GARDENING: A new threat to our ash trees is coming |

GARDENING: A new threat to our ash trees is coming

Don't fall for the resplendent green beauty of the emerald ash borer. This pest aims to kill all of our ash trees.
Colo. Dept. of Agriculture |

Last week, the emerald ash borer was identified in Boulder County. This wood-boring beetle is a beautiful emerald color and devastating to ash trees. It was first detected in North America in 2002 in Eastern Michigan where it was killing ash trees of all types; green, white, black and blue.

Infested trees lose leaves and their upper branches and twigs. Serpentine tunnels are formed under the bark by the larva. It is estimated that more than 50 million ash trees have died due to this beautiful beetle since its first discovery in Michigan in 2002. Since then this beetle has spread to 21 states including Colorado.

It will be interesting to see what this beetle does to the estimated 98,000 trees in the city of Boulder. Will the city be proactive and start a spray program or wait to see how much damage the beetle will do? Does Boulder even have the money necessary to conduct such a spray program? Once this beetle moves into the Denver Metro area, it will have an estimated one and one-half million ash trees to kill.

More than likely there will be a quarantine put in place to prohibit the movement of firewood from the area where the beetles were identified. However, quarantines don’t work when enforcement is lacking. More than likely this insect will be in western Colorado within the next several years. They might come along with ash firewood somebody brings on a camping or hunting trip to this area. It is equally likely beetles will be hitchhikers in a vehicle coming from the Eastern Slope. Let’s get real! No one is going to inspect all these vehicles. Property owners, to include municipalities, are on their own when it comes to preventing damage to their trees. There are treatments available to protect your trees and I would suggest if you have an ash tree, have a talk with your favorite arborist or pesticide applicator sooner rather than later.

I’m not sure how many ash trees are within the city limits of Grand Junction but it is not unusual for ash to comprise up to 80% of a community’s trees. These trees are very tolerant of urban growing conditions and have nice fall color. This is obvious when you drive up Fifth street in Grand Junction and observe the purple ash growing along the side of the road. The local parks are filled with ash.

It will be interesting to see how the various communities in western Colorado deal with this insect. The removal of public trees that are killed by the thousands will be extremely expensive. The cost to replace them could easily bankrupt a community. Denver expects the removal of public and private ash trees will cost approximately $435 million. The replacement of those trees will cost approximately $580 million. While Denver has a lot more ash trees than Grand Junction, many of our parks have high concentrations of ash.

The only way to prevent this beetle from killing your tree is with yearly insecticide treatments. Pesticides will need to be made on an annual basis for the remaining life of the tree. Waiting until the insects are actually identified in this area is most likely what communities will do. By that time, however, there could be a serious infestation already established.

The emerald ash borer is indicative of the global environment we live in. Last year, the Pine Wilt Nematode, another Asian native, was found in the Grand Junction area killing Austrian pines. No matter where you live in this small world, you will find pests that originated from some other part of the world in your yard. The only thing you can do is be observant of your trees and shrubs, and be proactive when you learn of a new threat to their health. Property owners and managers need to be cognizant of any new threats and be willing to take the time to learn how to deal with them. If you are not able to do that yourself I would suggest you visit with your favorite arborist or a consultant such as myself to determine what steps should be taken to protect your landscape from disasters such as the Emerald Ash Borer and Pine Wilt Nematode.

Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at, 970-778-7866 or check out his blog at He owns Swift Horticultural Consulting and High Altitude Lavender.

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