Forest & Shade Tree – Insect & Disease Conditions for Maine
August 26, 2024
Images: (left) The average difference in temperature in July 2024 compared to average normal temperatures from 1991-2020. July 2024 was about 3-5°F warmer than average for much of the state. (right) The percentage of normal precipitation compared to the normal precipitation from 1991-2020. Overall, the month of July 2024 had slightly above average rainfall for most of the state. Source: Northeast Regional Climate Center.
July 21-24 were the four hottest days on the planet in history since 1940, beating previous records set last year. It was the fourteenth consecutive month of the globally hottest months on record. Projections suggest that July will continue to become even hotter in coming decades. In Maine, the month was hotter than normal with an average monthly temperature of 69.6°F, 4.3°F above averages from 1991-2020. Some areas in southern Maine reached average monthly temperatures that were 5-7°F above normal. In Caribou, 21 out of the 31 days of July had temperatures above 80°F, one of the highest number of days with temperatures above 80°F recorded in the town since record keeping began in 1939.
Statewide, monthly rainfall averaged around 4.75 inches, which is only slightly above the normal rainfall average of 3.79 inches from data collected in July between 1991-2020, however precipitation varied considerably within the state. Some areas of central Maine had more than 200% of normal rainfall, while some areas in southern Maine only saw 25% of normal rainfall totals. This lack of rain contributed to abnormally dry and moderate drought conditions in southern Maine, the only area in Maine to have drought conditions in July 2024. Overall, 0.5% of the state had drought conditions in July, which is similar to this time last year, where no part of the state was experiencing drought according to NOAA data. The three-month forecast projects rainfall is leaning toward above-normal levels through November. Drought and soil moisture conditions for July were otherwise normal and are projected to stay normal through November (NOAA Climate Prediction Center).
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Image: At the end of July 2024, southern Maine had abnormally dry and moderate drought conditions; comprising roughly 0.5% of the state. Sources: NDMC, NOAA, and USDA.
American Pelecinid Wasp (Pelecinus polyturator)
During recent MFS survey work in southern Maine, staff noticed this unique wasp lazily drifting along the lower forest canopy. The American pelecinid wasp, Pelecinus polyturator, is a parasitoid wasp native to North America and is one of only three species in its family, the Pelecinidae. The remaining are in the same genus and are only found in Central and South America, making identification much easier.
The shiny black females use their long abdomens to lay their eggs inside the larvae of scarab beetles, particularly June beetles. One laboratory study found that females can probe their delicate abdomens up to 5 cm into the soil to reach these hard-to-find larval hosts. If you’re looking to spot one yourself, American pelecinid wasps can be found throughout the lower levels of deciduous woodlands in eastern US and Canada during late summer/early fall. Interestingly, males of this species are almost entirely absent from the landscape, leading most experts to believe this species reproduces asexually.
Image: The American pelecinid wasp, Pelecinus polyturator, is a large, slow-flying parasitoid wasp found throughout the eastern US and Canada.
Browntail moth (Euproctis chrysorrhoea)
Earlier this month, browntail moth caterpillars hatched from their egg masses. At this life stage, the caterpillars are very small, only about a quarter inch in length. Young caterpillars have two dark spots near their head and the two glands that comprise the characteristic orange spots are also visible near the tail end of the caterpillar.
These young caterpillars will feed communally on host plants from now until the end of September or early October, depending on environmental conditions. Damage from young browntail during this time of year typically doesn’t cause any lasting health issues for the tree since it is late in the season and the leaves are closer to dropping. At this time the caterpillars will also start building the winter web that they will shelter in during the next several months. This winter web composed of silk and leaves can start off looser and bigger but will condense by the time the caterpillars stop feeding.
Because of the browntail caterpillars' small size, their mouthparts are not large enough to consume entire leaves. Instead, they graze on the outer surface of the leaf, which causes sections of the leaf to die and turn a bronze or copper color. This damage is called skeletonization. When we perform our aerial browntail moth surveys in the late summer, we look for this copper-colored damage in the trees to help identify where browntail moth populations are severe.
