Forest & Shade Tree – Insect & Disease Conditions for Maine
April 29, 2022
Hello from your Forest Health and Management team! We are happy to provide you the first of the Condition Reports for 2022.
In This Edition:
“Caterpillar 101” Now Available Online
While no one knows for sure what this season will bring, we can say with some degree of confidence that it will be another busy year for defoliators such as browntail moth caterpillar and spongy moth caterpillar, possibly among others. Browntail moth caterpillar emergence and spongy moth egg hatch is already occurring in parts of Maine. The winter web pruning window for browntail moth has now passed, and this might be the last chance to get out and do some spongy moth egg mass scraping. With active populations of so many caterpillar species in a single season, we’ve aimed to try to make caterpillar identification a bit easier for the average homeowner. The ability to accurately identify caterpillars can help both you, the caller, and us, the forest entomologists on the other end of the line, have better conversations about your situation and get you the best management recommendations more quickly. If you have eight minutes to spare, please watch our short video on identifying some of the most commonly encountered hairy caterpillars here in Maine.
NOTE: Earlier this year spongy moth became the common name officially recognized by the Entomological Society of America for Lymantria dispar, formerly gypsy moth.
Browntail moth (Euproctis chrysorrhoea)
Our annual winter web survey wrapped up earlier this month thanks to our hardworking field crews. The data is now available on the browntail moth (BTM) dashboard. This survey helps predict the intensity of BTM infestations for the upcoming spring over a broad area. This is a coarse windshield survey however, and BTM webs are not always visible from a moving vehicle; even in surveyed areas, the information can be incomplete. For towns and homeowners, additional finer-scale surveys are required to make location-specific decisions about management activities.
Beginning the week of April 11, we received our first confirmed report of BTM caterpillars emerging from their winter webs, followed by more widespread reports the week of April 18. Like the past two years, we will continue with our network of monitoring sites throughout the season, posting developmental updates for the public and stakeholders on the Maine Forest Service BTM website and to do the Browntail News bulletin. The best opportunity to clip webs has passed due to both caterpillar and host development.
Dead browntail moth caterpillars on the outside of a winter web, Chelsea, Maine.
During preliminary checks at several monitoring sites, we observed many webs with dead caterpillars on the outside from late last summer; this appears to be a little more widespread than previously thought, at least throughout Kennebec County. We’re working with Dr. Angela Mech and her graduate students at UMaine to see if there are any pathogens viable for infecting the newly emerging caterpillars. We also plan to perform test inoculations of newly emerged caterpillars with material gathered from fungus-killed caterpillars last summer to see if this is a viable option for spreading disease. Wet weather, particularly in May and June, helps the fungus and virus that attack BTM to spread and proliferate. As mentioned in our conditions reports last year, we made regionally widespread observations of fungal and viral activity impacting BTM caterpillars from Blue Hill down through Jefferson and Liberty and many towns in between. Unfortunately, these outbreaks were confined to just a few trees at each site, but with the proper weather this spring and early summer, they are in good locations to spread and help crash the population.
Emerald Ash Borer (Agrilus planipennis)
The Maine Forest Service is once again soliciting participation in our Girdled Trap Tree Network to monitor for EAB this May/June. Anyone outside the generally infested area of Maine and who has an ash tree they are willing to donate to science is welcome to participate. We are especially seeking trap trees located in Oxford, Androscoggin, and Sagadahoc counties. However, trap trees are welcomed in all other counties outside of known infested areas.
Trap trees are perhaps the most powerful tool for towns, landowners/managers, land trusts, and others to monitor for EAB in their specific areas. Trap trees have often detected EAB one to two years before any external signs of infestation, allowing land managers more time to plan and manage their forests.
An ideal trap tree is:
- A healthy ash (green, white or black/brown),
- Four to ten inches in diameter at breast height (4.5 feet from ground level),
- Getting plenty of sunlight, either because it is dominant/co-dominant, or because it is along an edge,
- Safely and easily felled in the fall, and readily moved to a processing site.
