Differentiating the Common Wood Borers of Ash Trees

It is understandable for arborists and landscapers to assume that Emerald Ash Borers (Agrilus planipennis) are the cause when they observe branch die-back of ash trees (Fraxinus genus). During the 21st century, this invasive Asian tree beetle borer has killed many 100’s of millions of ash tree species as it has spread across much of the eastern half of the United States. However, it is important to remember some of the other wood borer species that cause ash decline, if not death. This blog will discuss the symptoms to distinguish between the Emerald ash beetle borer, the Ash/Lilac Clearwing moth borer (Podosesia syringae), the Banded Ash Clearwing moth borer (Podosesia aureocincta) and Ash Bark Beetle borers (Hylesinus species). Only the ash bark beetle species will have more than a single generation per season.

Symptoms on ash from banded ash clearwing moth borer infestation. (Photo Credit: Steven K. Rettke of RCE)

Although Emerald Ash Borer (Agrilus planipennis) will eventually find & kill most unprotected ash trees, other common ash borer species may infest some of the trees first. These can include the two species of clearwing moths, the ash bark beetles, & the ambrosia beetles. The ambrosia beetles are only likely to attack younger & stressed ash tress & are not likely to be a concern with larger, mature trees. There are other borer species of ash, but they usually only become active with severely stressed & dying trees.

The symptoms for an infested ash tree with Emerald ash borer (EAB) are well known & have been documented extensively in the literature during the past 15+ years. They include the following: 1-Flecking or “blonding” of the trunk & major branches by woodpeckers; 2-Branch dieback & thinning of upper tree canopy; 3-Capital D-shaped exit holes in the trunk & branches; 4-Epicormic twig growth emerging at trunk base & major branches; 5-Vertical splitting of bark where extensive shallow serpentine mining by EAB larvae has occurred. Short of identifying active larvae, if most of the above symptoms are observed, then this will probably confirm the Emerald Ash Borer as the cause of the ash decline. Though, observed D-shaped exit holes in ash trees alone is considered diagnostic for the presence of EAB.

Symptoms on ash from Emerald ash beetle borers. (Photo Credit: Steven K. Rettke of RCE)

Epicormic twig growth caused by Emerald ash beetle infestation. (Photo Credit: Steven K. Rettke of RCE)

Avoid the mistake of automatically assuming any ash tree species decline is from EAB. If many of the symptoms listed above are not observed, then it is important to investigate further. As always, first attempt to determine if abiotic, physiological causes are the problem (e.g., drought stress, compacted soils, etc.). Also examine the tree for the possibilities of disease pathogens, such as Ash Yellows.

Inspecting the tree for the possibility of other species of wood borer infestations is important. Arborists & landscapers should be familiar with the two clear-winged moth species that commonly attack ash. They are the Banded Ash Clearwing Moth (Podosesia aureocincta) & the Ash/Lilac Clearwing Moth (Podosesia syringae). Both borer species will cause some of the same symptoms seen from Emerald Ash borers. Like EAB, these clearwing moth caterpillar borers will cause upper crown thinning, which may eventually result in the death of major branches. The symptoms often produce a “staghorn” type appearance of the tree canopy. Multiple dead branches may be seen sticking out above the leaf canopy.

Row of ash trees infested by banded ash clearwing moth borers. (Photo Credit: Steven K. Rettke of RCE)

Although misdiagnosed as Emerald ash borers, the banded ash clearwing moth infested trees were still rightfully removed. (Photo Credit: Steven K. Rettke of RCE)

Other than uncovering bark & identifying larvae, the easiest way to determine the ID of the species is by observing the size & shape of borer emergence holes in the trunk & branches. EAB exit holes will have a capital D-shape with a diameter of approximately 3mm or 1/8th inch. During the first two or more years, most of the exit holes will be found in the upper tree canopy branches. As the infestation advances, then more emergence holes can be seen in the trunk. Since ash trees have deeply furrowed bark, it is difficult to find these exit holes. Perhaps the best way to find them, is to examine the smooth, lighter flecking areas created by woodpeckers. If EAB adults emerge out from these areas, then the holes will be clearly visible.

