Forest & Shade Tree – Insect & Disease Conditions for Maine, June 16, 2023

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Maine Forest Service

Forest & Shade Tree – Insect & Disease Conditions for Maine, June 16, 2023

Memorial Day Firewood Confiscations

Holiday weekends are always a nerve-wracking time for the Forest Health Division. We know that firewood travel miles will be in the tens of thousands as people head to their favorite outdoor spots carrying their firewood along. Although out-of-state firewood is of particular concern, even Maine-origin firewood can be just as problematic to our forests when it is moved long distances.

This Memorial Day weekend was no exception. We are especially thankful for the vigilant eyes of law enforcement at times like these. Pictured here is part of a load of Pennsylvania firewood that included many pieces of ash infested with emerald ash borer (EAB). The wood was being carried in an open trailer and was intercepted by a Maine State Trooper and then surrendered to Maine Forest Service Rangers.

Piles of firewood

Images: Roughly 25 percent of a large load of firewood intercepted by a Maine State Trooper over Memorial Day weekend was EAB-infested ash coming from Pennsylvania (left). The metallic green elytra, or wing coverings, of a dead EAB adult was found still stuck in its D-shaped exit hole (right).

Fortunately, no viable EAB adults or larvae were found in this particular load, which was promptly destroyed by incineration.Yet another reminder that although ash trees killed by EAB make prime firewood, they need to be used locally, period.

Although ash has been a prime focus, it is important to remember that other types of firewood may harbor damaging forest pests new to the region that have the potential to permanently harm Maine’s forests. Locally, people should be aware that your firewood could transport browntail moth pupae to new areas and could hasten the spread of emerald ash borer, European larch canker, southern pine beetle, and other locally established insects and diseases.

In addition to the catch by a trooper, on the same day, two full-sized pickup loads of firewood were intercepted in a day’s work by a team of Forest Rangers and Forest Health staff at the Kennebunk Rest Area.

The simple message is: Buy Local, Burn Local. If there isn’t a local source of firewood at your destination, use certified heat-treated firewood (and check any packages for hitchhikers such as browntail moth, spongy moth, and spotted lanternfly among others that can infest wood after treatment).


Acorn Weevil (Curculio sp.)

While out shaking oak trees for winter moth caterpillar collection (see Winter Moth article below), we were fortunate to encounter the charismatic acorn weevil. Many of you have likely seen acorns with small 1/8” holes in them, but perhaps you never knew the cause. Acorn weevil larvae develop inside acorns. When their larval development inside their acorn abode is complete, they emerge once the acorns have fallen to the ground to complete the rest of their life cycle in the soil over a 1–3-year period. When adults emerge, they proceed back up into the oak canopy to mate and search for new green acorns in which to deposit their eggs. This is precisely what these beetles were doing when they were rudely dislodged by a Maine Forest Service entomologist. For those collecting acorns in the fall to plant, make sure your acorns don’t float in water because these will often have an acorn weevil larva inside that has not emerged yet.

Beetle with large eyes and enormously long and thin snout, giving it a funny expression

Image: An adult acorn weevil with a comically elongated snout.

Weevils in general are known for their long snouts, giving them a somewhat comical, if not friendly, appearance that charms many people. Weevils in the genus Curculio take this look to the next level.

There are about 30 species of Curculio native to North America and approximately 350 species worldwide. Their host range is diverse, with North American host plants including pecans, oaks, walnuts, hazelnuts, hickories, and chestnut trees. The greater chestnut weevil (Curculio caryatrypes) is one species that is possibly extinct due to the functional extinction of its host, the American chestnut (Castanea dentata), due to an introduced fungus called chestnut blight that decimated American chestnut populations in the 20th century. The lesser chestnut weevil (Curculio sayi) seems to have fared much better, as it can use other hosts and was not as specialized as the greater chestnut weevil.