Image: Browntail caterpillars skeletonize oak leaves in Dedham, ME. Note the copper color of an older feeding site on the right side of the oak leaf.
The distribution of the browntail caterpillar population in Maine appears to be more sporadic compared to previous years, likely due to the pathogens that attack browntail caterpillars. We are soliciting reports from the public of recently hatched browntail caterpillars or browntail egg masses. Your observations will help us understand where browntail moth is most active in the state. Please send reports to foresthealth@maine.gov. If you can, attach a photo from a phone or tablet and let us know where you took the photo. This will make it easier for us to confirm identity and track where populations are being reported.
Learn more about the risks of exposure to browntail caterpillar hairs and management this time of year in our browntail moth bulletin from August.
Emerald Ash Borer (Agrilus planipennis)
Image: An adult EAB emerging from an urban ash tree in Portland, ME in late August 2024.
This submitted photo from late August is a timely reminder that emerald ash borer emergence is still active in Maine and can continue through early fall. Once September hits, we’ll begin to remove our statewide network of sticky purple prism traps to check for any adult EAB stuck to the surfaces. After that, we’ll move onto the processing of ash trees in our girdled trap tree network throughout the fall to assess those for any evidence of emerald ash borer larvae beneath the bark. As Mainers and visitors alike flock to our beautiful natural areas in late summer, especially Labor Day weekend, we cannot caution enough to leave the firewood behind and source your firewood needs as locally as possible to your destination. Maine continues to enforce a ban on all out-of-state firewood that has not been certified heat-treated to destroy insect and disease pests. Within Maine, all firewood that has not been certified heat-treated is prohibited from leaving the quarantined areas highlighted in blue on the included map. We thank you for your continued public reports of EAB in new places and appreciate any tips if you believe you’re seeing the illegal movement of firewood into the State or beyond Maine’s internal EAB quarantine areas.
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Image: Map of EAB quarantined areas in Maine. Firewood that has not been certified heat-treated is prohibited from leaving the areas shaded in blue.
Fall webworm (Hyphantria cunea)
This time of year, our native fall webworm caterpillars are busy making silk webs which are often mistaken for browntail moth caterpillar webs. Unlike browntail moth, fall webworm caterpillars are not invasive, they do not cause any harm to humans and create minimal tree health impacts. Their webs can get quite large and unsightly, sometimes covering multiple leaves and branches, or even encasing entire trees, but they are not a cause for concern. Once the fall webworm caterpillars become moths, they will not use the webs anymore, and the webs will break down naturally over time.
Image: A large silken web, typical of fall webworm caterpillars, incorporating multiple tree leaves and branches in Garland (Penobscot County).
There’s no need for any management of fall webworm. However, if web removal is desired, the webs can be pruned out of trees with a pole pruner or hand pruners where within reach. Alternatively, the webs can be manually destroyed using a variety of equipment – or even something as simple as a forked stick inserted into the webbing and then spun to collect as much webbing as possible. At this time of year, these native caterpillars are already quite large and have already completed most of the damage that they will cause in a season. There are always associated risks with pesticide use, therefore the benefits and the consequences of use should be considered. In this case, the tree damage is typically minimal, therefore, pesticide use targeting fall webworm caterpillars is not recommended.
We put together some information on the difference between browntail moth and fall webworm caterpillars and webs with photos that may be help identify which insect is making webs on your property. Although both browntail moth caterpillars and fall webworm caterpillars are making webs on the same host trees at this time of year, there are distinct differences between the two insects. Browntail moth webs do not get as large as those of the fall webworm; often, browntail webs are no larger than the palm of your hand. On the other hand, fall webworm webs can span multiple branches and are usually larger than the size of a basketball. If you are close enough to the webs, you may be able to accurately identify the caterpillars. Browntail caterpillars in late summer are small (about a quarter of an inch) with two dark spots near their head, whereas fall webworm caterpillars are larger and lack the two dark spots – instead, they will have rows of dark spots down their back. If you're not sure if you have browntail moth webs or fall webworm webs, you can always submit photos to foresthealth@maine.gov so we can help determine the insect making the webs.
