Forest & Shade Tree - Insect & Disease Conditions for Maine
July 22, 2009


Despite the rather long list of tree insects and diseases described in this Conditions Report, please keep in mind that once again, tree health statewide is quite robust this year.   While some individual trees bordering low, wet areas, swamps, and streams are already showing some flooding injury in many areas, long-term damage is expected to be limited. Christmas tree plantations that have suffered from “wet feet” in years past may also be experiencing some losses again this summer and fall.   On the other hand, the high moisture conditions have allowed spring-planted trees to become well-established, and have prepared the way for an excellent season for fall planting, as well.  With a few notable exceptions, established trees in ornamental plantings, plantations, and forests exhibit dense, lush foliage, and are in good position to counteract routine biotic and environmental stresses.  In any case, with a dreary July nearly behind us, it may be useful to remember the old adage “It’s not about waiting for the storm to pass, it’s about learning to dance in the rain (- unknown).”

ALB Adult.  Photo: PA DCNRS-Forestry Archive, Bugwood.orgInsects

Asian Longhorned Beetle (Anoplophora glabripennis) -

Not found in Maine yet.    Keep looking.

August is Asian Longhorned Beetle month and we encourage everyone to be on the lookout for the beetle. The adult beetles could have emerged by now in the southern half of Maine – if they are here – and soon could be found in northern Maine.

These invasive beetles are an inch or more in length, patent leather, shiny black with white spots and long black and white antennae.  Their preferred hosts are any type of maple but they also like birch, poplar, elm, willow, locust and other hardwoods.

Photo: ALB Adult, PA DCNRS - Forestry Archive,

*Birch Leafminers (Undetermined spp.) - Birch leafminer populations are spotty this year with some areas showing very heavy infestations and others very light. The sawfly adults lay eggs on the birch leaves and the larvae feed between the top and bottom layer of the leaf and then drop to the ground or pupate in the leafmines, depending on the species. Control should have taken place by now. If this insect was a problem this year, make plans to control it in May next year.

*Browntail Moth (Euproctis chrysorrhoea) – Larvae have finished feeding, pupated and begun to emerge as white moths with plump shiny brown bodies.  The moths are attracted to light so if you live in a browntail moth area, turning off lights at night may reduce the pest problem in your immediate vicinity- no guarantees. This year an infestation on Vaughn Island off Kennebunkport’s Cape Porpoise was reported.  The population has apparently been growing there for a few years as there is heavy defoliation on the island frequented by locals and people using the Maine Island Trail.  The hairs can still cause a rash if they are stirred up from the shrubs below the defoliated trees.  All the rain we have been getting will help wash the larvae skins and associated hairs down into the soil where people will not come in contact with them.

Emerald Ash Borer (Agrilus planipennis) – Although emerald ash borer has not yet been found in Maine, we urge our readers to be on the lookout for it.  This jewel-like green beetle usually begins to fly at about 450 base 50°F growing degree days.  Maine points from Caribou to Portland and beyond have exceeded that threshold ( 

What should you look for?  The small metallic green beetles are somewhat bullet shaped and will fit comfortably on a penny (1/2”).  Unlike some of the common beetles that look similar, the wings of EAB taper from the junction with the thorax to the rear of the abdomen.  The thorax and head are almost as wide as the wings at their widest point (see diagram below).  The legs of EAB are relatively short, and therefore often indiscernible from above, many of its lookalikes have long, obvious legs.

Potential EAB Silhouette

Schematic of potential EAB silhouette

Not EAB Silhouette

In addition, you can look for ash trees in distress.  Ash (Fraxinus spp.) affected by emerald ash borer may have one or more of the following:

  • Longitudinal bark splits
  • S-shaped larval feeding tunnels (often visible beneath the splits)
  • Dieback from the top down
  • Basal sprouts
  • ¼” D-Shaped exit holes
  • Woodpecker feeding damage

If you find an ash tree exhibiting one or more of those symptoms, please contact our Lab for further diagnosis (207) 287-2431 or  Emerald ash borer does not affect mountain-ash (Sorbus spp.), a common ornamental. 