Aspen Defoliators Active in Aroostook County – Large Aspen Tortrix (Choristoneura conflictana) and Forest Tent Caterpillar (Malacosoma disstria)

Not only has forest tent caterpillar returned in abundance in northern Maine in 2023, but large aspen tortrix has also made an appearance. Both species feed preferentially on aspen, so damage may be hard to differentiate this spring. Forest tent caterpillars were reported in mid-May and have now reached full size and are busy feeding. Large aspen tortrix caterpillars can often be found with multiple caterpillars suspended on strands of silk suspended beneath trees and can also be identified based on their characteristic leaf rolling. A relative of spruce budworm, large aspen tortrix can reach high populations in short periods of time and a swarm of dead moths was reported in Fort Kent in late June 2022. Prior to these mentions, large aspen tortrix has not appeared in our conditions report since May 2012. Both forest tent caterpillar and large aspen tortrix tend to experience high population outbreaks over a period of a few years, after which they collapse naturally.

A beetle on a hand, two photos  of trees and landscape showing defoliation damage

Images: An example of a large aspen tortrix caterpillar (left); a young aspen with trunk covered in mature forest tent caterpillars (middle); a current example of forest tent caterpillar defoliation on aspen in Aroostook County this spring (right). Photo Credit: Aaron Bergdahl, North Dakota Forest Service/North Dakota State University, (larva).

Browntail Moth (Euproctis chrysorrhoea)

Across all our monitoring sites, browntail caterpillars are, on average, 1.2” (31.5mm) in length and are anticipated to continue growing until they reach roughly 1.5” (38mm) in length in late June. This week we saw evidence of pupal cocoons as the caterpillars began to pupate. Beginning in late June and early July, adult browntail moths will become active, and you may start seeing them near light sources. We recommend keeping unnecessary outside lights off between 9:00 PM and midnight until the beginning of August to avoid attracting adult browntail moths to your property. If this is not possible, consider switching to yellow-spectrum lighting to reduce property attractiveness to dispersing moths.

Chart showing increasing numbers over time

Image: Browntail moth caterpillar body length showing growth at our monitoring sites across the state from April 20, 2023 to June 7, 2023. Note that there were no emerged caterpillars (no data) at our Garland or Turner sites on May 3, 2023 due to weather conditions.

At our monitoring sites, host trees are experiencing partial or complete defoliation, depending on the level of infestation. Although this looks alarming, healthy trees will flush out new leaf growth over the next few weeks once the caterpillars have ceased feeding. Browntail caterpillars are very good at traveling to new areas to feed on new trees and shrubs, otherwise known as hitchhiking – read more about this in our hitchhiking post. The best way to prevent these caterpillars from being introduced to new areas is to check for them crawling on your vehicles or equipment and taking precautions to remove them before you leave. This past winter, we documented winter webs in isolated areas such as Jackman, Greenville, and Rockwood, and we think these caterpillars more than likely hitched a ride to get there.

In the past few weeks, we have been finding some evidence of pathogen-related death at a few of our monitoring sites. We responded to a request from the manager of Eagle Island State Historic Site and confirmed high mortality of browntail caterpillars from the fungus Entomophaga aulicae. The recent cloudy and rainy days are helping contribute to the spread of fungal and viral pathogens that attack browntail caterpillars. Although pathogens can regulate some populations of browntail caterpillars, disease outbreaks can only happen with co-occurrence of rainy weather and the presence of the pathogen. Therefore, we should caution that pockets of disease like this epizootic event (disease outbreak) may be quite isolated.

caterpillars dead on a branch

Image: Fungal pathogens have caused browntail caterpillars to die in some locations. Eagle Island, ME.

Sign up for our Browntail Moth Updates to stay informed about this insect.

Elm Zigzag Sawfly (Aproceros leucopoda)

An elm pest new to the region has been increasingly found in neighboring provinces and states and beyond. The USFS requests your observations of damage consistent with this pest. Look for characteristic feeding in elm. You can submit your observations in this USFS ArcGIS Survey 123 form.

caterpillar eating a path into a leaf

Image: Elm zigzag sawfly larva feeding in the characteristic ‘zigzag’ pattern on an elm leaf. This damage in elm and related species may be the result of EZS (similar damage patterns are created in other species). Image Credit: Gyorgy Csoka, Hungary Forest Research Institute,

If you find one or more larvae along with the damage in Maine, collect them and contact us. You can preserve them in a cool place (next to your veggies in the fridge), or in alcohol.