Graphisurus fasciatus
One of our numerous native longhorn beetles that lacks a common name is Graphisurus fasciatus. This species is one of the more common members of the family Cerambycidae, though it usually goes unnoticed due to camouflage that allows it to blend in perfectly with the bark of trees like oak and beech. G. fasciatus is sexually dimorphic (males and females look different) and females possess a long protrusion at the end of their abdomen known as an ovipositor, used to probe cracks and wounds in the bark of dead and dying host trees in which they lay eggs. The grub-like larvae feed on cambium and wood tissue under the bark. While this species prefers oak, it can be found in a variety of hardwoods and interestingly, pine is also included in its host list. These beetles serve an important ecological role by utilizing dead and dying trees, which aids in decomposition, nutrient cycling, and forest succession. Adults can be found from April through October throughout much of its range and are often encountered at lights at night or when attracted to freshly cut trees. There is also a closely related species called Graphisurus despectus that is uncommon and can be distinguished by subtle differences in morphology.
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Images: A mating pair of Graphisurus fasciatus found on a fresh-cut firewood pile; note the female’s ovipositor in the lower photo.
Hemlock Woolly Adelgid (Adelges tsugae)
This is your annual reminder that we have reached the low-risk time of year for spread of hemlock woolly adelgid (HWA). The crawlers have settled and inserted their mouthparts at the bases of the needles and will be unable to move from that spot for the rest of their lives. It is now safe to do major work in your hemlock trees with minimal concern about spreading HWA to uninfested hemlocks. The settled nymphs will be dormant (aestivating) and very hard to see until October when they start to feed and grow new waxy ‘wool.’ This low-risk period will continue until late February or early March, when the adelgids mature and start to lay eggs which can easily be moved around. During winter when the ground is frozen is the recommended time to conduct major work in and around hemlocks, as there is less chance of compacting the soil and causing root damage.
Image: Aestivating hemlock woolly adelgid nymphs at base of needles.
The Wonderful World of Parasitism
While many of us may have seen a similar scene on a hornworm in our garden, the photo below illustrates that these dramas are very much a part of the natural world as well, happening all around us but very seldom noticed. This sphinx moth caterpillar, likely a great ash sphinx (Sphinx chersis), is covered in the pupal cases of a braconid wasp (likely Cotesia congregata), a diverse group of parasitoids that plays an important role in the natural control of caterpillar populations.
Image: A sphinx caterpillar on an ash leaf covered in the pupal cases of a parasitoid braconid wasp.
Many parasitoids require extremely sophisticated relationships with their hosts to overcome host immune defenses and the challenges that come with living on or inside another organism. When a female parasitoid lays eggs inside a caterpillar (known as endoparasitism), unique chemical compounds are often also injected at the same time to exert control over host behavior and immune response. This is especially important to prevent parasitoids’ eggs from being destroyed by the immune system of the host caterpillar. Some braconids possess entirely unique ‘bracoviruses’ for this particular purpose. For ectoparasitoids that reside on the outside of the host, paralyzing chemicals are often used to prevent eggs or larvae from being dislodged if the host were able to remain mobile. Even the larvae feeding within the host will selectively feed on non-essential tissue types first, meaning the host remains alive and viable for the longest time possible. Once development is complete, these braconids emerge from the host to pupate on the surface, resulting in the remarkable scene seen in the above figure.
Containing mostly wasps and flies, the number and diversity of parasitoids in the natural world is staggering, many with a highly specific host or host group. In the forest health world, the role of these parasitoids can be crucial to population dynamics and can contribute significantly to the collapse of forest pest outbreaks. For the Maine Forest Service, they are also an important strategy in our forest pest management programs, especially for emerald ash borer and winter moth.
