In terms of the damage that it can cause a tree and to a community, emerald ash borer (EAB) is in the same league as Dutch elm disease, oak wilt and gypsy moth. Fortunately, though it may be complicated and require persistence, EAB control is achievable.

Pockets of infestation

EAB was first discovered near Detroit, Mich. The lake states of Wisconsin, Illinois and Michigan, as well as the province of Ontario, are the hotbed of EAB activity. It is estimated that EAB is responsible for killing more than 30 million ash trees in this area. As is the case with other insect outbreaks, EAB can be transported to new locations. Currently, they appear to be spreading westward, and damage has recently been found in north central Illinois.

Host preference

Most insect pests tend to be attracted to weak or dying trees. Yellowing leaves and other biological signals are sensed by the insect, aiding them in finding a suitable host. However with EAB, healthy and stressed trees are equally likely to be attacked.

Further, almost all species of ash are susceptible to EAB, with white, blue, black and green ash most commonly infested. Native species and hybridized clones, such as Marshall’s Seedless, Patmore, Bergeson, Cimmaron, Summit, Autumn Purple, Autumn Applause, Autumn Blaze, Fallgold and True Blue, are equally likely to be attacked.

Damage symptoms

Adult borers are elongated, slender beetles, emerald green in color, about .5 inch in length. The first sign that an ash tree is infested is the appearance of weak and dying stems and branches in the crown. The bark on the trunk of the tree will typically be riddled with small holes, shaped like a capital letter D. Further inspection will reveal zigzag tunnels under the bark that are packed with frass (insect excrement and sawdust). These tunnels disrupt the cambium of the tree, rendering it incapable of transporting nutrients and water to the upper branches. Once this occurs, tree death ensues in a matter of one to four years.

Photo by Jim Kalisch.
Damage caused by emerald ash borer

Other symptoms of an EAB infestation include: metallic-green beetles observed feeding on the foliage, epicormic sprouting (water sprouts, suckers) around the trunk, split or loose bark and increase in woodpecker activity.

Life cycle

Emerald ash borers generally have one generation a year, but may require two years to complete their life cycle in cooler regions. They overwinter as prepupae under the bark of the tree. Adults (beetles) begin to emerge in early June leaving behind the characteristic D-shaped exit hole. Females mate then begin depositing their eggs in bark crevices on the trunk or branches. Larvae (borers) hatch from eggs in about seven to 10 days and begin tunneling just under the bark in the cambial region of the tree. EAB larvae are creamy white, flattened and elongated, and reach a mature length of just over an inch.

Control

Effective control of EAB remains difficult. However, research conducted in Michigan, Ohio and elsewhere suggests that satisfactory protection of ash trees can be accomplished through residual insecticide sprays applied to tree trunks and major branches, soil drenches and insecticide injections into tree trunks. Trees killed by EAB should be removed and the wood destroyed before emerging beetles can move to noninfested trees.

Sanitation is also an important component of the overall control program. Because EAB is easily transported to new areas in infested firewood, quarantines are in place by most states to keep the insect from spreading. Inspect all firewood that you cut or move to ensure it is borer free, especially if the wood is from an ash tree. Better yet, if you identify the dead or dying tree as an ash, don’t cut and sell it as firewood, burn it instead.

Other borers

Don’t forget about other borers that attack ash trees. These include the lilac ash borer and the banded ash clearwing, both of which are larvae (caterpillars) of day-flying moths, and several beetle species of round and flat-headed borers and bark beetles. All of these insects infest ash trees that are already in a weakened condition and rarely attack healthy trees.

When working with your clients, be sure to emphasize the importance of practicing plant health care, and treat their ash trees using an integrated pest management approach. Proper arboricultural practices, such as proper mulch placement, watering to reduce drought stress, removal of dead or damaged limbs, preventing development of codominant leaders and avoiding soil compaction, will go a long way toward reducing infestation and damage from these wood-boring insects.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.