperatures ranged below freezing in northern Maine during May as they typically do, but no freezing temperatures were recorded in Bangor or further south for the month. Ice out on northern Maine lakes was several days earlier this year than in the 1990s (see Hodgkins et al., 2002. Int. J. Climatol. 22: 1819–1827). The timing of leaf out was close to normal in southern Maine but was a couple of days early in northern Maine (U.S. National Phenological Network). Temperatures are projected to be likely above normal for the next 7 days, the next month, and the next three months (National Weather Service).

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

Precipitation in May for much of the state was only rain; no snowfall was reported at either Caribou or Bangor. Rainfall totals in much of the state were significantly higher than normal during May 2025, with some areas of northern and southern Maine receiving over twice the normal average amount of rain. Statewide, the average rainfall total for May 2025 was 5.04 inches, roughly 1.65 inches above normal historic averages. Frequent days with rain may support native fungal pathogens that can reduce browntail moth, winter moth and other caterpillar populations. Precipitation levels are projected to have equal chances to be below or above normal for the next 7 days and month (National Weather Service).

Drought conditions for southern Maine have remained consistent from April through May 2025, with roughly 3.5% in abnormally dry conditions and the rest of the state (96.5%) with no notable drought conditions. Occasionally windy dry periods created some days with a very high level of fire danger, which was mitigated by frequent days with rain throughout the month.

Insects

Browntail Moth (Euproctis chrysorrhoea)

Browntail moth caterpillars continued to defoliate host trees throughout June and reached their last instar in most areas across the state. We have noted a decline in browntail moth caterpillar populations at a few of our monitoring sites and have noted this decline in other towns this year. Viral and fungal pathogens contributed to browntail moth caterpillar mortality in Durham, Newport, and Old Town, and may be contributing to caterpillar mortality in other towns too. We welcome any reports from the public of diseased browntail moth caterpillars.

Some signs of pathogens may be:

• White/yellow fungal spores on caterpillars (Fungus)
• Swollen, puffy caterpillars (Virus & Fungus)
• Liquid expulsion of the caterpillar guts, often the caterpillar rests in an upside-down “V” shape (Virus)
• Caterpillars hanging onto branches with just their rear prolegs (leg like extensions of the exoskeleton) (Fungus)
• Dead caterpillars

In mid-June, we noted the presence of silky pupal packets, a protective structure created by tying a few leaves together with silk by the late instar caterpillars. These pupal packets can be difficult to see, which helps shelter them from predators as they undergo pupation into a flying adult.

Image: A late instar browntail moth caterpillar creates a pupal packet by tying leaves together with white silk.

Now that we have seen the creation of pupal packets, browntail moth adults will emerge upon completion of pupation. Although adult browntail moths look hairy, they do not have toxic hairs found in the caterpillar stage, but they may pick up residual caterpillar hairs when emerging from their pupal packets. Last year, the first report of flying adult moths was recorded on June 26. Adult browntail moths are typically active from late June to early August and are highly attracted to lights. If possible, turning off outdoor lights may help dissuade browntail moth adults from flying toward and mating in the trees around your house – reducing the number of eggs laid (and future caterpillars!).

Fall cankerworm (Alsophila pometaria)

Fall cankerworm is one of our native geometrid moths which is in the same family as the non-native winter moth. The populations of this species occasionally surge, with outbreaks occurring for two to four years. This season we have observed increased activity of fall cankerworm in some areas of Maine including reports from Bath (Sagadahoc County), Brunswick (Cumberland County) and Wiscasset (Lincoln County). We are also hearing of elevated populations elsewhere in the northeast including southern New England and Pennsylvania where a treatment program for fall cankerworm occurred this spring for the first time since 1996. We have also observed some fall cankerworm caterpillars during our winter moth collections. Due to the current outbreak of winter moth in Midcoast Maine, some of the damage from these two species may overlap and obscure the true scope of the fall cankerworm damage.

The caterpillars of fall cankerworm have a wide host range and feed on many deciduous trees including elm (Ulmus spp.), ash (Fraxinus spp.), maple (Acer spp.), oak (Quercus spp.), members of the rose family, walnut (Juglans spp.), and willow (Salix spp.). As the cankerworms mature, they devour all but the midrib and veins of the leaves. Fully grown cankerworms are about one inch long and vary in color from light green to brownish green, with a darker stripe running down the middle of the back and a white stripe along each side. They have three pairs of fleshy legs called prolegs near the end of the abdomen whereas most other lookalikes have only two. The most interior pair of prolegs is reduced compared to the other two.