Hemlock Woolly Adelgid (Adelges tsugae)

June is when the predatory beetle, Sasajiscymnus tsugae, is released as biological control for hemlock woolly adelgid (HWA). This tiny beetle feeds specifically on adelgids and is known to feed on all life stages of HWA. Sasajiscymnus tsugae, or ‘Sasaji’ as it is often called, is one of the very few biological control agents of forest pests that is available for sale to individuals. As with other wildlife, even though it can be purchased, it requires a permit from the Maine Department of Inland Fisheries and Wildlife and USDA APHIS for release. The Maine Forest Service will work with private buyers to cover permitting.

Two photos of larvae on a hemlock stem

Image: Adult (left) and larval (right) Sasajiscymnus tsugae feeding on hemlock woolly adelgid. Image credit: Carole Cheah, Connecticut Agricultural Experiment Station, (larva).

The Maine Forest Service has been working with several land trusts and private landowners over the past winter to select suitable sites for biocontrol release. Land trusts, in return, have been passing this expertise on to private landowners and helping them combine predator beetle orders to obtain bulk discounts. During June, these predators are being released at multiple sites, both private and public, throughout the range of HWA in Maine.

This predator will never eradicate HWA, but we know it has become established at many release sites and has been found at Maine sites up to 18 years following initial release. It may be helping keep hemlock trees alive at some release sites. Although there is no guarantee that ‘Sasaji’ alone will keep hemlock trees alive, biological control is one of the very few management strategies that has any hope of providing long-term control of forest pests, including HWA.

Slow Start for Spongy Moth? (Lymantria dispar)

As we prepared for a third season of defoliation damage from spongy moth in western Maine, there was some light at the end of the tunnel in the form of early signs of population decline in 2022 in the form of evidence of disease and pathogens among the spongy moth population. While oak trees in western Maine might look like they’ve already been attacked by spongy moth in 2023, the true cause for widespread oak leaf damage in is a hard frost that killed developing oak leaves across large portions of the state (see Freeze Damage to Trees article below). Typically, the spongy moth reports start increasing in late June and early July as caterpillars mature and defoliation damage becomes highly visible. The question this year is just how much the frost damage will affect the development of young spongy moth caterpillars. They are quite flexible when it comes to host plants they can develop on, but perhaps many will be affected by low temperature, poor nutrition, and weather-supported disease outbreaks. The outcome will be clear in a few short weeks. We welcome your observations.

We have had reports of patches of oak mortality in western Maine from the ongoing spongy moth outbreak coupled with drought impacts. Even if spongy moth does not have the opportunity to defoliate many oaks this season, the defoliation from frost damage is still significant for trees already running low on resources from multiple seasons of drought and spongy moth defoliation. The overall effect of these compounding stress events is hard to predict, but significant additional oak mortality in various areas of Maine is certainly possible in 2023.

A caterpillar in the claws of a spider, both on a leaf

Image: One less young spongy moth caterpillar on the landscape thanks to a crab spider.

Winter Moth (Operophtera brumata)

Many Midcoast towns, especially on the peninsulas, are experiencing severe defoliation from winter moth, including West Bath, Phippsburg, Southport, Bristol, South Bristol, and Boothbay, among others. Now that winter moth caterpillars have completed or nearly completed feeding, the trees that have been severely defoliated will flush out another set of leaves, which will be helped along by some of the rain we’ve been receiving.

Biocontrol agents, like the winter moth parasitoid fly Cyzenis albicans, do take time to get established and show results. The fly is not commercially available we must rely solely on our previous release sites as a source of this agent for release in Maine. MFS staff, along with our colleagues at UMass Amherst, engaged in our annual winter moth caterpillar collection on May 23 and 24 at previous biocontrol release sites, which included Boothbay Harbor, Bath, Cape Elizabeth, South Portland and two sites in Kittery. A separate, smaller collection was made at our newer release sites on June 8 in East Boothbay and South Bristol to determine establishment. Approximately 12,500 caterpillars were collected during these three days. Some of the caterpillars collected are parasitized by C. albicans, a parasitoid fly that is very host specific to winter moth. Flies reared from parasitized caterpillars will be the source for next year’s release. These collections will also show where the parasitoid has been established and the proportion of the winter moth population parasitized at each site. In the spring of 2024, we will release the collected parasitoids in West Bath.