Capital D-shaped Emerald ash beetle borer exit hole (3 mm) is easily observed on the smooth flecking areas created by woodpeckers. (Photo Credit: Steven K. Rettke of RCE)

The emergence holes created by both clearwing moths will be more oval shape & about twice the diameter size (6 mm or ¼ inch) compared to the EAB exit holes. Furthermore, these holes will be concentrated within the lower main trunk of the trees with fewer being found in the scaffold branches. Also, unlike EAB, the clearwing moth tunneling larvae will expel sawdust-like frass from trunk openings they maintain. This frass can be observed down the trunk & can also accumulate at the bottom of the tree. As clearwing moth adults exit the tree, they will push their pupal skins out of the holes & these skins might be seen extruding from the trunk. These pupal skins are ephemeral & will break apart easily. The lilac/ash clearwing borer adults emerge in the spring (May) while the banded ash clearwing adults emerge in late summer (September).

Banded ash clearwing moth pupal skin emergence from trunk. (Photo Credit: Steven K. Rettke of RCE)

Banded ash clearwing moth adult exit hole (1/4 inch). (Photo Credit: Steven K. Rettke of RCE)

There are two species of ash bark beetles (Hylesinus species) in northeastern US & the symptoms they produce on infested ash trees (green & white species) are distinct from the other borers previously discussed. Also, unlike the other borer species, they typically have two generations per year. Ash bark beetles infest stressed trees & they concentrate activity within lower branches, although they can be found in the trunk. Ash trees are not particularly tolerant to their own shade & the lower, shaded branches may become stressed & hence, more vulnerable to attack by the beetles. The bark beetles will also be attracted to drought or disease infected trees. Ash trees experiencing infestations by ash bark beetles can still typically have a relatively healthy appearing upper crown but show symptoms of numerous dead or dying lower branches.

Ash bark beetle borers infest lower branches.

Ash bark beetles infest mainly lower branches, as upper crown still appears healthy. (Photo Credit: Steven K. Rettke of RCE)

Within branches that are mostly one inch in diameter or less, adult ash bark beetles will cut egg galleries across the grain perpendicular to the branches. These branches are sometimes girdled & cause them to easily break off the trees 2 or more years later. After egg laying, dozens of hatched larvae will tunnel at right angles away from their galleries along the grain of the branch. When ash bark beetle adults emerge from the tree, they create numerous round exit holes (<2 mm) in the wood.

Ash bark beetle adult exit holes (<2 mm). (Photo Credit Steven K. Rettke of RCE)

Ash bark beetle egg gallery girdling twig branch. (Photo Credit: Steven K. Rettke of RCE)

In New Jersey, ash bark beetles have 2-generations per year & therefore treatments applied in June may need to be repeated in August. Late instar larvae or adults can overwinter in the trees. Adult beetles can bury themselves deeply within the tiny bark fissures in the trunk. During the early spring, woodpeckers often create the flecking or “blonding” areas on the bark as they search for the beetles. This can mimic the same appearance found with Emerald ash borer infestations, as woodpeckers in this case, search for overwintering EAB larvae. A common misconception is ash trees having numerous flecking areas on the trunk is proof for EAB. In some instances, it is the presence of ash bark beetles that is the reason for the flecking.

An example of the importance for a correct diagnosis of the ID of an ash wood borer species was illustrated a few years ago at a townhouse complex. The photograph below shows just 3 of over 100 green ash trees located at the development that the HOA had tagged for removal. They were incorrectly determined to be infested with Emerald ash borers & were ordered to be removed. In reality, they were infested with ash bark beetles & only the lower branches were showing die-back & decline. Unlike EAB, ash bark beetles are not life-threatening to the tree. Fortunately, the trees were saved & subsequently are being protected against both borer species.