A trap tree is created by removing a cylindrical section of bark from around the trunk in late May or early June. The tree stays alive throughout this growing season but is stressed, causing it to release scents into the air, making it attractive to EAB in the area. If present, EAB are more likely to come to this tree than a random ash. A trap tree does NOT draw EAB into an area from afar but may help protect other ash in the immediate area. Some landowners will girdle trees they plan to cut for firewood, allowing these trees to do double duty.
To girdle a trap tree:
- Peel a 10 to12-inch-tall section of bark from the lower stem.
- Peel to the sapwood between early-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 callus 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 to 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, and it will no longer attract EAB.
- An alternative to the drawknife is a pruning saw or other tool to make two parallel cuts 10-12-inches apart. 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.
A short video on how to girdle a tree is available.
If you girdle an ash tree for EAB monitoring, please use the online form, EAB Trap Trees 2022, to report it to the Maine Forest Service. You can also reach out at foresthealth@maine.gov 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. We will send out information in the autumn on how to fell and section trap trees. We ask 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.
(Left) known locations of emerald ash borer in Maine; (right) a girdled trap tree. (Photo: E. Carle)
European Wood Wasp (Sirex noctilio)
Along with the new species detected through the 2021 EDRR and the detection of southern pine beetle, the European wood wasp (Sirex noctilio) is yet another newcomer to Maine (see the Early Detection – Rapid Response Survey Underway and Southern Pine Beetle sections in this report). This new detection occurred unintentionally, so to speak, as the two specimens recovered were “by-catch” from another project. The wasps emerged from white pine logs taken from the Massabesic Experimental Forest (York County) and held in barrels over a two year period to monitor insect emergence. Finding this species came as a bit of a surprise because neither white pine nor the site are considered ideal for this species. Regardless, the samples were soon after confirmed as European wood wasp. Interestingly, this species was confirmed in New Hampshire for the first time in late 2021.
European wood wasp is native to Europe and Asia, but is now distributed worldwide due to multiple introductions, including to the continents of Africa, Australia, South America, and North America. This species typically attacks hard pines and has caused the most damage in its introduced range in hard pine plantations in the southern hemisphere. The amount of damage has been much less in North America, however there is still much to learn about this species’ behavior here and forest health concerns still exist, especially in southern states.
Now that we know European wood wasp is present within Maine’s borders, our first order of business will be to perform additional survey work in 2022 to better determine its distribution. This will include visual damage surveys of areas with extensive red pine resources, as well as careful screening of by-catch from our other projects to look for European wood wasp. We do have other native wood wasp species that look similar to Sirex noctilio, and the large, round exits holes resemble those of native wood wasps and long-horned beetles. Unlike the all-black females, males are more easily distinguished from our native species by a large orange band on the abdomen. This insect can be notoriously hard to detect in the absence of obvious damage, just like many of the other concerning invasive wood-boring species.
Although this insect is federally regulated, a comprehensive set of rules governing domestic wood movement is lacking. Maine pine producers sending products out of state should be aware that several states have regulations at the state level (at least Arizona, Iowa, Mississippi, and North Carolina). New permits may be required if shipping products to these states, so exporters should inquire directly with regulatory officials in the destination state for this information. This detection appears to have less impact on international export of pine products.
Dorsal and lateral views of adult female (left) and male (right) European wood wasps (Sirex noctilio), (Photos: Steven Valley, Oregon Department of Agriculture, Bugwood.org)
Hemlock Woolly Adelgid (Adelges tsugae)
Now that spring has arrived, we are in the season of high risk for transmission of hemlock woolly adelgid (HWA). Tiny mobile crawlers and readily dislodged eggs can be easily picked up simply by brushing against an infested tree. It is best to delay major work in areas with hemlock until the fall when all crawlers have settled and become permanently attached to the trees.
