Forest And Shade Tree Insect and Disease Conditions Report -- May 2021

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Maine Forest Service

Forest & Shade Tree Insect & Disease Conditions for Maine

May 20, 2021

In this issue:

As insect and disease activity continues to pick up this year, we remind you that YOU are an important part of Maine’s Forest Health Monitoring Network. From Asian longhorned beetle to zigzag sawfly and everything in between in the alphabet, if you see tree or forest damage of note from insects, diseases or abiotic factors, please be sure to let us know. You can use our on-line report form, email us at or call (207) 287-2431.

Business hours are 7:30 am to 4:00 pm, Monday through Friday, except for holidays. We are currently working remotely and DACF buildings are closed to public access. You can still reach us at: (207) 287-2431. Status of building closures will be updated on the DACF homepage.

Join the Girdled Trap Tree Network Now

If you are interested in girdling an ash tree to monitor for emerald ash borer, now is the time to do so.

With emerald ash borer (EAB) spreading in both northern Maine and southwestern Maine, there is urgency to delimit the boundaries of the infestations and to spot any new outlier infestations as quickly as possible. Knowing where EAB is will help target management, such as biological control release, and potentially slow the spread. Girdled trap trees are proving to be one of Maine’s most effective tools for finding EAB early, often 1-2 years before any visible symptoms of EAB are apparent. This can give landowners and managers extra time to plan and act. Also, natural enemies of EAB can only be released after it has been confirmed in a forest setting. Many of our biological control release sites were found using girdled trap trees.

To create a trap tree, a section of bark is removed from around the entire circumference of the tree. The tree stays alive throughout the growing season, but is stressed, causing it to release chemical cues into the air that make it attractive to EAB in the area. If present, EAB are more likely to come to this tree, rather than another random ash tree. 

Many people ask whether girdling a trap tree will draw EAB onto their property from great distances, to which the answer is no. A girdled tree will only draw EAB in from about 100 yards. If EAB is close enough to be attracted to your girdled tree, it is almost certainly already on your property at levels too low to be detected. In fact, a girdled tree is likely to protect other ash on your property, since it can act as a sink, drawing EAB away from your other trees. Girdled trees can be used as a management strategy in the early years of an infestation, to prolong the life of surrounding ash trees.

We will send out information in the fall on how to fell and process trap trees. We recommend that volunteers who girdle a tree participate in one of the Maine Forest Service log-peeling workshops or bring their tree bolts in for peeling (depending on location). If you girdle an ash tree for EAB monitoring, please use this this online form to report it to the Maine Forest Service. You can also reach out at or (207) 287-2431 if you have trouble with the form, if you have any questions about selecting or girdling a tree, or if you need assistance.

If you are able to girdle and fell your tree by yourself, please follow the instructions below and fill out the online form. If you need assistance in girdling or felling your tree, please contact us at  before girdling your tree so we can confirm we can assist.

How to Select an Ash Tree:

  • Any species of ash (Fraxinus spp.) can be used (not mountain ash).
  • Trees that have at least one side open to sunlight such as those at the edge of road, field or stream, or above the forest canopy, are much more attractive to EAB and make better trap trees.
  • Tree should be healthy prior to girdling.
  • Tree stem should be 4 to 10 inches in diameter at about 4.5 feet from ground level. Larger trees can be used but are more work to process.

How to Girdle an Ash Tree:

  • A short video on how to girdle a tree is available here.
  • Peel an 8- to 10-inch section of bark from the lower stem.
    • Peel to the sapwood between mid-May and mid-June. At this time of year, the bark will slip away from the sapwood easily. When this happens, you know you have peeled deep enough.
    • Young, vigorous ash will sometimes callous over the girdle, this reduces the stress and the tree’s attractiveness to EAB. Therefore, make sure the girdle is clean, and is at least 10-12 inches tall all the way around the trunk. 
  • Use a drawknife or other precision tool to perform the cuts for girdling.
    • Be careful not to cut into the sapwood. Once the bark peels away, do not cut any deeper. If you cut into the sapwood, you will disrupt the cells that transport water. This can cause the tree to die too quickly to be attractive to EAB.