Biosurveillance for Emerald Ash Borer -This summer, Maine is monitoring for emerald ash borer (EAB) using Cerceris fumipennisCerceris is a non-stinging wasp that hunts metallic wood-boring beetles related to EAB, and is also very efficient at hunting EAB when present.  Rather than us looking directly for EAB, which can be hard to detect, we find colonies of the wasp and see whether EAB is among its prey.  We have volunteers throughout the state, which have ‘adopted’ colonies in their towns, and will spend a few hours this summer monitoring them.  The wasp has been very late in emerging this year, due to the cold, wet weather but we expect it to emerge well before the end of July, and it will be present until late August. 

We have found most of our Cerceris colonies in baseball and softball diamonds.  Information on what the wasp looks like and how to find it can be found on our website,  If everyone checked one or two baseball fields in their town, we might find many more colonies to help us monitor for this very destructive pest.  So check your local ball field or other sunny, sandy locations and let us know if you find any colonies. 

*Fall Webworm (Hyphantria cunea) Small fall webworm nests are beginning to be apparent and numbers are expected to be high again this year.  Same drill as in past years - look for loose tents containing tiny, grayish, fuzzy caterpillars on alder, apple, ash, beech, birch, cherry, elm and oak.  Clip and destroy these small developing tents to help reduce the problem locally.  Carbaryl (Sevin), Diazinon, acephate (Orthene) and Bt are registered for use against this pest.

Hemlock Woolly Adelgid (Adelges tsugae) – Wayne Searles was out checking hemlock woolly adelgid conditions around Chases Pond in York Maine last week and discovered eggs from the spring generation (Progrediens) as well as some settled nymphs from the overwintering generation (Sistens).  This means crawlers, one of the most easily transported stages of the adelgid, are still present.  Take appropriate precautions when working or recreating in potentially infested hemlocks including cleaning equipment and clothing before coming in contact with uninfested hemlocks.    As a result of the eggs, the wool on last year’s hemlock growth is puffy and readily visible.  The nymphs settled on this year’s growth are barely visible with the naked eye and will not begin producing new wool until mid to late October when they will begin to feed.

Japanese Beetle (Popillia japonica) –   Beetles are late emerging this year – by about 10 days at my house and not as numerous as past years.  Many of the beetles I have observed have one or more small white dots on their back.  The dots are the eggs of a parasitic fly, Istocheta aldrichi that will kill the beetle in 5-6 days (healthy adult beetles can live 4-5 weeks).  Leave any beetles with white dots on their backs so that the parasites can spread to other Japanese beetles. This parasite was introduced in southern States back in the 1920’s.  It does not control Japanese beetles well down there as the flies emerge before the beetles.  Up here in Maine and in surrounding states the flies and beetles are synchronized and are having an impact on the population. Support your local flies! Department of Agriculture (pdf)

Mosquitoes – they are thick.

Mountain Ash Sawfly (Pristiphora geniculata) - The larvae of the mountain ash sawfly are feeding now and damage is apparent on trees infested with this pest.  If numbers do not appear to be that great, wait until next year and either clip off the infested branches or treat when the larvae have just hatched in late June.

*Oak Twig Pruner (Anelaphus parallelus and A.  villosus) – Oak branches on the ground this time of year are usually a clue that longhorned beetles have been boring in the branches. A look at the end that has been cut will reveal a smoothly chewed end with an oval hole packed with what looks like sawdust. Break the branch and it is hollow, split up the branch and there will be a larva in there all set to overwinter.  Picking up the branches and destroying them will help reduce the beetle population if the dropped branches are an aesthetic problem.

Oak Skeletonizer (Bucculatrix ainsliella) – The first generation of oak skeletonizer larvae are finishing up feeding on the leaves of oak trees along the coast.  They will then spin down on silken threads and form cocoons that look like ribbed grains of rice. A second generation of larvae will begin feeding in August. The larvae feed on the underside of the leaves leaving the veins and making lacy looking damage on the leaves. In some years damage can be heavy enough to weaken trees and cause branch dieback. There are recommendations for raking and destroying leaves to reduce the population but I have observed so many cocoons formed on other trees, screens and other objects that I am not sure how effective it would be (Forest Pests.Org Site).