Beech Leaf Disease (Litylenchus crenatae mccannii)
Beech leaf disease (BLD) continues to be found in more areas of Maine. Franklin County remains the only county in the state with no positive detections of BLD recorded to date. The total number of towns statewide with positive BLD reports is 174, with 77 new towns confirmed so far in 2024. The disease was first confirmed in Waldo County, Maine, in 2021. The Maine Forest Service BLD website continues to be updated with the most recent information about BLD and its distribution in the state.
Image: Late summer symptoms of BLD on an American beech seedling. Note the chlorotic (yellowish) interveinal (between leaf veins) bands on the upper side of leaves. This symptom is prevalent in some trees and leaves, while absent in others.
Image: Dark interveinal banding as seen looking up at the underside of the leaves in an American beech canopy.
Chaga/Tinder Conk (Inonotus obliquus)
The Maine Forest Service is interested in learning more about the practice of chaga cultivation in Maine and is seeking cooperation from landowners who have enrolled their land in this new practice. We would like to monitor the progress and health of chaga-inoculated trees, the production of the chaga conk itself over time and other aspects of this incompletely understood fungus and chaga cultivation process. If you have had some of your trees inoculated with chaga and are open to helping us understand more about the pathogen that causes the chaga conk and its impacts on trees, please contact Aaron.Bergdahl@maine.gov, (207) 287-3008.
Image: A chaga conk on a yellow birch.
Image: A sawn white birch log with a chaga conk on it (orange arrow) showing the extent of internal decay associated with the causal fungus.
Also related to chaga, the Maine Forest Service has partnered with the US Forest Service Pathologists in Durham, NH, The Massabesic Experimental Forest, and the University of Maine Forests to establish two chaga inoculation sites to closely monitor disease development associated with infection by the pathogen in white and yellow birch trees. The trial includes trees inoculated with three different local strains of the fungus that causes chaga formation and control trees that have not been inoculated with the fungus for comparison. Several tree health metrics were recorded on all included trees at the beginning of the trial to monitor tree health impacts over time. It is hoped that this long-term project will contribute to the relatively small body of information on aspects of I. obliquus disease development and resulting chaga formation as well as provide insights into the impacts of the practice of chaga farming on the surrounding forest.
Stress and Pathogens (Botryosphaera spp., Cystospora spp., Phomopsis spp.)
Opportunist pathogens, like those mentioned in the title of this section, are typically not able to parasitize their tree hosts unless they gain an advantage over host resistance mechanisms via sources of tree stress. Sources of tree stress can include environmental or human-caused factors (abiotic sources of stress). Sources of abiotic stress include drought/too much water, soil compaction, poor soils, improper planting, herbicide damage, physical damage to functional parts of the tree, including the roots (via wind, hail, collision, excavation, etc.). Alternatively, sources of stress may be living agents (biotic sources of stress), typically insects or diseases. Less-virulent (less aggressive), opportunistic pathogens are those that cannot successfully attack vigorous trees. However, under the right conditions of stress, these pathogens can cause significant damage to trees.
During a recent field visit to check extensive fir tree mortality in a planting in Androscoggin County, inspecting the aboveground portions of trees revealed cankers that were later identified as those caused by a fungus belonging to the genus Botryosphaera. This was unusual, since species of Botryosphaera that attack woody plants are generally regarded as weak, opportunist pathogens. This indicated that there was likely a significant stressor involved. Following pulling five dead/dying seedlings from the ground and removing the soil from the root zone, it was clear that root growth had been restricted. This was associated with some fabric that had arrived on the seedlings and was left on the root ball at planting. The fabric had not broken down in the three years since planting and seemed to be the significant tree stressor making the tree vulnerable to attack by a stress-related fungal pathogen like Botryosphaera spp. This situation provides a good reminder that when planting trees, making extra efforts to plant them well -- in the right place, at the proper depth and with proper post-planting care. This is the best way to ensure vigorous growth that will prevent establishment of opportunistic pathogens and insects that may lead to the failure of plantings and the loss of potential tree services.