You can also use a pruning saw or other tool to make two parallel cuts 10-12 inches apart around the tree circumference. Then use a chisel or other tool to remove the bark between the cuts. A chainsaw is not recommended because it can be difficult to avoid damaging the sapwood.

Thank you for your participation! The data from your trees will help us manage EAB.


Balsam Gall Midge (Paradiplosis tumifex)

For those paying attention to balsam gall midge populations in their Christmas tree and wreath brush plantations, here’s a friendly reminder that the treatment window for emerging adult midges might already be here in some areas of Maine or fast-approaching in others. Occasionally, the small, orange female midges can also be observed on new shoots on calm and sunny days. Otherwise, the beginning of adult emergence can be documented fairly easily using yellow sticky card traps or simple emergence traps consisting of an open-bottomed, opaque box outfitted with a transparent collection jar on the side (e.g. small Ball jar with lid attached around hole in box) placed on the ground below trees with known populations of balsam gall midge in the previous fall.

Balsam gall midge commonly affects both balsam and Fraser fir and surveys to document balsam gall midge populations should be performed prior to the damaged needles dropping in the fall. Damaged trees should be flagged for spring since most of the evidence will disappear with needle drop. High levels of damage by this insect were observed in several parts of Maine last year including broad stretches of Downeast Maine and in the Fryeburg area in 2019. Damage was spottier in other areas of the state in 2020. 

Those who have routinely treated with Chlorpyrifos in the past should pay special attention in future treatment seasons, as this chemical is currently under consideration to be banned for use in Maine.

For more information, please consult our Balsam Gall Midge factsheet

Browntail Moth (Euproctis chrysorrhoea)

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A cluster of rapidly maturing browntail moth caterpillars clinging to the remains of their winter web before spreading out to feed throughout the host tree.

Browntail caterpillars across the state have been feeding and growing for about a month now and at some of our developmental monitoring sites they have grown considerably. Many of the sites reported fourth instar caterpillars, which are distinguished by their white tufts on each body segment by the second week of May which is a couple of weeks earlier than in previous years. One explanation for this is that the mild early spring may have coaxed the caterpillars to emerge and therefore begin feeding earlier. There is also quite a bit of variability in size even within a single nest with some caterpillars being as large as 17 mm while others that were 7 mm look they had not molted since they emerged in April.

We have been receiving reports of wandering caterpillars for about a week and a half now, which usually doesn’t occur until the end of May. They may be outstripping the host plants they emerged on and need to find better quality food, regardless it is much earlier than anticipated this year.  Swarming caterpillars can be removed from hard surfaces such as decks, siding and driveways using a HEPA filter-equipped wet/dry vacuum with a few inches of soapy water in the basin. Let the caterpillars soak in the soapy water at least overnight and check to be sure they have died before discarding.

In other news, University of Maine Assistant Professor of Forest Entomology  Dr. Angela Mech has a fully-funded PhD opportunity working on the ecology and management of browntail moth. In particular, research will investigate the use of sex pheromones for both developing a monitoring program and as a potential mating disruption tool. Interested candidates should reach out to Dr. Mech. 

Eastern Larch Beetle (Dendroctonus simplex)

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Heavy woodpecker feeding on a larch affected by eastern larch beetle revealing evidence of beetle galleries below the bark.

Eastern larch beetle is a common, native bark beetle in Maine that has been a chronic influence on larch stands growing on poorly drained sites throughout the state. Populations of eastern larch beetle have been observed to build with changes in precipitation patterns and therefore water table. It seldom does widespread damage and the primary host tree, tamarack (American larch, Larix laricina), is of limited economic importance here in Maine. While we historically average just a report or two each year of eastern larch beetle damage, the number of reports rose during winter and spring of 2021. We’ve also had other forest health colleagues from the Northeast reach out due to a similar increase in reports.

Eastern larch beetle has been known to undergo periodic outbreaks throughout its native range from northeastern North America to Alaska, but there has been an apparent change in eastern larch beetle population dynamics, especially in the upper Midwest. While certain insects are unable to alter their reproductive strategies, others are able to take advantage of favorable climatic conditions and produce additional generations each year. Researchers at the University of Minnesota have documented that changing climatic conditions have allowed eastern larch beetle to do just this, resulting in the devastation of Minnesota’s tamarack resource. The current outbreak of eastern larch beetle in Minnesota has been ongoing since 2001 and has now affected some 666,000 acres, representing roughly half Minnesota’s tamarack-dominated forests.

