Forest & Shade Tree – Insect & Disease Conditions for Maine; September 29, 2023

Forest & Shade Tree – Insect & Disease Conditions for Maine; September 29, 2023

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Maine Forest Service

Forest & Shade Tree – Insect & Disease Conditions for Maine

October 3, 2023

In This Edition:


Browntail Moth (Euproctis chrysorrhoea)

As the young browntail moth (BTM) caterpillars continue feeding and building their winter webs, we are seeing a high amount of skeletonization in many of the areas harder hit in early summer. As we saw a couple of years ago, the young caterpillars at this stage can also succumb to pathogen activity, so we are monitoring for any of this mortality. A clearer picture of the impacts of fungal and viral pathogens on the BTM population this past spring and summer will come to light once we begin our winter web surveys this January. For updates on this survey, please check out the browntail moth page on our website later on this winter/early spring.

Brown patch on a leaf

Image: Young browntail moth caterpillars feeding, creating characteristic skeletonization, Dedham ME. (Maine Forest Service)

Great Ash Sphinx Moth (Sphinx chersis)

During this time of the year, we encounter many native species of caterpillar in our daily field work. While taking down emerald ash borer traps in Blue Hill, we came across a great ash sphinx caterpillar. The larvae of this species can be distinguished from other similar caterpillars in this family by the following features: seven long diagonal lines down the length of its body, elongated spiracles (breathing tubes) that are black and ringed with white, and a blue horn at the tail end. The larval hosts include cherry (Prunus), ash (Fraxinus) and quaking aspen (Populus tremuloides). In the north, adult moths are active in May and June with southern states having two broods per year and therefore another set of adults present in August.

Two photos of a large green caterpillar

Images: Great ash sphinx larva. (Maine Forest Service)

Hemlock Woolly Adelgid (Adelges tsugae)

It seems hemlock woolly adelgids (HWA) are having a bad year. First there were those sudden, record-breaking cold snaps in January and February. That led to high winter mortality of overwintering adelgids (sistens generation) in many areas. The survivors laid eggs and the spring generation (progrediens) developed normally. Currently the nymphs of this year’s winter generation (sistens) are in their summer aestivation (hibernation) period. However, the frequent and heavy rains we have had this summer may be leading to higher-than-normal summer mortality. On at least two sites, many of the aestivating nymphs appear to be recently dead. We do not know how widespread this is, nor have we quantified mortality. However, similar trends have been observed by colleagues in New Hampshire. There are fungi capable of killing adelgid; possibly this is what is causing mortality, helped by the wet summer weather.

If you have HWA on your trees, it can be very difficult to tell if they are living or dead at this time of year, but as we move into later October and early November, surviving adelgids will come out of aestivation, start growing obvious wool, and become very visible. At that time, it would be worthwhile to check your trees to see how many adelgids have survived the summer.

Two photos of tiny nymphs on hemlock twigs as seen through microscope

Images: (left) Healthy HWA nymphs (photo: Maine Forest Service); (right) dead HWA nymphs. (Jesse Wheeler, NPS)

Jumping Worms (Amynthas spp.)

Jumping worms (Amynthas spp.) are a nonnative and invasive worm species that are now considered to be widespread in Maine. Currently, they have been confirmed in 13 of 16 counties, with most populations occurring in southern and central Maine. Jumping worms rapidly decompose the leaf litter layer and change the texture of the soil into dry, loose castings (worm poop) that resemble coffee grounds. Loose soil in a forest setting may cause soil erosion, exposed plant and tree roots, nutrient loss, and may cause plant and tree death, which may allow for invasive plant species to outcompete our native species. It is unknown how these worms will affect our forests in the long term. The Maine DACF is currently conducting preliminary surveillance in the counties where they have not been found yet (Piscataquis, Aroostook, and Washington) to understand their progression in the state.

At this time of year, the majority of jumping worms are adults and will have a milky white clitellum that wraps all the way around the worm. They often exhibit thrashing and “jumping” behavior if picked up or disturbed. If you believe you have jumping worms in your woodlot, or maybe you see them on a hiking trail, please fill out the Maine Jumping Worm Report Form!