HWA is one invasive species in Maine which is limited by our cold winter weather. In past years, winter mortality was sometimes well over 90 percent. However, we now no longer get the consistently cold winter weather that used to be more common. This year we examined populations at six sites and found that winter mortality ranged from 47 percent to 87 percent, with an average of 65 percent. Although 65 percent might sound like a fairly high mortality rate, HWA is a prolific species with asexually reproducing females and multiple generations per year. This means HWA populations will likely continue to expand this year.
(Left) Tiny HWA crawlers can be easily moved; (right) Waxy ‘wool’ pulled back to show HWA eggs.
Southern Pine Beetle (Dendroctonus frontalis)
On October 20, 2021, Caroline Kanaskie of the University of New Hampshire found 13 southern pine beetles (SPB) in one of her traps at the Waterboro Barrens, an area owned by The Nature Conservancy (TNC). Kanaskie also found southern pine beetle in her traps in New Hampshire in late 2021, representing the first state records for both Maine and New Hampshire. A ground survey was performed in the area of the trap catch, but no infested trees were found. While this new finding might be alarming, it is more of an early warning for a larger future population of SPB that could attack and kill trees in Maine. Going forward, the Maine Forest Service will adapt its monitoring program to better cover fall dispersal of SPB, whereas previous monitoring had focused on spring dispersal.
SPB is a small beetle (around 3mm long) native to the southeastern US that has been expanding its range northwards due to milder winter temperatures. It is among the more aggressive bark beetles able to attack healthy pine trees with a preference for “hard pines” such as red, pitch, and jack pine. Especially at risk are Maine’s hard pine resources, such as the inland pine barrens (a globally rare ecosystem), the pitch pine communities along Maine’s rocky coast, and red pine plantations statewide. However, there can be a silver lining to this story, as forests properly managed through thinning or prescribed fire are more resilient to SPB attack. In the absence of natural fire, disturbances like these are necessary to maintain the health of fire-adapted ecosystems like the inland pine barrens. As a landowner, TNC has been proactive in managing the pine barrens by using fire and thinning to reduce stand density and increase resilience to the beetles.
Winter moth (Operophtera brumata)
On October 30, 2021 we placed an emergence cage containing 329 Cyzenis albicans pupae in the ground at our newest biocontrol release site in South Bristol. Cyzenis albicans is a host-specific parasitic fly which is the biocontrol agent of winter moth. MFS has been releasing it in Maine since 2013, working our way up the coast with each successful establishment of the fly. The South Bristol emergence cage will be opened in mid to late May once the flies have fully developed, allowing them to emerge under their own power and disperse.
Cyzenis albicans emergence cage in South Bristol, ME.
Like browntail moth caterpillars, winter moth caterpillars often begin feeding before leaves expand. We have scattered reports of defoliation by this species from Kittery to Mount Desert. We would appreciate your reports of suspected winter moth feeding damage in Maine, so we can better target our surveys to deploy biological control.
Winter moth caterpillars start feeding before the leaves are fully emerged.
Early Detection – Rapid Response Survey Underway
Field season is officially upon us and our first trapping survey is already underway with the installation of our Early Detection-Rapid Response sites in mid-April. This will be the second consecutive season where Maine takes part in this federally sponsored survey program targeting invasive ambrosia beetle and bark beetle species. Overall, we collected 24,097 individual ambrosia beetles and bark beetle samples belonging to 58 species in 2021. Though most of these are native species, four of the species recovered in 2021 were new State records. These include Cyclorhipidion bodoanum, Hylesinus pruinosus, Xylosandrus crassiusculus (granulate ambrosia beetle), and Ips avulsus (the small southern pine engraver). While they may not belong in Maine, fortunately none of these are of serious concerns as forest pests. There exists a somewhat greater concern over the potential for some of these new species to become nursery or orchard pests here as has been witnessed elsewhere. While we look forward to the results of this season’s survey, we hope not to add any more species to Maine’s ever-growing list of non-native insect species.