Hands holding a pile of green caterpillars

Image: Winter moth caterpillars gathered from tree branch sampling.

Top view of two buckets containing leaves and twigs

Image: Collection buckets containing winter moth caterpillars.

Diseases and Environmental Issues

Beech Leaf Disease (Litylenchus crenatae ssp. mccannii)

Reports of beech leaf disease (BLD) continue to be submitted to the Maine Forest Service by email to, via our online reporting form, and directly to personnel. In late May 2023, two years after the first report of BLD in Maine in Waldo County, the disease was found in Kennebec County. This brings the county total to 7: Hancock, Kennebec, Knox, Lincoln, Penobscot, Waldo, and York. The public has been instrumental in determining the current range of BLD in Maine. The Maine Forest Service thanks everyone who has reported BLD this year and in past years. Your vigilance is key to the early detection of damaging forest pests. For current information about BLD, please refer to the Maine Forest Service Beech Leaf Disease website.

Cedar-Apple Rust (Gymnosporangium juniperi-virginiana)

The fungus that causes cedar-apple rust most commonly overwinters on galls on eastern red-cedar (Juniperus virginiana). Currently, twigs are developing the conspicuous bright orange, jelly-like tendrils that produce the spore stage, which can infect apples and crabapples (Malus spp.). Despite the dramatic appearance of the sporulating galls, the disease is of little consequence to juniper health. On the other hand, infection of apple/crabapple (and sometimes hawthorn, Crataegus spp.) leaves can result in leaf spots that may lead to heavy, pre-mature defoliation. Although fungicides can be used to control cedar-apple rust on apples, the best solution is to plant apple varieties that are resistant or nearly immune to infection. Common apple varieties resistant to this disease include Liberty, Delicious, McIntosh, Macoun, and others. Many newer apple/crabapple varieties have also been bred for cedar-apple rust resistance.

Two photos of galls with orange sprouts, one photo of a leaf with yellow spots.

Images: An overwintered cedar-apple rust gall prior to tendril elongation and spore dispersal (left); Gelatinous spore tendrils of the cedar apple rust fungus in early June (middle) (J. Chatfield, The Ohio State University, Bugwood); Numerous lesions on crabapple tree leaves leading to premature defoliation (right).

Emerald Ash Borer, Dead Ash Trees and Decay

A recent study by the University of Minnesota looked at decay fungi associated with emerald ash borer (EAB)-killed ash trees. The findings of the study are useful for land managers, municipalities and property owners as it describes what happens to the trees after EAB attack and mortality. The results also stress the importance of removing EAB-killed trees, especially nearby infrastructure and high human-use areas, as these trees will lose their structural integrity via decay processes and may soon become hazardous if branch, limb and trunk failure occur. The findings, also published in a research brief on the University of Minnesota website, will help those who care for ash trees make appropriate and safe decisions when managing EAB-killed trees.

Freeze Damage to Trees

A freeze event during the week of May 14 impacted trees throughout Maine, with reports ranging from Moscow (Somerset County) to the North, North Berwick (York County) to the south, east to western Hancock County and west to the New Hampshire border (Oxford County). Reports of severe damage were widespread in western Maine, while reports throughout the rest of the described region were scattered and correlated with exposed areas and cold draws where cold air settled and sat overnight. Conversations with forest health colleagues in neighboring New Hampshire indicate frost damage affected the entire southern half of the state there as well.

Symptoms also ranged widely, from mild discoloration (mostly reddish coloration) to dead leaf tips and margins to full leaf wilting and death. Some trees were fully wilted, while others only suffered freeze damage at the tops or bottoms of trees. The species and individual trees affected were those at a particular early leaf maturation stage, which was highly susceptible to damage from the sub-freezing temperatures that dipped into the mid-to-upper 20s in some areas and persisted for several hours. In a survey of damage shortly after the freeze event, species that were observed to have sustained damage included apple (mostly blossoms), beech, black locust, poplar, red maple, red oak, shagbark hickory, silver maple, striped maple, sumac, sycamore, and white oak. Damage to oak was most frequently encountered.