Images: Fall cankerworm caterpillars. Right image, prolegs indicated at arrows. Proleg at 3 is reduced compared to those at 1 and 2.

Although fall cankerworm and winter moth aren’t closely related within their family, there are some similarities between the two. Both species have wingless adult female moths that are active in the fall/early winter; however, fall cankerworm females lack the small vestigial wings that winter moth has. The female moth crawls up a host tree trunk and lays eggs in clusters on tree branches and the main stems. The eggs hatch the following spring, synchronized with the flush of leaves. After maturing, the caterpillars drop from the trees and pupate in the soil and adults emerge the following fall.

Management of this species generally isn’t warranted, as it has its own suite of native predators and parasitoids that help control outbreaks including two Scelionidae wasps which parasitize the eggs.

Friendly flies (Sarcophaga aldrichi) – A friendly face in the forest

If you happen to notice an uptick in large, slow-moving gray flies loitering around your woodlot in Aroostook County, you’re not alone. These are likely “friendly flies,” and their sudden appearance is no coincidence; it’s a direct response to the recent forest tent caterpillar (FTC) outbreak affecting parts of northern Maine.

Image: Friendly fly adults will roam around in search of forest tent caterpillar cocoons.

Friendly flies are native to Maine and have coevolved alongside FTC over time. The adult flies deposit maggots preferentially on or near FTC cocoons as hosts for their young; the larvae enter the cocoon and feed on the developing pupa, ultimately killing it before it can emerge as a moth. This relationship helps naturally regulate FTC populations following an outbreak, so while their presence can be a nuisance, these flies are a signal that natural checks and balances are in motion in the forest. Maine DACF does not release this species.

Despite their unsettling scientific name, the Latin prefix sarco- meaning “flesh” and suffix -phaga meaning “eater”, friendly flies are completely harmless to people and pets. They neither bite nor sting, and their common name likely comes from their tendency to land on people.

So far, the flies have been observed in multiple locations in Aroostook County, as well as Big Six Twp in Somerset County where a satellite population of FTC defoliated swaths of sugar maples in 2024. Anecdotal reports suggest reduced FTC defoliation in both counties compared to the previous year, though upcoming aerial surveys should shed more light on the extent of this year’s damage. One thing is for sure though, there are still plenty of caterpillars out and about crossing roads, scaling buildings, and climbing trees. Northern residents annoyed by such behaviors may now be encouraged by the increased presence of a caterpillar scavenger on the scene.

Image: Forest tent caterpillars are still occurring in large numbers throughout parts of northern Maine, such as this drive-through in Caribou.

As the outbreak enters its next phase, the appearance of friendly flies is yet another reminder of the complex interactions at play in our forests. Keep an eye out for this friendly insect as the season unfolds, and with knowledge of its biology, you may see it in a new light.

Hemlock Woolly Adelgid (Adelges tsugae)

A research article was recently published by the US Forest Service that has generated interest.

In this article the authors showed that thinning of hemlocks not yet infested with hemlock woolly adelgid (HWA) increased the live crown ratio and crown area of trees (and possibly, but not statistically, the diameter growth), thus potentially making them more resilient against HWA if they were to become infested.

We should remind readers that this is different from thinning hemlock stands after they are infested with HWA. Thinning tends to result in increased foliar growth, which tends to lead to increased infestation by HWA that feed on new growth. For a similar reason, treating infested trees with a nitrogen fertilizer is not recommended. As a side note, if you are releasing predatory beetles for HWA management, thinning or fertilizing with nitrogen can be beneficial, since trees with more new foliage will support more HWA which can lead to increased feeding and egg-laying by predators, and may encourage them to stay on those trees instead of dispersing to other areas.

Research has also shown that in hemlock seedlings, 30% of full sunlight or more led to reduced HWA infestation and enhanced carbon balance and growth. Research is also ongoing into the effect of clearing large areas around individual infested hemlocks in a forest setting to look at the effect of sunlight on HWA and mature trees.

Maine Forest Service and other forest health entities have long recommended that if costly integrated pest management strategies are going to be employed against HWA, land managers should choose to work in hemlock stands that are the healthiest and most robust, with the best access to sunlight, water, and good soil conditions, reasoning that these trees will have the best chance of surviving. We are still figuring out the complete story of sunlight, hemlocks and HWA. Increasing sunlight alone is unlikely to protect hemlock trees but sunlight could be a component of integrated pest management.