Two photos of worms on the ground

Images: (left) Loose, “coffee-ground” consistency of the soil caused by jumping worms. Note the early stages of erosion causing exposed tree roots, Oakland, ME; (right) A jumping worm; these worms have a clitellum that is milky white and fully encircles the worm. It is often found on the 12-14th segment toward the head, whereas a common nightcrawler has a clitellum that is saddle-shaped (it does not fully encircle the worm) and are found around the 32-37th segment in the middle of their bodies. (Maine Forest Service)

Light Trapping Program

Sample collection from the annual Light Trap project is now completed and cooperators have submitted captured moths from all over the state, from York County to northern Aroostook. All that’s left to do is sort! The species counted include many types of defoliators, such as the rosy maple moth, forest tent caterpillar, and browntail moth, as well as some larger, more charismatic lepidopterans like sphinx moths, luna moths, and other saturniids. Data collected in this program is important to help predict forest pest outbreaks and monitor invasive species. It also happens to be the longest continuously running dataset in the Forest Health and Monitoring division – 77 years of moth flight data! This program would not be possible without the help of the Maine Forest Service’s citizen collaborators who take the time to operate light traps and submit all the different species they collect.

Insects in a display; man at table sorting insects

Images: (left) Map of all light traps operated in the state this year; (middle) identification of collected samples; (right) some moth target species monitored as part of the program. (Maine Forest Service)

Southern Pine Beetle (Dendroctonus frontalis)

Our annual monitoring program began the week of September 18 and consists of a total of 21 traps at 14 sites placed in our pine barrens and other hard pine resources. These traps will run until the middle of November to capture the fall dispersal period of southern pine beetle and help us determine where this new forest pest occurs in Maine. Through the help of our cooperators, including The National Park Service, The Nature Conservancy, and The Department of Defense, we are able to run these traps in many varied locations. This year we are also participating in a lure study which adds an additional nine traps. The purpose of this project is to develop an enhanced lure that is suited to early detection and response.

Spotted Lanternfly (Lycorma delicatula)

Although tourism season is winding down in Maine, the continual threat of invasive forest pests arriving in Maine is as constant as ever. In an unusual case of invasive insect hitchhiking, we recently learned of at least one adult spotted lanternfly (SLF) found aboard a cruise ship docking in Portland, ME in late September. Prior to this, the ship sailed from Newport, RI and likely visited other ports as well on its journey. SLF continues to spread and take hold in southern New England. News of three additional established populations of SLF in nearby central Massachusetts was released in late September as well. While we have semi-regular interceptions of SLF arriving from other infested states, there are still no documented established populations of SLF in Maine. Let’s keep it that way.

SLF is an incredibly challenging pest to deal with, considering its wide range of host plants. Fortunately, Maine remains on the cusp of suitable climate for this insect, but that does not ensure SLF cannot become established here as well. While SLF is still of concern to our forest trees in Maine, it is of much greater concern to our agricultural commodities, especially fruit crops. SLF is known to cause damage to plants by sucking out the sap (full of plant nutrients) which may lead to wilting, leaf curling, dieback, or death. As we all make our apple-picking plans for the upcoming weeks, this classic fall pastime provides a great opportunity for public participation in SLF visual survey around the state. Keep your eyes peeled as you pick for this easy-to-identify insect and report anything suspicious. At this time of year, we would expect to encounter adults or egg masses, pictured below. Egg masses are often found on outdoor flat surfaces; firewood, tree bark, house siding, fences, boats, patio furniture, outdoor gear, etc. If you can, try to capture a specimen, but capture photos at a minimum and submit your reports to Because SLF heavily prefers to complete its life cycle on tree-of-heaven (Ailanthus altissima), we are also requesting that folks report this tree location as well. Your local apple growers will thank you!

Three photos of adult and nymph insects, and insects on a tree

Images: (left) An adult SLF at rest. When outer wings are spread, bright red underwings are also visible; (middle) An example of a heavy SLF infestation covering the trunk of an apple tree in Pennsylvania, where SLF populations are well-established; (right) SLF egg masses on the trunk of a tree-of-heaven in Pennsylvania (Lawrence Barringer, Pennsylvania Department of Agriculture,; Erica Smyers, Penn State University, Emelie Swackhamer, Penn State Extension).

