Asian longhorned beetle (Anoplophora glabripennis) – Asian longhorned beetle (ALB) has not been found in Maine. We need your help in keeping a look out for it! ALB hit the news last month when an adult beetle and associated tree damage were discovered in Hollywood, South Carolina. This detection is a good reminder that our hardwood trees and forests are vulnerable to ALB, even if we feel far removed from any infestations (the nearest known infestation is in and surrounding Worcester, MA). This time of year, activity from larvae feeding within trees would be visible around infested trees, including dripping sap and piles of coarse, wood-shaving-like frass. In addition, the showy adults would be flying if present. ALB attacks many types of hardwood trees and appears to prefer red maple in the northeast. If you suspect you have seen ALB please try to capture the beetle, at a minimum in pictures, but preferably in a sturdy container, and report it as soon as possible. You can learn more about Asian longhorned beetle and report suspected observations at www.maine.gov/alb.

Image: Asian longhorned beetle next to a penny for scale. Joe Boggs, OSU, bugwood.org

Balsam Fir Sawfly (Neodiprion abietis) – District forester Terri Coolong responded to a report of scattered dead conifers on a woodlot in Lincoln, Penobscot County. On site she found scattered dead balsam fir and on a live fir, aggregations of balsam fir sawfly larvae. This is an insect that is not reported to our office very frequently (no reports in 26 years of data).

Balsam fir sawfly is a native insect that sometimes has short duration outbreaks. Unlike many other defoliators of fir, larvae feed preferentially on the older foliage, leaving the current year foliage intact.

The last localized outbreak recorded in Maine was in 1992–1993, with defoliation seen in coastal pockets from Jonesport to Milbridge. Prior to that, there was an outbreak of this insect in coastal regions from Portland to Rockland in the 1940’s. At the time of the 1992–1993 outbreak there was a concurrent outbreak of hemlock looper which also feeds on fir. The trees impacted by sawfly could be readily told from those impacted by looper based on the presence of healthy, green current-year growth on defoliated trees.

On Christmas tree farms and in other settings where this defoliation is unacceptable, balsam fir sawfly can be managed with an appropriate contact or stomach insecticide when large numbers of the larvae are detected. Scouting for larvae should start in the latter part of June. Because this is a sawfly, products with the active ingredient BtK will not be effective for management.

The fir mortality that prompted this discovery did not appear to relate to the sawfly presence but was likely due to moisture stress. The site was ledgy, and as reported in the disease portion of this bulletin, parts of Maine have experienced several years with significant periods of scarce moisture.

Image: Messy feeding on older fir foliage is characteristic of balsam fir sawfly damage. The larvae have dark heads and dark, longitudinal stripes on their bodies. MFS, T. Coolong

Browntail Moth (Euproctis chrysorrhoea) – Browntail moth (BTM) pupation had occurred at all monitoring sites by June 26, first pupation was noted the week of June 15. Adult BTM had begun to appear at all monitoring sites from Old Town south to Portland and west to Turner in the first week of July.

We recommend limiting outdoor lighting always, but especially during BTM flight period through the end of July/early August. Flight is concentrated during the hours of 10pm to midnight. Winter webs (therefore summer egg masses, and the source of spring discomfort) are often found concentrated in trees near outdoor lighting. Anecdotal evidence suggests more males come directly to lights than females, with the females spending time on host foliage near the lights. For this reason, among others, bug zappers and homemade light traps are not effective in controlling BTM. Additionally, these systems are non-selective and kill many beneficial insects including those that attack BTM and other insect pests.

Egg-laying was observed in Manchester on July 14. Where eggs are observed, the leaves or egg masses may be removed and destroyed or soaked in soapy water. Although control of the early feeding larvae which will hatch in August is possible, it is often difficult to tell which hosts are impacted before the most effective window for management is closed.

Image: (top) Female browntail moths depositing eggs; (inset and bottom) Egg masses of browntail moth, eggs are covered with the setae (hairs) from the adult female’s abdomen.