Beech Leaf Disease
Beech leaf disease (BLD) was first found in Maine in 2021 in the town of Lincolnville. Subsequent delimiting surveys revealed that the disease was found throughout Waldo and Knox counties with later detections in Lincoln and Penobscot counties. While information is building about BLD, there is still much that is unknown. Recent scientific efforts have indicated that the causal agent of BLD may not be solely the plant pathogenic nematode, Litylenchus crenatae mccannii, and may involve a combination of the nematode and other microorganisms (See journal article). However, just how BLD spreads is still a mystery. Despite the gaps in knowledge, MFS will continue efforts to better understand and monitor BLD in Maine in 2022. These efforts will consist of continuing assessments of the BLD long-term monitoring plots established in 2021. These were established in areas with and without current BLD symptoms. Also, BLD survey will continue to determine the extent of the disease in Maine. The most current information about BLD in Maine can be found on the recently updated BLD webpage on the MFS website. For questions about BLD, please contact Aaron.Bergdahl@maine.gov, (207) 287-3008.
Exposure to De-icing Materials
In late winter and early spring the insect and disease lab begins to receive calls from citizens concerned about the condition of roadside trees and shrubs. They describe pale to orangish foliage and express fear that mass amounts of roadside trees are dying. These visual symptoms by roadways are very often due to exposure to de-icing products, primarily those containing sodium chloride. This year, fewer reports were received and less damage reported, but salt damage remains a significant stressor to trees near rights of way where deicing products are used.
When sodium chloride dissolves in water, sodium and chloride separate. When the positive sodium ions enter the soil, they prevent plant roots from absorbing as much needed nutrients like calcium and magnesium as they would in the absence of high-sodium soil levels – this can lead to nutrient deficiency. The chloride is also absorbed by plants and can accumulate in leaves/needles, causing chlorosis (yellowing) and necrosis (death) of leaf tissues. Further, salt deposited directly on evergreen foliage can desiccate it, resulting in pale to orangish coloration. This foliage may recover, or if exposure is high enough, evergreen foliage can die, leading to thinner looking crowns. When salts are deposited on the coming year’s buds of deciduous tree, emerging leaves may be deformed or buds may even die, delaying leaf out as the tree has to set new buds. This is a significant tree stress. Flower buds are particularly sensitive to salts and may be killed via salt exposure. Logically, trees and tree parts closest to the point of exposure will be more heavily impacted than trees and tree parts further away from the exposure point. Thus, it is not uncommon to see road salt exposure symptoms expressed decreasingly with increased distance from the road. Also, it is typical to see roadside trees showing salt exposure symptoms on the road side of the tree, while the opposite side may show less severe or even no symptoms.
There are a few ways to mitigate salt damage. First, barriers can be used to protect trees from salts in a yard setting. One commonly used barrier is fabric, such as burlap, that will also help to limit winter burn (winter burn is a type of winter damage to trees that can be related to salt exposure, but also may simply be due to warming aboveground temperatures stimulating physiological activity in the foliage leading to moisture loss, while the water to replace that lost in transpiration is still unavailable as ice in the root zone). In a landscape setting, deep watering can help flush salt from the root horizon of the soil. Also, salt-tolerant plants should be used in high salt-exposed areas. There are alternatives to plant-damaging de-icing materials like potassium chloride, calcium chloride and calcium magnesium acetate. However, these formulations are often significantly more costly.
Finally, the reason I believe salt damage symptoms were less reported and seem to be less severe this year in many parts of the state is the periodic rains experienced several times during the winter months. Rain events in many parts of Maine in January, February, and March likely washed away salt accumulations on foliage, lessening symptoms. The impacts of high soil salinity may yet be seen, with symptoms often noticed in lower areas where salty run-off accumulates.
 (Left) Salt damage to roadside hemlock foliage; (right) Salt damage to a fir tree showing more severe damage on the side facing the road (this tree faced west and was well sheltered from direct sun).