This is concerning given the similarities between our northern climates, and the fact that northern climates appear to be disproportionally affected by ongoing climate change. Maine is fortunate in that our tamarack resource is substantially smaller than that of Minnesota, but we still have a substantial resource with around 104,000 acres of this forest type. Aside from that, tamarack is simply a beautiful tree.

The best time to look for tamarack damage is during the winter months when snow blankets the ground. Just like with emerald ash borer, woodpeckers attacking infested trees are the best giveaway that eastern larch beetle is present. Extensive woodpecker activity reveals the reddish inner bark along with large piles of bark chips on the ground below, which stand out well against a snowy background. If you observed tamarack being attacked by woodpeckers this past winter, please reach out to Mike Parisio who will be keeping track of whether tamarack mortality due to eastern larch beetle is becoming more common in Maine. If not, please be on the lookout whenever the snow flies again. 

Emerald Ash Borer (Agrilus planipennis)

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Example of a metallic green adult emerald ash borer (actual sixe of abut 0.5 inches).

May first marked the beginning of the 2021 regulatory flight period for emerald ash borer in Maine and once again you will soon begin to see purple prism traps hanging in ash trees all over the State. These are an important part of our detection network and we remind folks to please not disturb these traps despite their odd appearance. Biological flight period, when adults are out, will begin in southern Maine in late-May or early June, depending on temperatures.  This activity begins closer to late June or early July in northernmost Maine. To slow the spread of this pest, it is essential that ash material is not moved unless absolutely necessary during the high risk months of May through October. Some movement of ash tree material is prohibited under state quarantine rules.

If you think you see an EAB adult, please try to capture a sample or document it with a good-quality photograph. There are several other bright, metallic green look-alikes in Maine, and this can help us to obtain a proper Identification.

Elm Zigzag Sawfly (Aproceros leucopoda)

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Elm leaf demonstrating the signature zigzag feeding pattern of a tiny elm zigzag sawfly larva. Credit: Véronique Martel, Natural Resources Canada.

Be on the lookout for yet another species newly introduced to North America – the elm zigzag sawfly. So far, the only known populations of this insect in North America are in Quebec. Researchers there have reached out to neighboring states and provinces for assistance with determining its distribution. If you happen to see yellow sticky card traps placed in elm trees this summer, you’ve probably stumbled upon a monitoring site. 

As the name implies, larval feeding on elm leaves create a distinct zigzag pattern. Keep in mind there are many other insects that feed on elm leaves in Maine, such as elm leaf miner and elm leaf beetle. Feeding damage from these other insects will appear as irregular patches of skeletonized tissue and lack the distinctive shape of feeding damage from elm zigzag sawfly. For additional information and photos of feeding damage, please visit the Canadian Food Inspection Agency’s comprehensive webpage.

If you think you’ve observed this type of feeding damage in Maine, please attempt to document it with photos and alert the Maine Forest Service Insect and Disease Lab at or 207-287-2431.

Hemlock Woolly Adelgid (Adelges tsugae)

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A colony of hemlock woolly adelgid at the base of hemlock needles is revealed by their namesake white covering.

We’d like to remind people that we are in the middle of the high-risk season (late March to the end of July) for moving hemlock woolly adelgid (HWA) and starting new infestations. The tiny, mobile crawlers are now present, and since they are all female and do not need to mate, all it takes is for one crawler to be accidentally moved to a new tree to start a new infestation. Whenever possible, try to conduct all tree work involving hemlocks between August and February when the risk of spreading HWA is generally lowest. 

We have been receiving a large volume of calls about HWA this spring. This may be due in part to the mild winters over the past two years. Each March we look at infested hemlock branches from several sites to determine what proportion of the population did not survive the winter. In some years winter mortality has been over 90% (less than 10% survive the winter). However, over the past two milder winters, winter mortality has been around 60% (40% survive the winter). Although 60% winter mortality may sound like a lot, when you consider that each female can lay up to 300 eggs and that there are two generations per year, you can understand why HWA populations have been growing in the past couple of years.

For those wishing for more information about HWA, we have a fact sheet summarizing their biology as well as some control possibilities and more detailed information, and guidelines on hemlock management, on our website.

