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Big Bad Bug Blog – Episode #2: The Emerald Ash Borer

Updated: Mar 15, 2022

The Invasion

2002 was a bad year for ash trees. An alien invader, living undetected since the 1990s, was about to take center stage in single-handily decimating their population in North America. Before that fateful year, the emerald ash borer was never found outside of Asia. Hailing from northern China and Korea, they also inhabit eastern Russia, Japan, and Mongolia, where they don’t seem to pester their native trees all that much. But boy, is it a troublemaker here in the ol’ US of A.

Identified first in southeast Michigan, researchers think the emerald ash borer arrived in Detroit several years earlier, probably as a stow-away in wooden packing materials aboard a ship. The original infestation area has grown since 2002, largely because of people moving infested firewood. On its own, the EAB only flies about 1/2 a mile in a year. But tucked away in a piece of ash firewood, it can move hundreds of miles in a single day in the trunk or truck bed of an unsuspecting camper. The EAB has now hitchhiked its way into 33 states and 3 Canadian provinces, killing up to 99% of all ash trees in its path and continuing on in search of more.

Adult emerald ash borers are a bright metallic green color, like a deadly diva dressed for disco, and when they flare their wings, you can see just a hint of their violet abdomen. An EAB can easily fit on top of a US penny, with room to spare! The larvae grubs are white in color and live underneath the bark, feeding on the nutrient rich layer of trunk that lies just below. When they do this, they cut off the flow of water and nutrients in the tree. Most trees die after about 3 years of infestation.

The Life Cycle

Adult beetles begin emerging from an infested tree in May, with peak emergence in June. You’ll find most of the beetles in late June and early to mid-July. Females usually begin laying eggs about 2 weeks after they leave the tree. Eggs hatch in 1-2 weeks, and the tiny larvae bore through the bark and into the cambium and phloem, the tree’s nutrient superhighway. The larvae feed under the bark for several weeks, usually from late July through October.

The larvae typically pass through four stages, eventually reaching a size of roughly 1 to 1¼ inches long. Most EAB larvae spend the winter in a small chamber in the outer bark or in the outer 1/2 inch of wood. Pupation occurs in spring and the new generation of adults will emerge in May or June, to begin the destructive cycle again.

The Damage

Sometimes ash trees produce epicormic (there’s a new word for you!) sprouts or “water sprouts” on the trunk or on large branches where EAB damage is heavy. Epicormic sprouts are shoots coming from old wood. The bark of the tree may crack over the larval galleries – the places where the larvae are burrowing. Adult beetles leave a characteristic exit hole that looks like a capital “D”, you know for death, roughly 1/8 inch in diameter, when they emerge in June.

Woodpeckers often attack larvae, especially during the winter, flaking off the bark in small strips as well as making holes. Woodpecker damage is larger and easier to see than the D-shaped exit holes. Several infestations have been discovered because people noticed woodpecker damage on ash trees and then took a closer look. Another common symptom of EAB infestation is yellowing or dying branches at the top of the tree. Be aware of EAB imposters, though. Other insects like lilac/ash borer, ash bark beetle and flat-headed appletree borer may look like EAB or cause similar tree symptoms.

The Defense

There is a national effort to limit the spread and impact of EAB. A national plan, coordinated by the United States Department of Agriculture, Animal Plant Health Inspection Service (APHIS), guides what federal, state and local officials must do to manage this insect. Infested areas are quarantined, which means that things like ash firewood, nursery trees, and ash logs may not be moved out of areas where there are EAB. To help identify ash trees, CSFS and Colorado State University Extension have developed a free app that will allow anyone to use their mobile device to quickly ascertain whether a tree may be a potential target for the pest.

Avoid Planting Ash Trees

Ash trees have been widely planted in Colorado, but because EAB is always fatal to ANY untreated ash trees, avoid planting any true ash species (genus Fraxinus). Instead, focus on tree and landscape-plant diversity. No one tree species should comprise more than 10 percent of the planted trees growing in any urban or community setting.

Now is a great time to ‘plant ahead’ and get new trees in the ground that can someday replace ash lost to EAB, and also replace the shade and other benefits they provide.

An ash tree replacement tool, which includes a list of trees suitable for ash replacement, is available on the Colorado Tree Coalition website at

Management Options for Homeowners

In Colorado, EAB has only been detected in the Cities of Boulder, Lyons, Longmont, and Gunbarrel but its further spread is imminent. While there are effective insecticides available to protect ash trees from EAB, other management strategies also exist for dealing with the pest, including:

  • monitoring trees for the presence of EAB,

  • removing and/or replacing ash trees, and

  • planting new trees preemptively in an effort to get them established before the arrival of EAB.

Decisions about how to manage ash trees should take into account the overall health of each tree and its value to the property owner.

The closer ash trees are to an area of known infestation, the higher the risk that they will become infested by EAB through natural spread. Also, trees within or near the EAB Quarantine area are at a higher risk of infestation through human-assisted spread of the pest, because infested wood can legally be moved throughout the area.

Consumers should educate themselves when purchasing chemical products to protect trees against EAB. Click here for more information about chemical treatment options.

If hiring someone to apply pesticide treatments to protect ash trees, the applicator must be licensed by the Colorado Department of Agriculture as a Commercial Pesticide Applicator.

Don’t Move Firewood!

Never transport hardwood firewood or any other raw wood products from ash trees, as this is the most likely method of accidental spread.

A quarantine is in place in Boulder County and the surrounding areas to try and prevent the human-assisted spread of EAB. Regulated articles include EAB specimens, ash nursery stock, ash logs, branches and chips, green lumber, all hardwood firewood or any other article, product or means of conveyance that may present a risk of spreading EAB.

To Learn More

For current information about the current quarantine in Boulder County and surrounding areas, visit

If you think you have detected EAB in your ash trees, please contact the Colorado Department of Agriculture at 888-248-5535 or email

Contact a licensed tree care or landscape company. Response time is critical when it comes to saving your landscape from the mouths of these eating machines. If you see signs of emerald ash borers, contact Plant Escape immediately, so that our trained horticulturalists can begin evaluation and treatment of your plants, shrubs and tress, ASAP.


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