The Emerald Ash Borer

With spring right around the corner, and trees a critical part of the beauty of the Southern Gables neighborhood, you should know about this destructive, invasive species. The Emerald Ash Borer is responsible for the deaths of tens of millions of ash trees across the United States… It has been found as near to us as Arvada, and it’s on its way south.  

By Marcel Guajardo

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The Emerald Ash Borer, or EAB for short, is a tiny beetle about 0.3 inches long that originated from northeastern Asia. Known for its shiny, emerald color, it was first discovered in the United States in 2002 in Michigan. It likely was introduced from shipping materials and pallets from overseas. In its native habitat in Asia, the ash tree species have evolved to resist EAB, and predators include a species of parasite wasps that were only recently introduced as a biological trial in certain areas of the United States. Outside of its native habitat, it is highly destructive to ash trees throughout Europe and the United States. It has spread throughout the U.S. and has been found in every state east of Colorado. In Colorado, the Boulder area was the first “ground zero” for the EAB. The city of Boulder has spent a considerable effort to manage the spread of EAB, including measures such as removing ash trees, chemically treating healthy ash trees in public right of way, and educating the public.

Leaf cluster. Photo from Colorado State Forest Service.

First though, do you know if your or your neighbor’s tree is an ash? In the spring when leaves come out it’s easy to tell – the individual leaves are over with a pointed tip, and grow in groups with leaf pairs joining to a woody stem directly across from each other and a single one at the end. In winter or early spring it’s not as obvious but you can see the branches are all across from each other rather than staggered, and the bark of a mature ash will have diamond-shaped ridges.

Diamond pattern bark on mature ash tree. Photo from Colorado State Forest Service.

The beetle kills the ash trees by laying eggs into the crevices of the bark of the tree. Once the larvae hatch, they chew on the bark and the outer layers of the tree trunk, creating tunneling that prevents the tree from distributing water and nutrients properly, thus killing it gradually.  Once a tree is infested, it is only a matter of time until the tree dies, and this varies between 2 to 5 years. Once a neighborhood is infested, all ash trees are expected to die within 10 years unless mitigation is taken.

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Adult beetles prefer to lay eggs on stressed trees but don’t avoid healthy trees altogether. If you have an ash tree that you don’t regularly water, it will more than likely be susceptible to EAB infestation.

While there are various Ash tree types, including green ash, black ash, white ash, and blue or sometimes known as purple ash, the green ash and black ash are the most susceptible to EAB. The blue or purple ash tends to exhibit the highest resistance to the EAB, likely due to a higher amount of a chemical called tannin which the EAB does not prefer. The EAB can survive in outdoor temperatures down to about -22F, so a significant, sustained cold snap is required to mitigate the population.

There are a few actions you can take to protect and mitigate your ash tree. First of all, you want to regularly water and prune the ash tree to keep it healthy. The healthier the tree, the less likely that the EAB will be able to wreak havoc on it. However, it is expected that all ash trees will eventually become infected and die due to EAB, so you might be just buying time and making it less susceptible that your ash tree is one of the first to be infected. To give your ash tree the best chances of surviving EAB, you can have a certified arborist apply an insecticide. For our single, majestic green ash tree, we have a company called The Natural Way come every other summer and inject an insecticide into the trunk of the tree. We’ve been doing this for the last 4 years, and so far, there is no evidence of EAB in the tree. Doing this in perpetuity will prolong the life of the tree by avoiding EAB infestation. Another option is that you may elect to remove the ash tree, especially if it already shows signs of infestation or stress. The common sign of this is a tree whose canopy is dead, and new branches or “shooters” are growing at the bottom of the trunk.

Marcel Guajardo is originally from Houston but came to South Lakewood early enough to say he grew up here. He and his family are writers, artists, and musicians, and love the Southern Gables area for its arboreal beauty. Marcel has a green thumb and is a reliable authority on tree care.

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