Winter moth (Operophtera brumata)

This past May, Maine Forest Service staff, along with our cooperators from UMass Amherst, collected winter moth caterpillars at our previous biocontrol release sites to enable rearing of the biocontrol agent, a host specific parasitoid fly called Cyzenis albicans. The team collected over 9,000 winter moth caterpillars from nine sites which yielded 8,647 live pupae, of which a percentage had been parasitized by the fly. After these pupae were dissected to determine which ones were parasitized, we wound up with 1375 Cyzenis, which is quite a bit more than we’ve had in the recent past, with the last few years averaging about 450 Cyzenis pupae. We will receive these fly pupae soon and will install an overwintering emergence cage in the ground at our next release site in West Bath for a May 2024 release.

Woolly Alder Aphid (Prociphilus tessellatus)

If you have taken a look at the alder in your yard recently, you may have noticed white cotton-like flocculence produced by woolly alder aphids. These aphids have a somewhat complex lifecycle that involves multiple hosts. The lifecycle begins with the eggs that are laid on silver maple (Acer saccharinum) and occasionally red maple (Acer rubrum). These eggs hatch in the spring, and the juveniles begin feeding on the new tender leaves. At this stage, the aphids are all female and they reproduce parthenogenetically (without mating), which allows populations to grow quickly, as they are able to clone themselves. As summer gets underway, a winged generation of both males and females is produced, which fly to alder; in Maine, this is typically speckled alder (Alnus incana). As fall approaches and the length of daylight becomes shorter, another generation of winged males and females is produced. Occasionally males and females are produced when they first arrive on the alder; however, they only become reproductively mature once they receive the shortened daylight cues. These end-of-season reproductive individuals return to their silver maple hosts, mate, and lay eggs in sheltered places on the bark.

White fuzziness on a small tree trunk, the exudae of the adelgids

Image: Woolly alder aphid colony on speckled alder, Blue Hill ME. (Maine Forest Service)

Diseases and Environmental Issues

Amendment to last month’s edition: In the August edition of this publication, in the article “Butternut Canker Disease”, I wrote, “There seems to be no resistance to the disease in the native population of butternuts…”. As pointed out by one of our readers, this statement is incorrect. A better explanation would have been “no native butternuts have been found that are immune to infection by the butternut canker fungus.” When one is lucky enough to find several butternuts in an area, it’s likely apparent that the individual trees are not equally severely infected. This is likely in part due to differing levels of basal genetic resistance to general forest pests. What I refer to here is basic resistance, not co-evolved disease-specific genetic resistance. The lack of co-evolved resistance is exactly why butternut canker is incrementally eliminating native butternut from its native range in North America. Additional components to resistance are environmental elements that may increase or reduce host tree resistance (there also may be varying levels of hybrid genes in populations, further complicating this issue). Thus, perhaps the best way to address this is to use the word ‘tolerance,’ which well describes the butternut tree’s ability to persist on the landscape, grow and even reproduce in spite of butternut canker disease. The words used to discuss issues related to host/pest interactions can be a bit confusing, but these words help to characterize complex relationships clearly. Thus, I appreciate the reader feedback, as it provides the opportunity to highlight this.

Early Senescence and Defoliation Due to Foliar Diseases of Hardwoods

The concerning trend of early senescence in hardwood species across many areas of Maine began in late August and continued into September with symptoms in apple/crabapple, ash, horse chestnut, linden, oak and maple. The species-specific causal organisms most frequently seen associated with defoliation are detailed in the photo caption below. In a typical year, primary infection by fungal leaf disease that overwintered on leaf litter or other tree parts (depending on the tree species and the species-specific fungi) begins around the time of leaf expansion. In the cooler, wetter months of spring and early summer, some foliar pathogens may be able to initiate recurring infections after colonizing new host tissues. This reinfection ability is more prevalent in some leaf fungi and is referred to as polycyclic infection. Again, in a typical year, warmer and drier months prevail as summer progresses and the conditions are no longer ideal for recurring infections. This year, however, the persistent wet weather appeared to allow for multiple cycles of infections and reinfections, driving the severity of several leaf diseases throughout much of the growing season. The heavy pathogen loads by late summer are thought to be responsible for a large proportion of the early senescence and defoliation seen at this time, but not all of it. Some species of trees and shrubs seem to be defoliating with minimal pathogen load, leading to uncertainty about this phenomenon that some long-time forestry professionals say they have not seen in their lifetimes.