Branch with leaves that are brown and drooping

Images: A new set of oak leaves flushing from below blackened and curled freeze-killed tissues (left); Ash trees had yet to break bud in most areas impacted by the freeze; however, this Waldo County ash tree had its leaves killed by frost and is now in the process of forming a new set of leaves (right).

Only time will reveal how the trees will respond to this event. Healthy trees are expected to produce a new flush of leaves, although these may be smaller than usual. However, trees already experiencing additional stressors, such as inadequate moisture or poor health prior to the freeze, may struggle to recover from the damage. Recent survey has shown damaged trees have already set new buds and some are even flushing a new set of leaves.

It is important to monitor the condition of your landscape trees and provide them with appropriate care. Consulting with a licensed professional arborist or tree expert can help assess the extent of the damage and determine the best course of action. Consider working with a licensed professional forester and allowing time to gauge recovery before making management decisions about trees in the woods.

Collage of pictures of trees with drooping brown leaves

Images: New oak growth and leaves with symptoms of freeze damage (upper left); an oak-lined road in York County with all oaks heavily damaged by the hard freeze (lower left); a stand of oaks in Fryeburg (Oxford County) with heavy leaf damage due to freezing temperatures during early stages of leaf expansion (middle); the tips of beech leaves damaged by the freeze event (upper right); a line of shagbark hickory trees with severe crown damage due to freezing temperatures in mid-May (lower right).

Caring for Defoliated Trees

When trees are defoliated due to environmental causes, severe foliar diseases or insect feeding, several present and future issues should be considered regarding tree health. First, in the short term, defoliation represents serious tree stress, as a tree’s leaves are rich in resources. In the years following defoliation, trees can be expected to have less overall leaf tissue, slower overall growth and wound closure, and twig and branch dieback. These impacts will vary depending on the tree’s vigor prior to defoliation, the severity of defoliation and the number of defoliation events (for example, subsequent years of spongy moth defoliation or the double defoliation of browntail moth caterpillars in one season are particularly detrimental). Second, the timing of defoliation is also important: defoliation of new leaves or needles earlier in the growing season is much more detrimental to tree health than late-season defoliation when leaves have already done a large proportion of their photosynthetic work. Third, weather conditions also play an important part in a tree’s recovery from defoliation, as drought following severe defoliation can lead to rapid decline. Alternatively, adequate moisture will be beneficial to tree recovery and efforts to produce a second flush of leaves.

For highly valued landscape trees, replacing nitrogen lost to defoliation is recommended. Apply a readily absorbable fertilizer (water-soluble rather than slow-release) at a rate of 2 to 3 pounds nitrogen per 1,000 square feet. Do not apply fertilizer after the end of June, since this may stimulate new growth that will not harden off before the first frost, further harming the trees you are trying to rehabilitate. Refer to the example below from the University of Massachusetts Amherst for calculating the nitrogen fertilization rate.

Chart: 3 lbs of nitrogren needed times 100% divided by 30% of nitrogen = lbs of fertilizer to apply to 1000 square feet

Prompt and appropriate tree care after defoliation is critical to rehabilitating affected trees in hope of a second flush of leaves and lowering the chance of attack by secondary agents of tree decline. Typical secondary agents of decline include insect attack by borers, bark beetles and root disease, primarily Armillaria root disease (aka shoestring rot). Monitor your trees following defoliation to look for symptoms and signs of secondary pests and take appropriate measures to mitigate their effects.

Maine’s Moisture Status and Future Implications

Maine’s moisture status at this point of the 2023 growing season is favorable for tree health in most parts of the state, except in a band from northern half of Aroostook County extending along the norther border with Canada. Abnormally dry conditions also occur in southern Hancock County and most of Washington County.

Maps of Maine showing a band of drought affected area mainly in the north

Images: Total May precipitation (left); U.S. Drought Monitor for Maine, June 13, 2023 (right).