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Image: Washing the soil away from the roots of an impacted fir sapling revealed root restriction on one side of the root system due to a fabric mesh that came with the seedlings and not removed at the time of planting (arrow points to the mesh and associated reduced root formation in comparison to the opposite side of the seedling).
Image: A thumb tack is placed at the margin (border) of a fungal canker, with sunken, diseased tissue to the left of the pin. Growth of the fungal canker was progressively girdling the tree. Notice the small specks on the parasitized, sunken tissue, which are the spore-producing structures of the causal fungus.
August 15, 1979
“Bark Lice (=Psocids) seem to be a source of interest lately as clusters appear on the bark and leaves of various trees. They are small (3/16” long), grey insects with white banding and often with dark smoky grey wings. These clusters may get to be fairly extensive (6-8” across and 12-14" long) and move about frequently, especially when disturbed. Bark lice are interesting but not harmful to the trees. They simply feed on lichens and other surface material on the bark. At this time of the year it is suspected that clusters become very visible as they move on to the lower trunks of the trees.”
September 7, 2024, 8:00 AM to 3:00 PM, Forestry Field Day
Maine Woodland Owners is hosting this year’s Forestry Field Day at the 2024 Outstanding Tree Farm of the Year winners, Larry and Barbara Beauregard at their woodlot in Old Town, Maine. This free event will feature multiple hour-long walking tours, including a forest pest and disease tour led by Maine Forest Service Entomologists Brittany Schappach and Gabe LeMay. Registration is not required. Find additional information on The Maine Woodland Owners webpage.
September 20, 2024, 1:00 PM, Common Ground Fair, Unity, ME
Tom Schmeelk, hosted by Maine Woodland Owners, will present Maine Forest Pest and Disease Update in the Low Impact Forestry Tent at the Common Ground Fair.
September 21, 2024, 10:00 AM, Common Ground Fair, Unity, ME
Colleen Teerling of the Maine Forest Service and John Daigle of the Ash Protection Collaboration Across Wabanakik (APCAW) will present information about the emerald ash borer in the Low Impact Forestry Tent at the Common Ground Fair.
October 2, 2024, Noon to 1:30 PM Virtual Meeting
Forest Pests: What They Are & What We Can Do About Them is the next installation of the Northern Forest Center’s Building the New Forest Future Webinar Series. We’ll discuss forest pests in the Northern Forest and what they mean for the trees, the landscape, and the forest economy. Panelists include Allison Kanoti, Director of Forest Health and Monitoring, Maine Forest Service, Kyle Lombard, Administrator, NH Forests and Lands – Forest Health Bureau, Kathryn Aroian, State Plant Health Director for NH and VT, U.S. Department of Agriculture, Michael Bohne, Forest Health Group Leader New England and New York Region, U.S. Forest Service.
Registration Required
October 3, 2024, Maine Chapter, American Public Works Association Fall Meeting, Augusta
Allison Kanoti will present Municipal Response Options to Browntail Moth and Emerald Ash Borer as part of the fall meeting of the Maine Chapter of American Public Works Association at the Augusta Civic Center.
October 8 to 9, 2024, Wells Conference Center, Orono, ME and Virtually: The Future of Brown Ash: Weaving Indigenous Knowledge and Western Science to promote resilience amidst Emerald Ash Borer Invasion
A two-day gathering at University of Maine, Orono, will take place to inform people about current research about brown ash forests and reconvene the community of people who care for brown ash. We will have discussions, talks, meals, and posters. This meeting is hosted by APCAW.
Register or More Information
Conditions Report No. 4, 2024
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, Mike Parisio, Brittany Schappach, Thomas Schmeelk, Colleen Teerling, and Andy Whitman
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