Six photos of leaves with various blemishes and colors

Image: (top row, from left to right) Maple leaves showing symptoms of maple anthracnose and some that have blotches without clear evidence of fungal infection; full ash petioles appearing scorched due to environmental conditions (no clear fungal infection); heavy apple scab (Venturia inaequalis) infection on a red-leaved crabapple cultivar. (bottom row, from left to right) Ash leaves with ash anthracnose lesions (Gnomoniella fraxini); various other unidentified leaf spots and blotches on horse chestnut foliage; American basswood with lesions caused by linden leaf blotch (Didymosphaeria petrakiana). (Maine Forest Service)

Trees in a forest, one red and one green

Image: (orange arrow) A young sugar maple showing early fall coloration, and in comparison, another young sugar maple (green arrow) showing no early coloration. Both trees appeared equally healthy, highlighting the sometimes-unpredictable differences in early senescence of trees. (Maine Forest Service)

Eastern White Pine Cone Production (Pinus strobus)

In their travels, field staff have observed very large quantities of mature female cones in the upper canopies of many eastern white pines. This is particularly noticeable in the Lyman-Sanford-Waterboro area, but is occurring over most of Southern and Central Maine. These large numbers of cones can even be seen from the air. Field staff noticed that the crowns of white pine appeared to have a distinct brownish tint during routine aerial surveys for insect damage this past month.

Cone production peaks every three to five years, with 2017 and 2020 being high production years in New England. These cones develop over a two-year period. They start as small, purple-colored cones the first year, which then develop into larger green cones until finally transitioning to the familiar brown cones as they mature, dry out and eventually drop their seeds. These seeds will be dispersed by the wind and can travel quite far from their mother tree. Homeowners have reported noticeable smatterings of pitch falling from the cones onto decks and other underlying structures. These mast years are cyclic, so these effects should be less drastic for the next few seasons.

A branch of a pine tree, with many cones

Image: Many people have noticed heavy cone crops on their white pine trees this year. (Maine Forest Service)

Northern Tooth Fungus (Climacodon septentrionalis)

The northern tooth fungus has been observed more frequently this year than in the past several years. Typically found impacting mature sugar maple, this heart rot fungus develops dense, overlapping shelves of white fungus that are often larger than a basketball. C. septentrionalis can also infect birch, elm, oak, apple, and hickory, although I have only seen this on maple in Maine. The spore-producing structure of the northern tooth fungus often emerges from a tree wound and is not usually found near the ground, so it is difficult to observe up close without a ladder. The fungus will also form mushrooms inside hollow trees. The underside of the mushroom surface is different than many other decay fungi, as it has an undersurface of dense tube-like structures that release spores rather than the more common pores or gills seen in other decay fungi. These are called ‘tooth-like’ gills and are responsible for the fungus’s common name. As the fungal growth ages, it may take on a yellowish color, transitioning to greenish brown, then black. It is worth remembering that when you see a decay organism fruiting (producing a mushroom), there is significant decay both above and below the area where the mushroom is growing. There may be no signs of infection before mushroom formation. While some trees infected with C. septentrionalis will remain standing for many years, others could suffer unpredictable branch or stem failure.