White Pine Blister Rust (Cronartium ribicola)
Early spring is an ideal time to scout woodlots for currants and gooseberries (plants in the genus Ribes). Plants in this genus serve as an alternate host for the fungal pathogen that causes white pine blister rust, a serious disease impacting the highly valuable (economically and ecologically) white pine resource in Maine. Ribes plants are some of the first vegetation to leaf out in early spring, so locating and physically removing or managing Ribes plants with herbicide will enhance control of white pine blister rust in a specific area. Removal of all Ribes species within 1000 feet of susceptible pines will greatly reduce the likelihood of infection by the white pine blister rust pathogen.
Control of white pine blister rust can be achieved by removing Ribes, since the fungus needs both a Ribes host and a white pine host to complete its complex life cycle and cause disease. Mid-May is typically a good time to scout for the orange pustules that are a sign of the white pine blister rust fungus infecting pine. These structures, called aecia, are typically found at the base of younger white pine trees and at branch junctions/whorls. The aecia produce the spore type that will infect Ribes leaves, the fungus will then produce different spore-producing structures that will eventually produce spores that reinfect white pine. The orange aecia are a sure sign of infections. Infected trees will eventually die and should be removed to prevent further spread of white pine blister rust.
A young white pine tree with orange spore-producing pustules (aecia) erupting through the bark. (Photo: William Kragnes)
Strict laws are in place regulating the sale of Ribes in Maine (see the Ribes Regulatory Reminder section in this report). There are also several native species in the genus Ribes common throughout Maine that can be alternate hosts for white pine blister rust. However, cultivated varieties of currants and gooseberries are highly efficient alternate hosts for white pine blister rust, which is why Maine state law prohibits bringing these plants into Maine and growing them throughout most of the state. Some other New England states and New York have allowed the cultivation of Ribes specifically bred to be highly resistant or immune to white pine blister rust disease. However, a more virulent strain of the pathogen that causes white pine blister rust was discovered in 2011 in New England, and cultivars of Ribes, especially the European black currant, are now very likely susceptible, and represent a significant risk of spreading the disease to white pines. This is why European black currants are prohibited throughout the entire state of Maine.
Ribes Plant Sales and Planting – Regulatory Reminder
Plants in the genus Ribes (currants, gooseberries and their hybrids such as jostaberries) are an alternate host for the rust fungus that causes white pine blister rust, a devastating disease of white pine. Although regulations surrounding the sale of currants and gooseberries in Maine have been in place since 1979, these plants inevitably still show up on nursery shelves each and every spring. Fortunately, DACF also now regularly receives reports when they do, so we are able to take immediate action and prevent these plants from being sold and planted around the state. Already in 2022, we have intercepted red currant, black currant, and jostaberry plants. If you see any of these plants for sale, please do not purchase, and alert the DACF Horticulture Program. The quarantine regulations can be summarized as follows (for official language, see the Secretary of State Office’s website on rules, 01 669 Chapter 1):
- The sale, transportation, further planting or possession of plants of the genus Ribes (commonly known as currant and gooseberry plants, including cultivated, wild, or ornamental sorts) is prohibited in the following Counties in the State of Maine: all of York, Cumberland, Androscoggin, Kennebec, Sagadahoc, Lincoln, Knox, Waldo, Hancock, and parts of Oxford, Franklin, Somerset, Piscataquis, Penobscot, Aroostook, and Washington.
- The planting or possession of European Black Currant, Ribes nigrum, or its varieties or hybrids anywhere within the boundaries of the State of Maine is prohibited.
Winter Animal Damage
Each winter voles, squirrels, porcupines and other assorted animals damage trees. Sometimes the damage is a result of feeding on bark or foliage. Other times the damage results from squirrels or porcupines that clip large amounts of branch tips and larger branches for unclear reasons. Sometimes the damage is severe, resulting in large wounds and significant foliage loss that stress trees significantly – mortality is common.
