Emerald Ash Borer

Emerald Ash Borer (EAB) information for Northern Colorado.

Last Updated: July 4, 2016

Even if you haven’t heard of Emerald Ash Borer before, it will soon be something you can’t ignore. With as many as a quarter of our urban trees consisting of ash, the potential for losing a significant portion of our urban forest is very real. The Emerald Ash Borer is the most destructive tree insect we have ever seen.

Emerald Ash Borer has been in Boulder since at least 2011, possibly longer, and has been spreading at a quick pace ever since it was discovered. The situation in Boulder is now dire and EAB can be found all over town. On June 8th, 2016 it was discovered in the middle of Longmont. It is only a matter of time before EAB shows up in Loveland, Windsor or Fort Collins, and it may already be in Berthoud. What we’ve seen in other infected areas across the country is that it’s always there before we know it.

Once it’s discovered in a new area it is usually already established, and at that point it’s a race to catch up with it. No community has ever been successful in “getting ahead” of Emerald Ash Borer.

I don’t think it’s too early to start thinking about treatment for Emerald Ash Borer if you’re in Northern Colorado. In fact if you’re south of Highway 34 in Loveland, I’d say it’s time to start treating your trees or make the decision to let them go. The closer you are to Longmont, the more important taking action is, but remember that EAB doesn’t necessarily spread with linear progression and could just as easily skip Loveland and go straight to Fort Collins if someone were to transport infected wood there.

One last point to be made. I can’t stress enough that taking no action is actively deciding to lose your tree to Emerald Ash Borer. I don’t necessarily mean right now, but when it’s shown up in your neighborhood and you’re still thinking you’ll take your chances, please understand that the chance of your untreated tree not dying from EAB will be zero. This bug will kill every single untreated ash tree in it’s path: young and old, healthy or not.

Although Emerald Ash Borer is by far the most destructive tree pest ever seen in North America, treatments for the borer are effective, and Arborists have several options available.


How can I protect my trees from Emerald Ash Borer?
What should I do if my ash tree has Emerald Ash Borer?
Background information on Emerald Ash Borer
What does an ash tree look like?
What does Emerald Ash Borer look like?
What happens to a tree with EAB?

How can I protect my trees from Emerald Ash Borer?

Fortunately there are effective treatments against Emerald Ash Borer. It may not be feasible to treat every tree, but I think homeowners should be able to save their ash trees, especially if they just have a few. Front Range cities such as Boulder realistically won’t be able to save all of their trees, nor will many home owner’s associations and businesses. Even if saving every tree isn’t the goal, treatments can allow us to manage resources better by staggering the removal of trees across a number of years, rather than all at once.

I believe it is imperative to begin treating trees before they show visible decline. The sooner we start treatments, the more likely we can save the tree. And getting started now treating for Ash Lilac Borer is a great idea, as that bug can reduce the effectiveness of the EAB treatment. Be aware that treating trees for Ash Borer is a long-term commitment: not any less than 10 consecutive years.

There are a few different options for treating trees. The most effective treatment, emamectin benzoate (brand name Tree-age ®), is also the most expensive and must be applied by licensed pesticide applicators as it’s use is restricted by the EPA. It is injected into the trunk of the tree and when the high rate is used, will last two full years. This is the only good option for large trees. This treatment also controls aphids and Ash Lilac Borer.

A soil-applied treatment can also be done and is best for small trees only. Consider that a tree with a 14″ trunk has four times as many leaves as a 7″ tree (not twice as many as you might think), and you can start to understand that this treatment on the bigger tree won’t work as well as on the 7″ tree. The label allows us to double the rate for trees over 15″ but when you get up to a tree with a 24″ or larger trunk, doubling the rate just doesn’t do it anymore. Although this soil drench treatment doesn’t work quite as well as the above treatment, it has been shown to keep smaller trees (under 24″) alive through an Emerald Ash Borer outbreak, with perhaps 20% thinning of the canopy. This treatment must be done every year. It controls aphids but NOT Ash Lilac Borer, which should be treated for with a spray.