A large mushroom on a tree trunk

Images: (left) A northern tooth fungus growing from a wound in a mature maple tree; (right) A close-up of the spore producing mushroom/conk of the northern tooth fungus. (Maine Forest Service)

Puzzling Maple Defoliation Continues

While most of the defoliation we have been seeing in Maine’s tree species leading to premature defoliation can be linked, at least in part, to heavy infection by species-specific foliar diseases, the defoliation of sugar maples has been less straightforward. Some maples have indeed been defoliated due to maple anthracnose, a very common leaf disease this season. However, in a smaller proportion of cases the shed maple leaves have no lesions or other indications of fungal or insect disorders. With no earlier precedent to compare this to (and again, having spoken to some senior land managers and forest professionals who have not seen this in their long careers), one would think that environmental conditions are to blame. However, this is far from a clear explanation. This summer has been very wet, with frequent and sometimes very heavy rains leading to inundation of soils that typically stay well-drained in a typical year. A tree’s reaction to stress from too much water and inundated soils can be very similar to a tree’s reaction to drought – they drop their leaves. One observation that is particularly puzzling is that in some rows of older sugar maples, some trees have very high levels of symptoms and defoliation, while some remain green and healthy looking, especially in comparison to their early-senescing neighbors. This could be due to genetic differences in susceptibility/tolerance to disease or environmental conditions, or perhaps the tree’s site and particular microclimate (or potentially a combination of all of these factors). Whatever the reason, this trend is concerning from the fall foliage perspective. An additional issue of concern is the amount of overwintering inoculum (sources of fungal spores for reinfection in spring) that has high infection potential if fungus-favorable weather prevails next spring. This is a good reminder to collect and dispose of leaves this fall to reduce the infection potential next spring. However, with the high amount of diseased leaves falling early also in the forest, there is sure to be plenty of overwintering inoculum ready for reinfection if the correct conditions prevail in spring 2024.

Trees near a house

Images: (left) Three sugar maples (1–3) showing early defoliation and one red-leaved Norway maple (4) showing no defoliation, highlighting the difference in symptoms among maple species; (right) Prematurely shed sugar maple leaves accumulating under a mature maple. (Maine Forest Service)

Small tree that is missing most of its leaves

Image: A Norway maple variety with alternating branches holding either variegated or non-variegated leaves. Interestingly, only the variegated leaves were defoliated and had severe fungal lesions. The non-variegated leaves had smaller fungal lesions and remained green and on the tree. This phenomenon was seen/reported in Wayne, Sanford and Belfast and likely occurred elsewhere in Maine. The reason for this large difference in susceptibility to fungal infection is unclear. (Maine Forest Service)

Why Most Horse Chestnut Trees Look Really Unhealthy in Late Summer

Have you ever wondered why many horse chestnut trees seem to senesce rapidly in late summer, with foliage appearing discolored, blotchy, and sometimes fully scorched? There are at least three possible reasons for this: non-parasitic scorch, Guignardia Leaf Spot (Guignardia aesculi) and Anthracnose disease (Glomerella cingulata).

Non-parasitic leaf scorch can result from environmental conditions and is often said to be related to lack of moisture. Since this could not be the case in Maine this year due to the high rain amounts, perhaps the reason is overwatering (trees express similar symptoms in conditions of drought and inundated soils). Salts have also been implicated in late-season scorch of horse chestnuts growing on roadsides. Perhaps the most interesting thing about non-parasitic leaf scorch of horse chestnut is that there are some trees that do not show these symptoms in absence of the diseases to be discussed next. Those who have these trees are very fortunate.

Guignardia leaf blotch (Guignardia aesculi) affects trees in the genus Aesculus and some other plants and vines (Virginia creeper for one). Like most fungal leaf diseases, this leaf spot is favored by prolonged periods of wet weather, with reinfection building throughout the year. Guignardia leaf blotch can be differentiated from anthracnose fungal infection by the yellow halo often surrounding lesions (although this is not always clear).

Horse chestnut anthracnose (Glomerella cingulata) is also favored by wet weather and multiple re-infections over the course of the year, culminating in coalescing lesions leading to scorched leaves and early defoliation.

The thing to remember when you see a scorched horse chestnut in late summer is how fantastic the trees look in spring and when in full bloom. Smaller trees can be treated with fungicides to prevent fungal infection. Further, like most fungal leaf diseases, thorough removal of leaf litter in the fall will remove large amounts of infection potential (fungus overwintering on the leaves, to produce spores in spring).

