Emerald Ash Borer is an invasive and destructive beetle from Asia. It was first found in the U.S. in Michigan 2002, and it arrived in Indiana in 2004. It can now be found in almost every county in the state. Emerald Ash Borer, or EAB, is a threat to urban and natural forests all over the country because they attack and kill our native ash trees (and now possibly white fringe tree). Until EAB arrived on the scene, ash trees were among America's favorite city trees because of their attractive appearance, ability to tolerate urban conditions, and lack of serious pest or disease issues. In some communities, up to 40% of the street trees are ash trees, and they are all at risk of dying. Let's learn a little more about this insect, what it does to our trees, and how to stop it.
As their name suggests, Emerald Ash Borers are bright, shiny green beetles. They are fairly small, only about 1/3" long and 1/16" wide. The EAB life cycle begins in late May when the adults begin to emerge to mate and lay eggs. The adults emerge in several waves, so it's possible to spot them through July if you look carefully. The eggs are laid on the bark of ash trees. Once the eggs hatch, the pale white-ish larvae burrow into the wood of the tree to feed. They spend the rest of the summer and fall eating the critical layer of wood just under the bark. This thin layer of wood under the bark is responsible for transporting sugars, water, and nutrients throughout the entire tree, so as the larvae continue to feed they cut off more and more of the tree's ability to feed itself. EAB larvae stay under the bark over the winter until spring when the weather warms up. At this point, the larvae turn into pupae and morph into adults. By late spring or early summer, the adults begin to emerge to start the cycle over again.
You usually won't spot the adult beetles themselves, but there are other ways to help diagnose EAB. Step one is to identify your tree. While there have been some isolated cases of white fringe tree being attacked, it is the American ash species we need to worry about. Not sure if you have an ash tree? This bulletin will help. High amounts of woodpecker activity can be a sign of infestation as they search for delicious larvae to eat. When the adults chew their way out of the tree, they leave a D-shaped exit hole. As the infestation progresses, you will see the tree start to lose leaves and branches dying back from the top of the tree working down. It's common to see green watersprouts (also known as epicormic growth) shooting up from the base of the trunk or near where the main limbs start as the tree attempts to start new shoots away from the larvae. If the bark of an infested tree is peeled away, there will be zig-zagging tunnels or galleries in the wood from the larvae feeding. Click the picture below to see larger photos of the damage EAB causes.
What are the chances of survival once EAB has attacked a tree? It all depends on how early you catch it. There are effective insecticide treatments, but they work best if you catch the insects early and continue to treat the trees for several years afterward. This may sound like a lot, but compared to the cost of removing and replacing a tree, it is actually less expensive to do insecticide treatments, especially since we now have some very effective options that only need to be used every two or three years. A tree can be saved if no more than 1/3 of its canopy has died back. For small trees, you may be able to treat it yourself by soaking the soil with specific insecticides. For medium or large trees, trunk injection of insecticides is often required (see image to left), and that is a job for tree care professionals. If you are interested in information on treating trees to protect against EAB, check out the links at the end of the post.
There is some good news in the fight against this destructive beetle. It looks like this insect will not become permanently established in our communities. The EAB population in any given area will reach its peak when about 50% of the ash trees are infected. After that, their numbers start to drop off as they run out of food. If you have decided to protect a tree with insecticides, you will still need to treat it afterward to protect it from any stragglers, but you won't have to treat the tree as often.
There are two other reasons to be optimistic: promising results from biological controls, and possible resistance in blue ash. In Asia, where EAB is a native insect, their population is kept at a manageable level in part by parasitic wasps that prey on them. After extensive research to ensure that these wasps won't cause a whole new problem, the USDA has approved a plan to release these Asian wasps to see if they can help control EAB populations. We also have a species of wasp native to the U.S. that is starting to parasitize EAB, so maybe more of our insects will learn to prey on them. Forest managers have started to notice that blue ash trees seem to be more resistant to infestation than green, white, and black ashes. Researchers are trying to figure out why, and then they will work to breed new varieties of ash trees that are more resistant to EAB. Hybridizing our ash trees with Asian ash trees may also help us cultivate better varieties so we can keep these awesome trees in our cities and landscapes.
Even though the destruction from EAB has been devastating to our urban and natural forests, it serves as a reminder of some important facts. In an increasingly global economy, we must be more vigilant than ever when it comes to quickly identifying and containing invasive species. In planning our communities and landscapes, diversity of plants species is critical. The EAB outbreak would have been slowed down and had a reduced impact if we were not relying so heavily on one genus of trees. As new non-native threats to our landscapes and natural spaces inevitably emerge, we will see if we have learned from our fight with EAB and the many other invasive species that have become serious problems in the U.S.
Want to learn more? Check out these websites for information on how to protect your ash trees, photos and tips on diagnosing EAB, information about local initiatives to fight the pest, and more!