*White Pine Weevil (Pissodes strobi) - It’s not too late to correctively prune infested white pine or spruce leaders.  Although larvae have dropped out of the leaders in southern and central Maine, pruning will still help trees to retain their upright stature. Prune back to the first healthy whorl of branches and remove and destroy infested branches (the larvae may still be in the leaders in northern Maine, and destroying them will help reduce the population.).  Then select the best lateral branch of this top whorl and prune off all remaining laterals leaving the single best lateral to form a new leader. 

*Yellowheaded Spruce Sawfly (Pikonema alaskensis) – Although this insect is not in high numbers, it is still present.  The larvae are hard to see as they blend in with the needles and most people do not notice them until “overnight” the foliage disappears and all that is left are bare outer branches at the top and sides of the tree. If you have had defoliation on your open-grown spruce in past years, check now for larvae before they do any more damage. Larvae are gaining size now and defoliation is becoming obvious. Control now with Spinosad or carbaryl to limit damage.

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Diseases and Injuries

Arborvitae (Northern White-Cedar) Dieback – A widespread and damaging condition was observed during the late spring and early summer involving the partial dieback of Arborvitae.  Many hedgerow and foundation plantings have been affected, and many plantings have required replacement.  The symptoms show as just one or a few stems of multiple-stemmed plants die and turn a bright red-brown.  This gives the affected plants a “sectored” look, with some stems of the plant remaining apparently healthy and green. 

Investigations so far have not uncovered a definitive cause, but root rot (Armillaria spp.) has been consistently associated with the dead and dying stems.  It is not known if the infections were initiated in the nurseries, or on-site after the trees had been stressed (possibly by poor handling of container stock; cold temperature extremes this past winter; or excessive moisture and soil freezing conditions).  Some hedge plantings that had been established for up to ten years have shown the symptoms, but most affected planting are younger than five years. 

It may be that the source of stock is an underlying, contributing factor to the problem.  Stock grown and shipped to Maine from other parts of the country, especially from the southern states, may be less able to adapt to the particular winter conditions here.  The cultural manipulation used for developing the multiple-stem condition of most ornamental arborvitae stock may also play a role.  Similar dieback symptoms have not been seen or reported to occur on northern white-cedar in natural forest stands, either in young sapling reproduction or in older trees.  

Arborvitae (Northern White-Cedar) Needle Blight (Phyllosticta thujae) – The pathogen causing this disease is now believed to have been misidentified in earlier Conditions Reports as an unknown species of Macrophoma.  A closely related fungus, Phyllosticta thujae is now considered the correct identification, based on recent investigations by Bruce Watt, University of Maine Cooperative Extension, and the APHIS National Identification Service. The disease has been reported in past years from the towns of Ashland, Augusta, Freeport, Harpswell, and Madison, and this year from Orono.

Ash Leaf and Twig Rust (Puccinia sparganioides) – Ash leaf rust has been observed causing moderate or heavier damage to individual trees in the mid-coast area.  Trees in the towns of Rockland and Thomaston have been damaged, and the disease likely occurs in southern coastal areas, as well.  The bright orange to yellow aecial cups of the fungus, which develop on the leaves, young stems and shoots provide an obvious and easy diagnosis, but are now largely past.  Premature leaf fall, brown leaf spots, and curling of the leaves and young stems are symptoms that are most visible now.  Some branch dieback is apparent in many ash stands in the affected areas.