A tree with red leaves; a close up of a leaf

Image: (left) A whole tree showing leaf scorch symptoms in early September; (right) Varying degrees of early senescence and leaf scorch symptoms in horse chestnut foliage. (Maine Forest Service)

Yellowing Twigs on Hemlock

Every year trees, lose some lower and/or inner twigs or branches as part of part of their natural growth. In hemlock, twigs with yellowing needles can often be seen in the summer. This year, the yellowing seems to be more prevalent, and we have been getting questions about it. Although this yellowing tends to be more common in years of drought, a very wet year after several years of drought may also cause tree stress, leading to various symptoms with no signs of insect or disease visible. This should not be a cause for concern, but it is worth monitoring these trees in the coming season to make sure more serious issues are not developing.

Hemlock twig with yellow needles

Image: Yellowing hemlock twigs. (Maine Forest Service)

This Month in History

From our Sept 1, 1993 Conditions Report:

“Fall Webworm (Hyphantria cunea) – Larvae of this species are still active and enlarging their filmy tents on a variety of deciduous trees and shrubs in central and southern Maine. Populations seem to be generally down this season except very locally in southwestern Maine.”


October 3, 2023, 11:00AM-12:00PM: USFS R9 Forest Health Webinar- Progress towards biocontrol of EAB

The US Forest Service Region 9 Forest Health and Monitoring Coordinator is hosting Dr. Julie Gould, lead scientist on the biological control of EAB, ALB, polyphagous shot hole borer, Roseau cane scale, and spotted lanternfly, with APHIS Science and Technology Laboratory in Massachusetts. Dr. Gould will be speaking to us on biological control of EAB.

Join Webinar

October 10, 2023, 8:00AM- 4:30 PM: Maine Division Society of American Foresters 2023 Annual Meeting – Orono

Location: Wells Conference Center, University of Maine,131 Munson Rd, Orono, ME 04460

This year’s theme is A Journey of Forest Health and Silviculture: Discussions to Guide Management Decisions in the Future.

Registration Required. View the Agenda (PDF).

October 23, 2023: Public Comment Session Ends for EAB Quarantine

The proposed changes to the emerald ash borer quarantine, Chapter 275, are designed to protect Maine’s forest and timber resources to slow the movement of the emerald ash borer and are in response to detections of the insect in additional areas of the state. Chapter 275 Proposed Rules Draft (PDF) / Proposed EAB Quarantine Expansion Map (PDF).

The public comment session on the EAB Quarantine Rule has been extended to October 23. The public is invited to submit comments to:

Gary Fish, State Horticulturist Department of Agriculture,
Conservation & Forestry 28 State House Station,
Augusta, ME 04333-0028
Telephone: (207) 287-7545
Fax: (207) 287-7548

October 25, 2023, 6:00-7:00PM: Speaker Series: Ever-changing Impacts to the Health of Maine’s Forests & Trees – Maine Audubon

Maine Audubon is hosting a discussion on Forest Health and Invasive Forest Pests with Director of Forest Health and Monitoring Allison Kanoti.

Event is free; donations are appreciated.

Register for event

November 9, 2023, 2:00-3:00 PM: EPA Hosting Webinar on Understanding Bulletins Live! Two

The U.S. Environmental Protection Agency (EPA) is holding a public webinar to provide an overview of the Bulletins Live! Two (BLT) system. The webinar will include information on the development of Bulletins and accessing Bulletins using the BLT system.

During the webinar, EPA staff will:

  • Describe how Bulletins relate to pesticide labeling.
  • Explain the use of BLT to determine if there are geographically specific mitigations for intended pesticide application areas.
  • Demonstrate BLT using malathion as an example.
  • Address frequently asked questions.

When directed by a product label, pesticide applicators are required to visit the BLT website and follow any mitigations specified for the intended application area. When users are directed to follow them on a pesticide label, bulletins are enforceable mitigations under FIFRA.

Conditions Report No. 5, 2023


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, and Colleen Teerling.

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

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