This year was the first time I witnessed animal damage that could possibly benefit a tree. The pine-pine rust (aka western gall rust) *Endocronartium harknessii causes gall formation on 2- or 3-year-old wood in 2- and 3-needle pines. This particular rust is microcyclic, meaning it does not need an alternate host (like an autoecious rust) and therefore lacks the spore types had by rusts that alternate hosts. Orange spore-producing pustules form on the bark of galls in spring. The spores reinfect the same tree, or nearby susceptible host trees. So, the missing bark on these galls prevents spore production and therefore decreases reinfection. The animal feeding will girdle and hasten the decline and death of the branch, but branch dieback is already often associated with gall rust infection.
 (Left) Heavy porcupine feeding on a young larch tree (Howard Carter); (right) The bark on this rust gall has curiously been a preferred food source for an unconfirmed animal (likely a squirrel). (Photo: Edwin Barkdoll)
*There is a possibility this gall (right picture) was formed by pine-oak gall rust Cronartium quercuum, although positive ID was not able to be made based on the picture.
From the April 23, 1991 Condition Report:
“Maine Forest Service Centennial and Arbor Week Activities- Governor John R. McKernan has issued a proclamation declaring March 25, 1991 as Maine Forest Service Day to honor the Maine Forest Service’s 100th year as a state agency providing leadership in forest conservation.
A number of activities are being planned around the state in celebration of the Maine Forest Service Centennial. These will include an historical photo exhibit which will be displayed at various locations during the year, and tours of Maine Forest Service facilities. The Maine Forest Service will also be distributing donated trees to schools throughout the state in an effort to encourage tree planting activities during Arbor Week, May 21-27.”
May 2, Emerald Ash Borer (EAB) and other Tree Health Concerns: Update for Maine Cities and Towns, 10:30am-12:00pm, Virtual. The Maine Department of Agriculture, Conservation and Forestry will provide an update for city and town tree and forest managers and public works staff on the known status of emerald ash borer in Maine, current state efforts, municipal opportunities as well as updates on other insects and diseases affecting forests and trees.
- Brief Introduction, Allison Kanoti, Maine Forest Service
- Emerald Ash Borer Update, Mike Parisio, Jan Santerre, Maine Forest Service
- Hardwood Defoliator Update, Browntail Moth, Spongy Moth, and Winter Moth. Tom Schmeelk, Maine Forest Service
- Tree Disease Highlights, Oak Wilt and Beech Leaf Disease, Aaron Bergdahl, Maine Forest Service
- Pesticide Rules Update, John Pietroski, Maine Board of Pesticides Control
- Wrap up, Allison Kanoti, Maine Forest Service
Connection Details
Microsoft Teams meeting
Additional Information
Contact foresthealth@maine.gov or 207.287.2431 for more information.
Continuing education credits:
- Licensed pesticide applicator 1.0
- Licensed Professional Forester, 1.5 hours Category 1
May 8, How to Create a Trap Tree to Monitor for Emerald Ash Borer Workshop, 9:00am-10:30am, Noyes Mt. Preserve, 172 Richardson Hollow Road, Greenwood, Maine. Join Maine Forest Service Entomologist Colleen Teerling and District Forester Mike Richard to learn how you can participate in the early detection of emerald ash borer infestations in Western Maine. Participants will learn about emerald ash borer's lifecycle and how to create a "trap tree" on their property that can aid in detecting infestations within the region.
For more information or to pre-register please call District Forester Mike Richard: 207-441-3276 or michael.b.richard@maine.gov.
This program is offered by the Western Foothills Land Trust, Maine Woodland Owners, Oxford County Soil and Water Conservation District and the Maine Forest Service.
June 17, Overview of Beech Leaf Disease - Forestry Friday Webinar, 9:00am, Virtual. Get an update on the current situation of Beech Leaf Disease in Maine from Cameron McIntire, Plant Pathologist with the USDA Forest Service in Durham, NH. Register online.
Conditions Report No. 1, 2022
On-line
Department of Agriculture Conservation & Forestry, Maine Forest Service – Forest Health and Monitoring.
Contributors: Aaron Bergdahl, Amy Emery, Mike Parisio, Colleen Teerling, and Thomas Schmeelk.
Unless otherwise noted, images by Maine Forest Service, Forest Health and Monitoring, DACF.
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