Orange-tailed Wood Fly (Blera analis)

Last week, during our winter moth collections, there was a multitude of interesting insects that fell onto the tarps. On one of the rainy days, we spotted an unusual fly on the tarp that had likely been taking shelter in the maples above. We were able to identify it as the aptly named orange-tailed wood fly, an uncommonly encountered flower fly in the family Syrphidae. Adult flies can be found mid-May through mid-July in hardwood forests and bogs, particularly where there are flowering trees and shrubs to feed on including hawthorns (Crataegus spp.), cherry (Prunus spp.) and viburnums (Viburnum spp.). The larvae of this genus of flies are an important component of the wood nutrient cycle in eastern forests. The maggots, as their common name implies, feed on rotting wood, helping to make those nutrients available again to other plants in the forest. These flies are typically residents of mature forests where there are more fallen trees and coarse woody material.

Image: Orange-tailed wood fly discovered in Boothbay, ME during winter moth caterpillar collections.

Spruce Budworm (Choristoneura fumiferana)

One of the most important spruce budworm (SBW) population indicators are the results of Maine’s L2 survey, which aims to quantify populations of dormant, overwintering larvae in their second instar (L2). During winter 2024-2025, the results of this L2 survey informed decision makers that management should be considered in 2025 to prevent a SBW population outbreak. An early intervention strategy (EIS) program developed in neighboring New Brunswick was determined to be the best strategy for protection and health of Maine’s forests. 

In early June, Maine forests experienced their first major aerial treatment program for SBW in decades, with approximately 240,000 acres of spruce-fir forest type in northern Maine treated to suppress populations before they outbreak. While the concept may be relatively straightforward, the correct timing can make or break a treatment program. For this project to be effective and successful, there were three main priorities: (1) the products used for SBW treatment must be applied at a time when larvae have emerged and are actively feeding, (2) must be applied to what they’re actively feeding on, and (3) ingested by SBW larvae to have any effect. To ensure proper timing of treatments, one of the most important phenological milestones to monitor after buds break dormancy is needle flare, when buds spread and individual needles finally begin to become recognizable. Maturing SBW larvae prefer this new foliage, and so this point is critical to ensure treatments will be most effective. Treat too soon, and small SBW larvae protected inside tightly closed buds will not be exposed to the treatment.

As we know, plant development during spring can vary significantly even across short distances, depending on factors like aspect or elevation. And as you can guess, bud development might be significantly different across a treatment area that spans more than 50 miles north to south and 50 miles east to west (furthest points, not contiguous). To get around this, Maine Forest Service built a visual survey tool to capture bud development and variability throughout the entire treatment area based on work from New Brunswick. Bud development was scored on a scale of 1 to 5 and three branches were evaluated at each site to develop an average bud score. Our target range and the ideal time to treat is an average bud score of 3.5 to 4. Thanks to a large team of dedicated surveyors, we were able to collect hundreds of data points using this survey tool to better understand the progression of bud development prior to treatment. Once we had sufficient data to demonstrate that development was at the right stage over the large geographic area, operations switched to all systems go.

Images: Examples of photos taken during bud phenology monitoring, ranging from premature for treatment with buds still closed (left two) to optimal for treatment with clear needle flare (right two).

Image: An example of data points collected just prior to the beginning of the treatment program to gauge when certain areas would be ready (target site average 3.5 or more).

Once buds and larvae were at the right developmental stages, a favorable weather window allowed for swift completion of Maine’s first SBW aerial treatment program in decades. A small area was treated on May 29 to ensure aircraft and equipment were working properly, followed by several days of inactivity due to unsuitable weather conditions. Treatments began in earnest again on June 2, and conditions were favorable on all but one day until the entire project was completed on June 9. The Maine Budworm Response Coalition, an organization of landowners, coordinated treatments.

This EIS method is a long-term process, and the true test of the efficacy of the treatment program will not come until we repeat this survey in winter 2025-2026 and compare the results from year to year. However, there is some opportunity to take an initial first look. Survey work by Dr. Neil Thompson immediately following treatment has indeed indicated good efficacy, as anticipated. Further, the later developing eastern black-headed budworm (Acleris variana) has not shown similar impacts, demonstrating the narrow window of insecticidal activity of the products used (Mimic 2LV and Foray 76B).

Images: Several examples of dead SBW larvae observed post-treatment. The photo on the right demonstrates how the remaining new foliage has been preserved now that larval feeding has been stopped.

Now that these treatments are complete, we are moving on to our other annual monitoring activities. We are currently installing our SBW pheromone traps across Maine to monitor adult moths that will be emerging in the upcoming weeks. During this installation and other survey activities, some of our staff in Downeast Maine have observed single SBW larva in the woods. If you see SBW larvae or pupae outside of the townships adjacent to the northern Canadian border, we’d be interested in your reports and photos to help plan monitoring efforts in fall.