Emerald Ash Borer (Agrilus plannipennis) ­– According to the latest degree-day map for emerald ash borer (EAB), adult beetles will have emerged throughout the state (a degree-day model looks at the accumulated heat during the season and uses this to predict the development of insects). Now is the time to watch for adult EAB. The look-alike six-spotted tiger beetle is also present during the summer months. It is the same brilliant metallic green as EAB and roughly the same size, although it has a broader shape and is very frequently mistaken for EAB (note that the white spots on the tiger beetle are not always as obvious as in the picture shown).

One of the easier ways for a non-entomologist to distinguish between the two is by their behavior. EAB adults are most likely to be seen on or near ash trees and will rarely be seen sitting on the ground. Tiger beetles, however, are frequently seen on the ground. They are very active hunters, and often hard to capture. If you walk toward them, they usually jump up and fly away 15 feet or so. If you continue to approach them, they will fly off a short distance again. If you observe this behavior in a brilliant green beetle, you can be fairly safe in assuming it is a tiger beetle and not EAB. However, as always, if you can take a picture of what you see, you can send it to us for verification.

In our ongoing search for EAB, Maine Forest Service field staff will now begin the important task of servicing the 200 purple prism traps hanging statewide to replace lures and inspect them for any adult EAB beetles. We’ll keep our fingers crossed as always that all traps come back negative, but positive traps will provide valuable information on new infestations located outside the Maine’s EAB quarantine area. Results of this mid-season trap check should be available in August.

Images: (left) EAB adult beetles; (right) A native tiger beetle. PA Department of Conservation and Natural Resources; University of Wisconsin Milwaukee

Fall Webworm (Hyphantria cunea) – Fall webworm feeding is becoming apparent. Eggs are deposited on the underside of host leaves in late-June or early July. When the caterpillars hatch, they begin to skeletonize the leaves and create their characteristic webs. Most folks notice the webs after they are well developed. Damage from this native insect does not significantly impact the long-term health of the host. Think of it as food to support generalist predators and parasitoids important in suppressing other significant pest populations. This is a good time of year to relocate or destroy webs developing in locations that are of aesthetic concern. A forked stick or a stick with a nail in it may be inserted into larger webs and by twisting it the loose webs may be pulled from the trees.

Image: Early feeding damage and webbing from fall webworm caterpillars in crabapple.

Gypsy Moth (Lymantria dispar) – Maine has enjoyed relative quiet in terms of gypsy moth caterpillar activity in recent years, however reports of mature caterpillars are now coming in at greater frequency this season compared to last. Damage may be noticeable in some locations but remains limited to our knowledge, including at regularly monitored locations that tend to maintain healthy populations of gypsy moth year after year. Some recent calls have come from new and unexpected locations and given the widespread distribution of these reports, ranging from Oxford to Aroostook counties and most counties in between, could begin to represent a statewide trend of increasing populations.

Gypsy moth is a cyclical pest that returns roughly every ten years and the last time Maine experienced anything resembling an outbreak was the early 2000s, where some 51.5 thousand acres were affected. Prior to that, massive outbreaks in both the early 1980s and 1990s both left more than half a million acres affected. It’s still too early to predict, but perhaps the current uptick in reports means that our luck has finally run out after escaping any major damage when we would have expected an outbreak about a decade ago.

Another possible explanation for the increased numbers of caterpillars in 2020 is the weather. Population control of gypsy moth is closely tied to spring weather conditions and whether or not cool temperatures and abundant moisture create conditions favorable for the spread of entomopathogens that kill caterpillars. One interesting observation in recent weeks is seeing oak stands that are co-inhabited and have been concurrently defoliated by browntail moth and gypsy moth. Since populations of both gypsy moth and browntail moth are regulated by many of the same environmental conditions, it’s likely that both insects experienced very low disease loads in 2020 due to prolonged dryness and therefore escaped high levels of mortality.

We have received a report earlier this week and accompanying photo of caterpillars likely afflicted by the fungal pathogen Entomophaga maimaiga from Eddington (Penobscot County). The forester reported many large caterpillars on the oaks hanging immobile, head down and filled with “brown goo.” Although the caterpillars were numerous, there was not obvious defoliation.

