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Emerald Ash Borer Update

Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture
jaschult@illinois.edu

Many of you probably remember last summer when the news came that the Emerald Ash Borer (EAB) had been found in Kane County, in Northern Illinois. News broadcasts on every network showed a tiny green insect and said that it was responsible for the death of 20 million ash trees.

Some people panicked–we had more than a few calls from people afraid this insect was killing their ash tree. My answer then is the same as it is today–there has been no confirmed discovery of Emerald Ash Borer in Macon County, or any county nearby for that matter. Currently the only counties with confirmed infestations in Illinois are in Kane and Cook Counties.

That doesn't mean Macon County will never see the Emerald Ash Borer–it is very likely that at some point it may arrive. If you have questions as to whether Emerald Ash Borer is affecting your ash tree, call our office at 877-6042 or 877-6872. We can help determine whether your situation warrants a call to the Illinois Department of Agriculture for further investigation.

Understand that if Emerald Ash Borer is ever truly suspected, it is a very serious situation that would elicit response from several government agencies and likely place a quarantine on the county.

Unfortunately the inspiration for this column was a Macon County resident who asked me about his ash tree, which had some dying lower branches. He told me he had hired a professional tree service to examine the tree, and he was told that "it must be that Emerald Ash Borer" and was charged over $200 for an insecticide treatment. But he didn't have Emerald Ash Borer.

There are a lot of reasons an ash tree would have dying branches–in this case, it was likely related to the spring freeze that came in April after most trees had budded out. That week of freezing temperatures killed buds and small branches on many trees and put stress on trees as they had to dip into their energy reserves to re-leaf out. In addition, the tree in question had pavement on two sides, which was very likely causing some stress to the tree and could contribute to dieback of branches. The resident said the tree did perk up after the $200 treatment, but that was most likely because the treatment also included fertilizer. Our advice for a stressed tree would include trying to remove the source of stress if possible, watering well and fertilizing.

The pattern of dying branches is important in diagnosing Emerald Ash Borer. The resident's tree in question had a few small dead lower branches, but the way EAB attacks typically results in the top of the tree dying first, not the bottom. 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 dying branches and ulimately 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.

The take home message is that no one, even a professional, should casually conclude a local tree has Emerald Ash Borer just because an ash tree has some dead branches. It is a very serious diagnosis, and if it is truly suspected, it needs to be investigated ultimately by the Illinois Department of Agriculture.

Scientists are scrambling to research possible control measures for Emerald Ash Borer. Insecticide treatments have unfortunately not proven to be a reliable control for this little green threat. Current data shows that insecticides work best in younger trees in the range of four to ten inches in diameter, and much be treated yearly, which can become expensive.

The most promising control for EAB may be another tiny insect, a parasitic wasp native to China, where the Emerald Ash Borer originated. These wasps are stingless and only about four millimeters long. They are estimated to be capable of killing approximately 75% of a given EAB population in an infested area. They are believed to be the reason why EAB has not devastated China's ash trees. While they don't kill every single EAB, they do keep the population in check, and the reduced population means that some trees can survive a minor infestation.

The wasps kill by laying their eggs in either the EAB eggs or larvae. They have incredible sensory power, being able to locate tiny EAB eggs hidden in the bark of trees, and some can even hear EAB larvae feeding beneath the bark, and lay their eggs directly into the larvae's body. Then, just like the movie "Alien" the wasp eggs hatch and they bust out of the EAB eggs or larvae, killing them. That may sound gross, but it may ultimately be how 8 billion ash trees in North America are spared from certain death caused by EAB.

The U.S. Department of Agriculture is in the process of evaluating whether releasing this tiny wasp in EAB infested areas is a reasonable and safe recommendation. Some scientists are wary of doing this, as many of our current insect pests, EAB included, are actually native to another part of the world. Would this parasitic wasp ultimately end up becoming a pest? Scientists are still evaluating the data, but many are supporting the idea of releasing the wasps as early as this summer. While many scientists believe the wasps will probably not be a pest in the U.S., there is some question whether they will control EAB in the U.S. as well as they have in China. The only way to answer that question is to try it. If the plan to release these wasps is approved federally, it will be up to each individual state to decide whether they want to use the wasps.

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