Pine Bark Adelgid (Pineus strobi)

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Trunk of a white pine tree bearing a heavy population of pine bark adelgid.

Although May snow is never off limits in Maine, the strange sight pictured above happens to be pine bark adelgid. Like certain other adelgid species, pine bark adelgid produces a white, woolly wax covering that is conspicuous on the trunks of pines as populations become large. Adelgids gather on the trunk in large numbers to feed on plant fluids, inserting their stylets (mouthparts) through the bark into the phloem tissue of the tree. Immature stages of pine bark adelgid are mobile, but also like other adelgid species, become sessile once feeding begins.

It’s not unusual or alarming to observe small populations of pine bark adelgid from time to time, but populations do fluctuate and occasionally reach the levels pictured here. These fluctuations are believed to be driven by weather conditions and predator populations, such as beetles and flies, which are voracious predators of aphids and adelgids. Regardless, pine bark adelgid seldom does significant damage to mature, vigorous trees. The damage is often seen on smaller branches and twigs, or where a large pine bark adelgid population overwhelms a small or suppressed pine in the forest understory.

The primary host is eastern white pine, however pine bark adelgid has also been known to infest Austrian and Scots pine that might be planted for ornamental purposes. Control measures for pine bark adelgid are usually unnecessary but are possible. Treating the trunk with a horticultural oil or insecticidal soap in late fall or early spring should help to kill a large proportion of overwintering nymphs. In both cases, temperatures should be above 40° F during application and remain above 40° F for 24 hours following application. Weather permitting, the first spring treatment should occur by mid-April when nymphs become active and subsequent spring applications may be required. Use of horticultural oils or insecticidal soaps should help to preserve more of the beneficial predatory insects that contribute to natural pine bark adelgid control.

Samples from the tree pictured here have been sent to USFS researchers attempting to obtain additional samples of a previously collected, unidentified fly species that appears to be a predator of pine bark adelgid. 

Spruce Budworm (Choristoneura fumiferana)

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Spruce budworm-related defoliation observed in Maine in 2020.

In July of last year, we reported on the first significant detections of defoliation by spruce budworm seen in Maine since the collapse of the last outbreak. This year, we are asking you to keep your eyes peeled for us and report your observations of defoliation of balsam fir and spruce in the woods of Maine.

University of Maine at Fort Kent Professor Dr. Neil Thompson is monitoring budworm larval development at a spot in Northern Aroostook County where overwintering larval (L2) sampling coordinated by the Cooperative Forest Research Unit revealed a building population. In the second week of May Dr. Thompson reported very scant webbing visible in the field, but abundant larvae emerging from pollen cones on branches brought into the lab for monitoring. We expect that careful examination of the upper, and outer branches of spruce and fir in the northern third of the state may reveal from trace to moderate defoliation of current-year shoots, as was seen in MFS mid-season defoliation assessments in 2020. The CFRU has posted a helpful video on spruce budworm identification on their YouTube channel featuring footage from Maine.

In related news, Dr. Angela Mech at the University of Maine has an opening for a Spruce Budworm Processing Lab Manager. The successful candidate will be responsible for processing spruce and fir branches from over 300 sites in Maine to determine overwintering spruce budworm population densities, and work with landowners, agencies, and researchers to evaluate management options. Applications for this position will be reviewed beginning on May 24. The goal is to have the position filled by July 1. Applications for this position will be reviewed beginning on May 24. The goal is to have the position filled by July 1. 

Southern pine beetle (Dendroctonus frontalis)


Example of a Lindgren funnel trap used in the Southern pine beetle survey program, in addition to several other surveys targeting beetle species.

Southern pine beetle monitoring traps were deployed the first week of May and will be checked every two weeks until July. Southern pine beetle has not been found in the state yet but is on our doorstep in southern New England. This trapping effort is focused on detecting it soon after its arrival. Southern pine beetle is an aggressive bark beetle native to the southeastern U.S. that threatens our hard pine resources such as coastal pitch pine and jack pine as well as red and Scots pine. Milder winters and high wind events like hurricane Sandy have facilitated its movement into parts of southern New England. The 11 traps are located at 10 different sites focusing on our southern coastal and inland pine barrens areas which are ranked S1, critically imperiled, in Maine.

