Jennifer Schultz Nelson
Extension Educator, Horticulture
Unless you've been away from absolutely all media sources in the past ten days, you've heard the news that the dreaded Emerald Ash Borer has hit Illinois. Many people have called my office panicked at the thought that their tree may be harboring this insect. It's not time to panic just yet. But it is time to learn what we're up against and what we can do now in relation to this tiny, yet surprisingly destructive insect pest that has killed many trees in the U.S.
The Emerald Ash Borer (EAB) is a native of Asia, specifically eastern Russia, China, Japan, Korea, Mongolia, and Taiwan. It was discovered in Detroit, Michigan in 2002. Experts believe it was present in Detroit at least ten years before its discovery, and that it entered the U.S. via solid wood packing materials. To date EAB has killed about 20 million trees in Michigan, Indiana, and Ohio.
EAB is a beetle that belongs to the insect family Buprestidae, which is also known as the metallic wood-borers. The insects in this family occur in a wide variety of colors with a metallic sheen. As its name says, EAB is brilliant metallic emerald green. It is in the same family as the bronze birch borer, and the two-lined chestnut borer, which are native to the U.S. Their life cycles are very similar.
In its native Asia, EAB is most successful infesting ash trees that are under stress. This is reminiscent of the bronze birch and two-lined chestnut borer in the U.S. Unfortunately for North American ash trees, they have never seen this pest before, and genetically, they are completely vulnerable. There is no known resistance in North American species. Scientists are searching for resistance in Asian species, and also testing insecticides to control the pest in North American trees. Hopefully, a reliable control will surface.
The EAB's life cycle occurs over one year. They can only survive on ash trees (Fraxinus spp.). Adult beetles are about half an inch long and emerge from under the bark of infested ash trees in May through early August. They leave a tiny hole, only 1/8" long, in the shape of an upper case letter 'D'. The flat side of the 'D' is always parallel to the ground. Adults feed sparingly on the tree's foliage until they mate. The female deposits eggs on the ash tree's bark. Scientists have observed the beetles infest a new ash tree at the top first, making early detection difficult.
The larvae hatch and tunnel beneath the bark, feasting on the nutrient-rich living tissues of the tree which occur immediately beneath the bark. This tissue contains the phloem, through which the tree pumps sugars to feed all parts of the tree. The larvae typically make a distinctive 'S' shaped path called a "gallery" under the bark. This is not always the case, as some larvae have been observed making random galleries without an 'S' shape anywhere. Larvae overwinter under the bark, and form pupas in the spring, emerging as adults a short time later.
The damage the EAB larvae do unseen under the ash tree's bark is what leads to the tree's death. Because of larvae eating their way through the phloem, the tree loses its ability to pump food above the damaged areas. Without food, the part of the tree above the larval damage dies. Since infestation commonly occurs at the top of the tree, dieback starting at the top of the tree is most often observed. As larval numbers and damage increase, so does the proportion of the tree that dies.
In advanced infestations, areas where the phloem has died beneath the bark dries out and the bark splits and may fall away from the tree. As increasing amounts of the tree's canopy dies, epicormic shoots, or "suckers" may develop at the lower trunk of the tree. Physiologically, the upper portions of the tree have been killed by the larval damage, but the roots and lower trunk are making one last desperate attempt at survival.
All of the signs and symptoms of EAB are physiological reactions that occur in the tree in response to the EAB larvae feeding on the phloem beneath the bark. They are not unique to EAB, which has complicated matters for homeowners. Do not panic at the first dead branch you see in your ash tree. There are several potential causes, and currently EAB is very far down on the list of possibilities for central Illinois. If you have concerns about your ash tree, please call the U of I Extension office at 877-6872 or 877-6042 and ask to talk to me or the Master Gardener at the Help Desk. We will be happy to assist you in diagnosing the problem.
Scientists have established that EAB can move a maximum of five miles under its own power in a season. Kane County is the only location in Illinois that has a confirmed infestation of EAB. Kane County is about 185 miles from Decatur. Even at the maximum travel rate, EAB would take 37 years to get here.
If EAB travel relatively slow, how did EAB spread through Michigan, Ohio, and Indiana, and enter Illinois so quickly? The answer, as with a lot of environmental problems, is man. Initial spreading was thought to have been through infected nursery stock and firewood. Shipment of ash trees and ash firewood are now strictly regulated in affected areas.
Firewood continues to be a problem, especially in situations where people transport firewood for personal use. The best advice is to buy and use firewood locally. We can all help slow the spread of this destructive insect by resisting the urge to bring firewood home from our camping trips this summer. When it's time for the fireplace this fall, remember to buy your firewood from a supplier that gets wood from local sources.
For more information on EAB and related topics, check out www.emeraldashborer.info.