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The Emerald Ash Borer Story (EAB)

Emerald Ash Borer, Agrilus planipennis

The emerald ash borer (EAB) is a destructive wood-boring insect that has killed millions of ash trees in North America. It was first discovered in Detroit, Michigan in 2002, and it likely came from wood packaging material imported from Asia. It has become widely established in 35 states and five Canadian provinces. As of April 2021, it has not been detected in Montana. Unfortunately, it is easily transported on firewood, so Montana is always just one visitor’s mistake away from EAB establishing here.

What’s at Risk in Montana?

Ash are the most commonly planted trees in many Montana communities east of the Continental Divide. Ash species represent more than 40% of all publicly-owned trees in 20 Montana communities. The pest is also easily transported through infested firewood. Emerald ash borer can also naturally spread by flying approximately 2 to 12 miles per year. Ash trees that are killed by EAB become particularly brittle and liable to breakage, which then threatens property and public safety.


Trees at Risk

Emerald ash borer attacks all true ash species (Fraxinus sp.) and the white fringetree, Chionanthus virginicus. Mountain-ash trees are not true ash trees and they cannot become infested with EAB. Proper identification of true ash trees is critical. Ash trees have opposite, compound leaves with short-stalked leaflets. Leaflet margins may be smooth or toothed.


 Green ash leaves (above-right). Photo by P. Wray, ISU,

Detection is difficult, so study the symptoms and be alert. The upper canopy of trees is attacked first, making it difficult to notice early stages of an infestation. Other factors can mimic symptoms of EAB. Ash trees in Montana may show evidence of decline and dieback for a variety of factors, none of which has yet been linked to EAB. The pest is often in the tree for up to four years before symptoms are visible. Green and purple prism traps are not yet proven effective for early detection nor mitigation of emerald ash borer.


The juvenile stage (larva) damages by feeding in the phloem and cambium, which interferes with the tree’s ability to transport nutrients and water. Ultimately, the branch and the trunk are girdled, causing dieback, canopy loss, and death of the tree. Infested trees will usually die within two to four years if left untreated.

Prevention and Mitigation

  • Diversify tree species in the community in both new plantings and regular replacement trees.

  • Remove unhealthy trees.

  • Don't bring out-of-state firewood into Montana.


What to Do if You Suspect Emerald Ash Borer

If your ash tree exhibits dieback, refer to all possible biotic and abiotic issues in this guide.

For further help, contact a certified arborist in your area. If you suspect EAB on your property or have a suspected EAB insect sample, contact your local extension agent, the Schutter Diagnostic Lab at Montana State University, or the Montana Department of Agriculture.


  • It is unnecessary to do preventive chemical treatments until EAB is confirmed within 30 miles.

  • Work with a certified arborist if infestation is suspected. Remove confirmed infested trees promptly.

  • Chemical treatments can be effective when properly applied

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