Winter Moth (Operophtera brumata)


Sign on Cyzenis albicans release cage.

The annual release of Cyzenis albicans flies, the biocontrol agent for winter moth, occurred this week. The release took place on May 17 in East Boothbay Harbor. This year’s host homeowners lovingly cared for the flies as they emerged and even fashioned a homemade sign to deter curious hands (see photo). We are gearing up to collect caterpillars from our previous release sites next week. Flies reared from those caterpillars will serve as ‘seed stock’ for next year’s release. In addition, these collections will show where the parasitoid established successfully and what proportion of the winter moth population it is parasitizing.

Diseases, Injuries and Environmental Issues

Beech Leaf Disease Long-term Monitoring Plots


Beech leaf disease symptoms. Jim Chatfield, OSU Extension.

In 2021, pathologists from the US Forest Service, Durham office, in cooperation with the Maine Forest Service, will be establishing nine long-term monitoring plots to monitor for beech leaf disease (BLD). While the disease has not been confirmed in Maine yet, the plots will provide set areas that will be checked at regular intervals for disease. Should the disease arrive in Maine, the plots will be used to assess the progression and impact of the disease.

BLD was first found in Ohio in 2012 and has spread eastward in the US and northward into Canada. The most recent detections in the US occurred in CT, RI and MA in 2020. This new disease is currently being studied and there are several unknowns at this time. Current understanding of the disease is that a microscopic roundworm, the nematode Litylenchus crenatae mccannii, a native of Japan, is the causal organism. Major questions remain unanswered regarding such things as the exact physiology of the disease in beech trees and even how the disease spreads.

BLD symptoms begin as leaf banding, progressing to severe leaf deformity and reduced bud formation. The disease symptoms are first seen in beech trees growing in lower levels in the forest leading to the most severe mortality in seedling and saplings. From there, it progresses upward affecting mature trees, which can be severely impacted leading to mortality from secondary agents of decline, if not from BLD itself. Symptoms are most easily observed by looking up into the canopy from below.

If you see symptoms like those pictured on planted beech or native beech in the forest, please contact Aaron Bergdahl, 207-287-3008.

Caliciopsis Canker of Eastern White Pine (Caliciopsis pinea)

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(left) Pine pitch streaking between branch nodes is a symptom of Caliciopsis cankers; (right arrows) The conspicuous, eyelash like spore-producing structures on the fungus causing Caliciopsis canker on a white pine; (inset) a close-up of the spore-producing structures.

The Maine Forest Service is assisting researchers from Michigan State University in a life cycle study of an incompletely understood fungal canker of eastern white pine. The disease under study is Caliciopsis canker of eastern white pine, caused by the fungus C. pinea. It causes numerous infections in the cambium forming cankers at each infection site on the main stem of white pine trees. A symptom of infection is exudation of whitish pitch, which is a tree defense response. Formation of numerous cankers, typical of C. Pinea infection, will inhibit movement of nutrients in the tree, leading to tree health decline. The spore release period for this fungus is currently not known. The planned epidemiological study is designed to provide a better understanding of this, which may have management implications for eastern white pine in Maine. Spore traps will collect spores from May to November and weather data will also be examined to determine possible relationships.

Cedar-Apple Rust (Gymnosporangium juniperi-virginianae)

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(left) Galls on eastern red cedar with spore tendrils beginning to extend from the gall; (inset) a close-up of a gall and tendrils; (right) late summer cedar apple rust symptoms on apple leaves.

Cedar-apple rust needs plants from two different families to complete its life cycle: the Cupressaceae family (including eastern red cedar, juniper) and the Rosaceae family (in the case of cedar-apple rust, apple/crabapple). Without species from both of these families the disease cannot cause damage. There are a few other similar rusts in this genus (Gymnosporangium) that have specific symptoms on cedar/juniper and on hawthorn and serviceberry, for example. This is the time of year when you may see the less serious impact of cedar-apple rust on redcedar and juniper in the form of galls that may have gelatinous, orange spore tendrils hanging from them. These spore tendrils appear as spikes (see figure, inset) in dry weather but will swell in wet weather and release spores for a period of several weeks in spring/early summer. The spores will infect a Rosaceous host which will form spore-producing structures whose spores will infect a Cupressaceous host later in summer.

