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The Emerald Ash Borer - Colorado's Newest Tree Pest

The Emerald Ash Borer - Colorado's Newest Tree Pest

Aug 22 2014

Photo Credit: Debbie Miller, USDA Forest Service, Bugwood.org

By Greg Howe, South Suburban Parks and Recreation Forestry Horticulture Supervisor

You may have recently read newspaper articles, seen TV newscasts or posts on social media about a new pest attacking ash trees in Colorado. The pest is called Emerald Ash Borer, commonly referred to as EAB, and it is responsible for the death of millions of ash trees and tens of millions of dollars in costs in more than 22 states.

In Sept. 2013, Boulder forestry staff noticed what they believed to be the “D” shaped exit holes created by the adult EAB beetle emerging from the tree. Upon further investigation the larvae infesting the tree were positively identified by State and Federal entomologists. Based on tree ring dating it is estimated that the insect arrived in Boulder in 2009 or 2010.

At this time, EAB remains undetected in the District’s ash tree population. This does not mean the pest is not here. It could mean that the level of the population is still very low and has not yet been found. 

Given the potential number of ash trees in the South Metro area, homeowners and businesses may wonder if their ash trees are currently vulnerable to the Emerald Ash Borer. The short answer is, possibly.

What is EAB?

The pest is a green metallic insect that attacks healthy, as well as unhealthy ash trees of the genus Fraxinus. The Mountain ash, genus Sorbus is not susceptible. The insect is from Asia and is thought to have been accidentally introduced to the U.S. via wood shipping materials in the mid to late 1990s in the Detroit area. 

The larva of the insect creates feeding galleries under the bark, boring tunnels through the tree’s water and nutrient carrying tissues. U.S. native ash trees have no natural defense against this exotic invader and when enough larvae are present in the tree, their boring activities disrupts the tree’s ability to distribute water and nutrients. This eventually deprives the tree of adequate water and nutrients causing the tree to begin to decline and eventually die.

It takes approximately 4-6 years from the time the pest is introduced to an area for the population to reach critical numbers and start killing trees in amounts high enough to be noticed. Once this population threshold is reached, the insect begins killing trees at an exponential rate, potentially leaving hundreds to thousands of standing dead ash trees across a community in just a few years.

Are South Suburban's ash trees at risk?

In Colorado, ash trees of the genus Fraxinus make up approximately 15-20% of the community tree population. For South Suburban parks, ash trees comprise only 7% of the known tree population. The South Suburban Forestry Division has been using a protocol developed by the Canadian Forest Service to search for the possibility of EAB in the District’s ash trees. 

As of April, Forestry staff has taken 220 branch samples from 104 ash trees strategically sampled from Sheridan to Lone Tree to look for the presence of larvae. In mid-May, 12 EAB adult traps were installed across the District. These traps were inspected on July 8 and will be inspected again in mid-September.

Currently, South Suburban has elected to be environmentally responsible and is not treating trees. Late in 2015, the Forestry and Horticulture Division will make a recommendation to the SSPR Board of Directors on the management of ash trees in the District.

Are my ash trees at risk?

Your ash trees could be attacked and killed by EAB; however, this is unlikely to happen soon based on our current understanding of the pest’s known locations. But, bear in mind this is apt to change as counties, cities and special districts sample trees and inspect their traps. So far the pest is limited to a few square miles within the center of Boulder. Currently ash trees are more susceptible to lilac-ash borer or ash bark beetle attacks which can be easily mistaken for EAB.

What can I do? 

For tree owners who wish to protect their ash trees, there are treatments available. Options vary based on chemical chosen, tree health, size and age. These treatments, once started, must be maintained for the life of the tree. Treatments can be expensive depending on the chosen type and if they are done annually or every two years.

The recommendation by the Colorado Department of Agriculture is, if EAB has not been located within five miles of your ash tree(s) no treatment is necessary at this time. It is advisable to inspect the overall health of your ash and determine its personal value.  Whether or not you should treat your ash tree(s) is an individual decision and based on how much risk you’re willing to assume concerning the preservation of your tree(s).

Where can I learn more?

For more information about EAB infestation and ash tree identification, view the EAB quick guide online at www.csfs.colostate.edu or pick up a free copy at the nearest Colorado State Forest Service district. For current information about the status of EAB in Colorado, go to www.eabcolorado.com.