Image: Spruce budworm pupa and feeding damage. Inset: spruce budworm larva (1 on scale is 0.5 inch).

In July, we’ll also take to the air to perform aerial survey over the areas that have been treated and over the areas where significant SBW defoliation damage was observed in northern Maine in the summer of 2024. We expect these flights will reveal less extensive defoliation damage this year, yet anticipate L2 surveys will reveal  areas that are eligible for treatment in 2026 under an EIS. You can find up-to-date information on this species at www.maine.gov/sbw and www.sprucebudwormmaine.org.

Winter Moth (Operophtera brumata)

Winter moth defoliation has hit the Midcoast again this year, with the most severe defoliation occurring in Georgetown, Phippsburg, Edgecomb, Bristol, Westport Island and many other towns throughout this region. Thankfully, the caterpillars are finished feeding for the year. Oaks and maples are fairly tolerant of defoliation and will push out another flush of leaves if the defoliation is severe enough. The regular precipitation we’ve had this spring will also help the trees overcome the damage.

For over a decade we’ve been releasing a biocontrol agent specific to winter moth – a parasitoid fly called Cyzenis albicans. We are limited in the amount of biocontrol we can access as it is not commercially available, and we must rely solely on collections from around our previous release sites in Maine. Unfortunately, the longer-term solutions like biocontrol do take a bit of time, but there has been success at our previous release sites.

Image: Collecting winter moth caterpillars during the deployment in South Bristol.

This year’s annual winter moth caterpillar collection to support our biocontrol program looked a bit different compared to past years. Maine Forest Service hosted eight forest health staff from different states, including Rhode Island, New Hampshire and Vermont. This was part of a two-day mobilization of the Northeastern Forest Fire Protection Compact Forest Health Working Team (FHWT) to bolster the number of winter moth caterpillars collected and thus the amount of Cyzenis albicans collected for biological control. The collection event focused on the peninsulas in the Midcoast where the cooler climate delays development compared to more inland sites. Samples were collected from Boothbay and South Bristol during the mobilization, with Bath and West Bath sites being unproductive due to timing. The purpose of calling for a mobilization of the FHWT was to collect enough Cyzenis to share with Rhode Island, which is currently experiencing an outbreak of winter moth. Although releases of this fly have occurred in Rhode Island, the parasitism rates are low.

Image: Group photo of the deployment team with Maine, Rhode Island, New Hampshire and Vermont represented.

On three separate days, Maine Forest Service staff additionally collected caterpillars in Lubec, Northeast Harbor, Boothbay, South Portland and two sites in Kittery. Our staff was joined at the latter three sites by our cooperators at UMass Amherst, who graciously donated their time and effort to help collect as well as rear caterpillars collected at all sites this year. More than 12,000 winter moth caterpillars were collected across the five collection events that happened this spring.

Diseases and Environmental Issues

Anthracnose Diseases, various species depending on host

Reports of Anthracnose diseases are on the rise in recent weeks. This was not unexpected, as cool and rainy conditions have prevailed this spring/early summer. This weather combination has been ideal for disease development among this class of pathogens causing leaf necrosis (brown patches of leaf tissue) leading to leaf deformation. For the most part, a specific fungus causes Anthracnose symptoms in each separate species, although some can infect one or two closer-related plants. In severe cases Anthracnose diseases lead to early defoliation (and refoliation in some species). When trees suffer defoliation, additional fertilizer with a higher nitrogen content can be considered to help the tree replace lost resources. However, at this point wait until next spring to do this, since adding fertilizer now could cause some tree species to push out new flushes of growth later in the season that may not harden off in time for the year’s first frost. Generally, pesticide applications for Anthracnose diseases are not practical or recommended; many trees (like oaks suffering from oak anthracnose) are quite resilient to this type of damage. Cleaning up fall leaves and disposing/efficiently composting them is a good cultural practice toward lowering local disease incidence. But if your trees are among many infected trees, you may just have to live with the symptoms and hope for a drier spring next year.

Images from left to right: birch Anthracnose, ash Anthracnose, oak Anthracnose.

Beech Leaf Disease (Litylenchus crenatae mccannii)

Beech leaf disease (BLD) continues to be found throughout Maine. Franklin County was Maine’s last county to be confirmed this spring. This year, MFS has confirmed BLD in 55 new towns for a total of 271 towns infected statewide. It has been demonstrated that BLD occurs at trace amounts well ahead of areas showing severe symptoms. This means that the disease could currently exist throughout much of the state.