Management of cedar-apple rust is difficult in Maine due to the abundance of both hosts in the natural and horticultural environments. The best cultural method of management is to not plant susceptible varieties near alternate hosts. The next best method is to plant varieties that are resistant to cedar-apple rust (there are several options available in the nursery trade). If practical, prune out galls on juniper. According to the University of Minnesota, fungicides containing copper or with the active ingredient Myclobutanil can be applied to protect landscape Rosaceous hosts (for aesthetics) during the time period when the spore tendrils are releasing spores (requires scouting of juniper galls following wet weather), which is typically 4 to 6 weeks – this may require several applications depending on the residual period of the product used. Use of fungicides for control on Cupressaceous hosts is generally not recommended and may be difficult to correctly time.

Spruce Needle Casts (Rhizosphaera kalkhoffii, Stigmina lautii)


Colorado blue spruce heavily impacted by spruce needle cast disease.

The infection levels of spruce by the spruce needle cast fungi, R. kalkhoffii and S. lautii, should be lower this year. This is because of the lifecycle of these diseases: symptoms seen this year originate from infections last year (needles stay asymptomatic for one year following infection). Otherwise said, the disease conditions of the current year are related to the weather conditions of the previous spring/early summer, which is the peak of the infection period. Thus, the drier weather of spring and early summer 2020 should mean lower infection rates and less severe symptoms. This does not, however, mean fungicide applications for disease control are unneeded to prevent further decline. For several weeks after budbreak, prolonged periods of wet weather could lead to new infections. In a landscape setting where aesthetics of Colorado blue spruce and white spruce is important, or full foliage is needed to maintain a privacy screen, experts suggest yearly application of copper-based fungicide in accordance with the product label (typically at two-week intervals after budbreak) through July and even into August. This is especially important in coastal areas with frequent periods of higher relative humidity (fogs). Protective fungicides are needed because there are enough trees in the general environment carrying high levels of inoculum (spore producing structures) for consistent re-infection. Spruce needle cast diseases continue to be found throughout Maine impacting the aesthetics of spruce trees.

Sapsucker Damage (Sphyrapicus varius)


(left) A yellow-bellied sapsucker and the characteristic damage it creates on trees (L. Huffman, Audubon Society); (right) Sapsuckers are notoriously shy and because of this, people often do not witness them causing damage to their trees (this is the best picture I have been able to get of these cautious migratory birds).

Sapsuckers are a subclass of woodpeckers that can damage trees by their feeding activity. Although sapsuckers do nest in many parts of Maine, many of these migratory birds are just passing through on their way to nesting areas further north. Most new damage to trees occurs in early spring, when the birds create nearly consistent and neat rows of cavities in the bark. Although sapsuckers have been documented feeding on over 250 species of native trees and vines in the US, favored trees in Maine include birch, hemlocks, aspens, and especially fruit trees (they seem to be particularly fond of apples/crabapples). Sapsuckers feed on the sap that flows to these wounds and also the insects attracted to the sap. Heavy damage has been observed in areas in Central and Southern Maine this year, but the bird’s range is wide, and therefore so is the damage. Sapsuckers are protected by the Migratory Bird Act. Thus, the only management option is to discourage them from feeding on your (and their) favorite trees. This can be done via physical barriers, like applying a sticky substance called tanglefoot to the feeding areas (a very messy, and likely a regrettable management tactic) or wrapping affected sections of tree trunks in burlap. Also, you can try to repel the sapsuckers via scare tactics like hanging old compact disks in trees or using predatory bird decoys. While these techniques may be successful, the sapsuckers typically acclimate to them over time or will simply fly to their next favorite tree in the area for an easier meal, thus damaging more trees. Usually trees are fairly resilient to the damage caused by sapsuckers, so tolerating and even trying to enjoy the presence of these shy seasonal visitors may be the easiest and most effective thing to do.


Details of upcoming events can be found by visiting Maine Forest Service's website.

Conditions Report No. 2, 2021


Department of Agriculture Conservation & Forestry
Maine Forest Service – Forest Health and Monitoring
Contributors: Aaron Bergdahl, Allison Kanoti, Mike Parisio, Thomas Schmeelk, and Colleen Teerling

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