The past couple of weeks have been characterized by overcast skies, frequent rains and cooler temperatures. These conditions favor the development of fungal diseases of plants. Foliar diseases that could become more prevalent in the coming weeks/months include the anthracnose diseases, rust fungi and other fungal leaf spots.

For the needle diseases of many conifers, weather conditions this year help us forecast the severity of these diseases next year. The current wet weather is occurring during the sporulation period of many of these diseases and newly emerging needles will be infected, showing no symptoms until next year. Examples of diseases that could increase in severity in 2024 due to current weather trends are needle casts and blights of spruce and fir, needle rusts and the white pine needle damage disease complex.

On the bright side, this moist weather also favors the development and spread of beneficial fungal diseases that infect some of our defoliating insects like spongy moth and browntail moth caterpillars. Given the right circumstances, there is a possibility that populations could crash due to disease in areas where these defoliating pests are found in high densities.

White Pine Needle Damage (brown spot needle blight, Mycosphaerella dearnessii/Lecanosticta acicula; Bifusella linearis; Dooks needle blight, Lophophacidium dooksii; Septorioides strobi)

Symptoms of yellowing and premature needle drop are steadily increasing throughout the white pine resource in Maine and will continue into early July. This results from infection by one or more of the fungi that comprise the white pine needle damage (WPND) disease complex.

Pictures of a landscape showing pine trees turning yellow

Images: Yellowing of eastern white pine foliage due to WPND fungal infection. Symptoms are often more severe by water bodies and on the coast where relative humidity stays higher for longer periods of time (top); Heavy shedding of needles due WPND infection (lower left); The yellow arrow shows a new pine shoot showing no symptoms, the orange arrow shows symptomatic second-year needles about to be shed and the blue arrow indicates the dispersal of spores from the infected needles to the new needles. The new needles will show symptoms of infection in one year’s time (right).

Due to the life cycles of the diseases involved, precipitation in May and June has been thought to strongly determine WPND severity in the following year (higher precipitation favors the ability of fungi responsible for WPND symptom development to produce spores and infect needles). Following this logic, the past few years of dry springs should have meant a lessening of overall symptoms in white pine last year and this year; however, this has not been observed. Current thought is that the amount of disease present in the white pine is high enough that lesser amounts of precipitation are needed to result in high levels of disease and symptom severity. WPND causes premature shedding of second-year and older needles, leaving infested trees with only current-year needles. This makes crowns appear thin and leads to lower rates of overall photosynthesis and less growth, representing a chronic stress that may make trees more susceptible to secondary pests and disorders.

This Month in History

From the June 20, 1990 Conditions Report:

“Euonymus Caterpillar (Yponomenta spp.): An infestation of euonymus by what appeared to be larvae of a species of ermine moth was reported from Wiscasset this past week. A dozen or more large shrubs over ten feet in height were stripped and “festooned” with webbing. Caterpillars were dropping in numbers on silk strands to find more food and/or pupate. This is the first record we have received of this pest from Maine and we will try to rear a series for more definite identification.”


Saturday, June 30, 9:00-11:30AM, Old Stagecoach Rd, Old Town, Maine

Enjoy a beautiful woodlot in Old Town, Maine and identify signs and symptoms of common forest pests and diseases firsthand with Maine Forest Service Entomologists Gabe LeMay and Brittany Schappach. This in-person workshop is designed to educate landowners on the identification of forest pests and diseases. We invite you to bring any questions or your expertise to walk and talk with us as we learn together. The woodlot has a wide, easy-to-follow trail and features red oak, ash, beech, hemlock, other softwoods and a large pollinator garden. Topics will include ticks, gall-making adelgids, tent caterpillars, white pine blister rust, white pine weevil, and more! 

Please review the map guide for instructions on how to get to the woodlot, where to park, and details on the trail we will take. This is an outdoor event in the woods; please dress appropriately for the weather and take precautions to prevent tick bites.

Please register for this event by emailing Larry Beauregard, or calling him at (207) 989-6158. We hope to see you there!

Register for Event

Conditions Report No. 2, 2023


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

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

Unless otherwise noted, images by Maine Forest Service, Forest Health and Monitoring, DACF.