By Steven K. Rettke, Agriculture & Natural Resources Program Associate, Rutgers Cooperative Extension of Monmouth County
It is understandable for arborists, landscapers, and homeowners to assume that Emerald Ash Borers (Agrilus planipennis) are the cause when they observe branch dieback and decline of ash trees (Fraxinus genus). During the 21st century, this invasive Asian tree borer beetle has killed hundreds of millions of ash trees as it spread across much of the eastern half of the U.S. Although emerald ash borer will eventually find and kill most unprotected ash trees, other common ash borer species may infest some of the trees first. This article will discuss the symptoms to distinguish between the emerald ash borer beetle, 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.
The symptoms for an infested ash tree with emerald ash borers (EAB) are well known and have been documented extensively in the literature during the past 15+ years. They include the following: 1) blonding or flecking of the trunk and major branches by woodpeckers; 2) branch dieback and thinning of upper tree canopy; 3) capital D-shaped exit holes in the trunk and branches; 4) epicormic twig growth emerging at trunk base and major branches; and 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, this usually confirms EAB as the cause of the ash decline.
Confirming the presence of other species of wood borer infestations is not difficult. Arborists and landscapers should be familiar with the two clear-winged moth species that commonly attack ash. They are the previously referenced banded ash clearwing moth and the ash/lilac clearwing moth. Both borer species will cause some of the same symptoms seen from Emerald Ash Borers. Like EAB, these clearwing moth borers (CWM) 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.
Other than uncovering bark and identifying larvae, the easiest way to determine the identity of the species is by observing the size and shape of borer emergence holes in the trunk and 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, emergence holes can be seen in the trunk. Since ash trees have deeply furrowed bark, it is difficult to find these exit holes. The best way to find them is often to examine the smooth, lighter flecking areas created by woodpeckers.
The emergence holes created by both CWM will be more oval shape and 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 found in the scaffold branches. Also, unlike EAB, the clearwing moth tunneling larvae will expel sawdust-like frass from trunk openings they create. This frass can be observed down the trunk and can also accumulate at the bottom of the tree. As CWM adults exit the tree, they will push their pupal skins out of the holes and these skins may be seen extruding from the trunk. These pupal skins are ephemeral and 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).
There are two species of ash bark beetles (Hylesinus species) in the northeastern U.S. and the symptoms they produce on infested ash trees (green and white spp.) are distinct from the other borers previously discussed. Ash bark beetles infest stressed trees and concentrate activity within lower branches, although they can also be found in the trunk. Ash trees experiencing infestations by ash bark beetles can typically maintain a relatively healthy-appearing upper crown but show symptoms of numerous dead or dying lower branches.
Adult ash bark beetles will cut egg galleries across the grain perpendicular to the branches one inch in diameter or less. These branches are girdled and can cause them to easily break off the trees two 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.
This article originally appeared in the August 2021 issue of Gardener News.