MFS has continued the Polyphosphite soil drench treatment trial in cooperation with Viles Arboretum in Augusta. Now in year three of the trial, the first of two 2025 soil drench treatments of Polyphosphite 30 was completed in early June. The second treatment will occur in July. No clear differences between control and treated trees have been noticeable. A trial in Ohio, on which our trial was based, did not see clear differences between soil drenched trees and controls until year five of treatment.

Year two of trialing macroinjections of Thiabenzadole (Arbotect 20S) to manage BLD in beech bark disease-resistant trees will begin in July. The single tree we injected in July last year has responded well and has significantly less symptoms than its untreated neighbors. Trees treated after July have not shown clear symptom reduction but can be expected to have far fewer symptoms in the following year, as this active ingredient will persist in the tree for two to three years after application. It was recently learned that Arbotect 20S injections should occur earlier in the growing season (best if done before the end of July), as this allows the active ingredient to arrive in the forming buds for next year’s leaves before nematode migration to the buds for overwintering (where their feeding damage leads to formation of leaf symptoms seen the following year).

The Connecticut Agricultural Experiment Station has recently released an overview document of their experiences with beech leaf disease treatment. When considering the treatment/management options in this bulletin, 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 applicators.

Image: From left to right: Moderate BLD symptoms of leaf banding; severe BLD symptoms of leaf deformation and bud abortion leading to very thin crowns.

White Pine Needle Damage (WPND), Brown Spot Needle Blight (Mycosphaerelladearnessii / Lecanosticta acicola); Bifusella linearis; Dooks Needle Blight (Lophophacidium dooksii); Septorioides strobi.

Similar to last year, white pines are currently showing symptoms of a disease complex called white pine needle damage (WPND), which causes impacted trees to drop their older needles due to infection by a group of native fungi. The most common disease in this complex is brown spot needle blight, although there are other similar fungi that can cause this.

The pines in many areas of Maine have been suffering from infection by this disease complex for several years to varying extents – in fact, many white pines in Maine have been impacted by WPND dating back to 2011, particularly in southwestern Maine. This year, the disease symptoms (needle discoloration/yellowing and needle drop) are quite severe throughout Maine where white pine grows and the Maine Forest Service has been getting many reports/calls/emails per day from concerned citizens.

The future of infected trees is uncertain. What we typically see with WPND is a loss of older needles and trees holding their current-year needles. The trees will appear greener when the infected, yellow/orange needles drop but they will have thin-looking crowns. Trees that do not flush adequate new foliage in 2025 that also experience severe defoliation of previous years’ needles will likely die. Trees on good growing sites tend to be more resilient to damage.

Images from left to right: A tree impacted by WPND showing more severe symptoms lower in the crown; heavily impacted needles of previous years’ growth with green arrows pointing to emerging new growth currently being infected by spore-producing structures on last year’s needles; discolored and shed needles with banding at the site of emerging spore-producing structures.

Personnel Update

Alyssa Kuskowski has joined the Forest Health and Monitoring field staff for the summer as a student intern. She is based out of the Maine Forest Service Central Region Headquarters in Old Town. Alyssa is from Bethany, Connecticut, and is a Woodbury FFA alum. She will enter her senior year at the University of Maine studying Forestry with a minor in Entomology. She has worked previously in Dr. Angela Mech’s spruce budworm and browntail moth lab. She is also conducting an independent study on browntail moth mortality. Alyssa will be helping in the regular summer field surveys for forest insects and diseases. She is excited to explore and learn more about the forests of Maine this summer with FHM.

This Month in Conditions Report History: June 19, 1978

Pine spittlebug – Spittle masses containing young, feeding nymphs are now visible on various conifers especially White pine. In one scotch pine plantation in Sedgwick, spittle masses were so abundant that they formed almost uniformly spittle covered branches. Small numbers do little damage and control measures are seldom necessary. When populations are heavy, control may be necessary especially if trees are under stress.

Calendar

FHM Staff will be at the Forest Stewards Guild Event in Warren (registration has closed) on Friday 6/20 and the Moose Permit Draw in Farmington on Saturday 6/21. Next month, we will attend parts of Maine TREE Foundation’s Teachers’ Tours. More forestry events can be found in the Maine Forest Service events calendar.

DACF Calendar

Conditions Report No. 2, 2025

On-line: https://www.maine.gov/dacf/mfs/publications/condition_reports.html

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

Contributors: Aaron Bergdahl, Amy Emery, Gabe LeMay, Allison Kanoti, Mike Parisio, Brittany Schappach, Thomas Schmeelk, Colleen Teerling, and Andy Whitman