What should I do if my ash tree has Emerald Ash Borer?

If you suspect your tree has Emerald Ash Borer, we first need to confirm this. Give me a call and I can make arrangements with the Department of Agriculture (at no cost) to do an inspection and confirm the presence of EAB. Healthy trees can usually survive a year or two of EAB infestation, but we would need to start treating with powerful insecticides right away.


What do ash trees look like?


The leaves of ash trees are compound; meaning they usually have 5 to 9 leaflets on each stem. Leaves can be confused with walnut, tree of heaven, and hickory, but those trees are far less common around Boulder than ash.

White Ash Bark

Young white ash trees can have either diamond shaped bark or smoother bark with plates and cracks as in this photo. As the tree matures the trunk takes on the diamond-shaped bark. The color of the bark varies; it can be white, grey, and pink or orange.

Green Ash Bark

Green ash tends to have oblong diamond shapes in the bark, and tends to be ash grey colored.

Autumn Purple Ash

The bark of purple ash tends to be more smooth. Since this tree is a variety of white ash, the bark can be similar. The branches above the trunk tend to be fairly smooth and either light grey or soft orange/pink. Foliage turns dark purple to bright orange in the autumn.


What does Emerald Ash Borer look like?


Adult Emerald Ash Borers are metallic green, and about half an inch long. They eat ash leaves during the summer and so can be found on the foliage.

Exit Holes

Emerald Ash Borers exiting the tree leave D shaped holes in the bark. This should not be confused with the native ash/ lilac borer that leaves O shaped exit holes.

Larvae Galleries

Larvae feed just under the bark and create a zig-zag or S pattern that starts small at one end and grows larger. A tree with an extreme infestation will start to shed bark, and woodpeckers can often be spotted blowing off big pieces of bark as they feed on larvae.

Epicormic Growth

Heavily infested trees will start to push new growth out at the base of the tree. Energy stored in the roots is unable to reach the canopy because the tree’s phloem is destroyed. This causes new growth to be pushed out of latent buds at the base of the tree.

Background Information

Emerald Ash Borer is an invasive pest that originates from China and other parts of Asia. It probably hitched a ride to the States in packaging material used in shipping. It was first discovered in Michigan in 2002, and has since spread like wildfire. Adult beetles have been observed to fly a half a mile in their lifetime, however, people have helped it spread much faster by moving firewood and through transporting nursery trees.

In Ohio the estimated (using mean average) economic cost of Emerald Ash Borer is pegged at $7.5 billion dollars for that state alone. The estimate includes tree removal cost ($2.9 bil), replacement costs ($1.3 bil) and property value loss ($3.4 bil). EAB is the most destructuve tree pest ever seen in North America.

Emerald Ash Borer was first discovered in Colorado in Boulder on September 27th, 2013.


What happens to trees with Emerald Ash Borer?

Ash trees in North America have no resistance to EAB, and so long as the insect’s population is sufficient, all untreated ash trees within an infestation area will eventually die. The process actually takes many years, as many as eight, but visually most of the damage takes place in about two years.

The first year or two of infestation, the tree may have little or no outside visual symptoms. As things progress, you may notice the canopy of the tree will start to thin. Once the canopy has died back by 40%, treatments are no longer effective, and the tree will likely die within the next year or so. When a tree is heavily infested, there are many other signs as well: Heavy woodpecker activity, suckers growing at the base of the tree, and bark splitting vertically.

Life Cycle of Emerald Ash Borer: Adults emerge in May and June and begin feeding on the foliage of ash trees. Females soon begin laying eggs on the bark of ash trees, and the eggs hatch a couple weeks later. Larvae bore into the tree (they are very small so we don’t see these holes) and begin feeding on the nutrient-rich tissue just beneath the bark. The larvae “molt” several times before overwintering in the wood and finish developing in the spring. As the flat-headed larvae chews it’s way out of the tree, it leaves it’s characteristic D shaped exit hole. At this point the larvae pupates and the adult beetle is formed.

defoliated ash tree