Mature gypsy moth caterpillars can be easily identified at this time of year by the five pairs of blue spots followed by six pairs of red spots on their backs. Although most caterpillars have reached full maturity at this point and the majority of feeding damage has been done for 2020, we encourage continued reports of large populations or defoliated areas in the event we can add new sites to our egg mass survey conducted during the fall and winter months.

Image: Overhead panorama of defoliation in an oak stand within Frye Mountain WMA (Waldo County), first caused primarily by earlier feeding browntail moth caterpillars and now experiencing some additional secondary defoliation due to gypsy moth caterpillars somewhat later to mature.

Dead Oaks in Portland

The Maine Forest Service forest pathologist and forest health technicians recently visited several sites in the Portland area to evaluate the reason for the mortality of oak trees reported by the City of Portland Forestry Department and a local nursery. The cause for greater concern than usual is the threat of oak wilt disease, which has not been found in Maine, but is a focus of early detection efforts. Fortunately, the reason for the oak mortality was not oak wilt.

Instead, a similar set of symptoms was seen at all the sites visited. The oak trees had numerous D-shaped exit holes all over the main stem. These holes are very likely made by the two-lined chestnut borer, a common pest of stressed oak trees. Even though the feeding on the cambium by the borer larvae prior to their emergence played a large part in the trees’ final demise, other stressors were likely what started the spiral of decline eventually leading to tree death. Such stressors include drought and site manipulation, both of which adversely impacted the evaluated oak trees. The impacted trees at one of the sites were growing next to a sidewalk that was recently renovated, which involved digging in the root zone and injury to roots. Another dead oak was in an area of high human traffic where soil compaction seemed to be a factor, while another dead oak had a large basal scar from an injury. All of the dead oaks had an additional significant stressor in common: water deficit.

For the past three years – including late May and June of this year – the southern part of Maine has experienced prolonged periods of very dry weather. Stress from water deficit is one of several factors that has been shown to increase susceptibility to borer attack. In summary, in these cases tree mortality has been a process that started in earlier times, not an event that happened at a recent specific point in time. When trees experience an initial significant stress – or a chronic stress – they become more susceptible to secondary agents of decline. While we cannot control weather-related and other natural stressors, this highlights the importance of cultural practices that avoid injuring trees and maintaining tree vigor.

In closing, to help the MFS in their oak wilt early detection efforts, please report all instances of unexplained oak mortality and any premature defoliation and/or wilting of oak foliage. If oak wilt disease does arrive in Maine, and we catch it early, we will be better able to manage it. All oak wilt inquiries can be directed to aaron.bergdahl@maine.gov, (207) 287-3008.

Images: (left) The silhouette of a dead oak tree in a high human traffic area; (right) The D-shaped exit holes left by the emergence of adult two-line chestnut borers.

Dutch Elm Disease (Ophiostoma ulmi, Ophiostoma novo-ulmi) – Reports of Dutch elm disease (DED) have been on the rise throughout Maine in late June to mid-July. Symptoms include discoloration of leaves, early defoliation and wilting branches/sections of trees, or even wilting and death of whole trees if the disease was transferred from an infected tree to a neighboring tree via underground root grafts. Typically, younger trees are not affected, although DED was confirmed on a less than 10-year-old tree in Manchester in early July. Despite the devastation of the American elm population via DED, there are still many American elm trees on the landscape throughout Maine. There are even mature American elm trees that have seemed to escape infection – or maybe they have some level of genetic resistance. The US Forest Service is trying to track large healthy “survivor” elms via an online fillable form. The user-friendly form allows you to locate the tree on a mapping application and add information about the tree for future reference. Maybe with your help, American elms with genetic resistance to DED can be identified and used to bring back this beautiful and iconic tree species.

Image: A elm tree dying from Dutch elm disease, flanked by already deceased elm trees that possibly infected the rapidly declining tree via underground root connections.