Botryosphaeria Canker of Oaks (Botryosphaeria  spp.) – Planted ornamental red oaks in Bangor have been damaged by a twig and branch canker disease caused by one of several species of Botryosphaeria, possibly B. obtusa.  This disease commonly affects twigs and branches that have been stressed by drought or winter injury.  A few days of extreme cold temperatures last winter may have triggered the infection.  Botryosphaeria canker is considered to be only weakly pathogenic, and the trees are expected to recover.  Numerous sprouts on branches and from the basal area of the stem were forming by early July.  Also, freezing injury was the apparent cause of mortality of planted ornamental oaks in Masardis.  Temperatures near or in excess of -40o F were recorded during the winter of 2008-2009.   This location is well beyond the natural northern limits of the range for oaks in Maine.  The original source of the affected trees is unknown.

Lophodermium Needle Cast of Scotch Pines (Lophodermium seditiosum) – Conifer needle diseases continue to be a significant concern this year, as the wet weather has continued from the spring into the summer  without much let-up.  One of these needle diseases, Lophodermium seditiosum (= L. pinastri), has been found causing severe defoliation of Scotch pines in the Houlton area, and reports of this disease have also come in from Solon and Byron.  Because spore release and needle infection occurs in mid- to late summer, applications (two or three about two weeks apart) of a fungicide (chlorothalonil or copper sulfate) applied now should give adequate protection to current-season needles. 

Oak Anthracnose (Apiognomonia quercina) - Oak anthracnose was identified from bur oak in Camden.  The disease, which is similar to the other hardwood anthracnose diseases, infects leaves in early spring.  Oak anthracnose is a little unusual because it seems to appear less commonly than other anthracnose diseases. Again, the excessively wet weather is likely responsible for its appearance.  The damage will be insignificant, except for reducing the aesthetic appearance of ornamentals. 

White Pine Needle Cast (Canavirgella banfieldii) – This needle disease has again been very prevalent this year, with the most severe infection occurring in the west-central and southern regions of Oxford County, Maine.  Infections have also been reported on white pine as far north as Baxter State Park and Millinocket.  Current-year needles become infected, but don’t show symptoms until the following spring.  During the early spring, before the new growth begins, the one-year old infected needles begin to turn a light tan or brown.  Heavily-infected trees appear as though damaged from salt injury.  Later, the new developing shoots and needles mask the one-year old needles, but the tree crowns still appear off-color.  This year, browning was very evident in late June.  By the end of the second week in July, the infected one-year (and two-year) old needles had been shed.  The tree crowns no longer appeared brown, but did appear very thin, since only current-year needles remained.  This disease has been common for several years in succession now, due to the excessively wet weather.  Conditions for needle infection this year are expected to have been ideal, so a high incidence of the disease is again expected next spring.  In some stands, white pine regeneration has also been affected when in the understory of older, infected white pines. 

To date, there has been no record of mortality occurring from this disease and, under more normal weather conditions, it is usually never seen.  However, because there have been several successive years of high infection levels, it may lead to growth loss and predisposition to other, secondary insect or disease factors.  Refer to the discussion in the next section on white pine decline for additional information.

White Pine Decline (Unknown Cause) - Several stands of white pine which exhibit rapid decline symptoms have been examined during the past several months.   Stands examined include one in each of the towns of Demark, Waterboro, and Norridgewock.  In each case, a small area from about one-quarter to two acres in size appears to have been affected.  Trees in the affected patches appear to have died rather suddenly, usually within a period of a few months, with the patch of mortality fairly well defined and contiguous. 

Site inspections have revealed no evidence root diseases or primary insects.  Lightning strikes have been considered, but not accepted as the cause, for lack of significant evidence.  The mortality is unrelated in symptomatology and appearance to the white pine decline of the late 1990’s, attributed to drought stress.  Two of the stands are located in areas of high white pine needle cast severity.  In two of the locations, a very limited amount of damage from secondary pathogens has been found, including that caused by Caliciopsis canker and Septobasidium infections, both of which suggest stand overstocking.  However, neither is thought to be the primary cause of the problem.  Recommendations for landowners have been limited to salvage of the dead or dying pines, but predictions of the progress of the condition are not yet feasible.  Landowners should be aware of this condition, monitor white pine stands regularly, and report similar conditions to the Maine Forest Service Insect and Disease Lab.

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Conditions Report No. 4, 2009

Maine Forest Service
Forest Health and Monitoring