Emerald ash borer (EAB) is a beetle, named for the appearance of the adult stage of the insect, which is a metallic emerald green. The adults are about 10-13 mm long and cylindrical (somewhat bullet-shaped). When the wing covers and wings are pulled back, a metallic pinkish-red colour can be seen on the upper part of the beetle’s abdomen. You may see adult beetles from late May until August, depending on the temperature conditions in your area.

Adult emerald ash borer beetle showing reddish abdomen. Image: David Cappaert, Michigan State University, Bugwood.org

The female EAB lay eggs in crevices or slits in the tree's bark. The tiny, oval eggs (about 1.2 mm long) are well hidden, so they are difficult to see. The eggs are white when they are first laid but turn brown after a few days.

Emerald ash borer eggs are white when first laid and turn brown after a few days. Image: Debbie Miller, USDA Forest Service, Bugwood.org

Larvae hatch from the eggs after 2-3 weeks. Larvae are translucent white, and have ten segments, the last one having two dark brown spines on the end. EAB has four larval stages (instars), so four distinct sizes of otherwise similar looking larvae can be found; fully-grown larvae are up to 30 mm long.

Emerald ash borer larva with 10 trapezoid-shaped segments. Image: Chris Gynan, Silv-Econ Ltd.

Newly hatched larvae bore into the inner bark layer of the tree where they start to feed. Larvae feed in a serpentine pattern leaving an S-shaped gallery; this gallery is distinctive and when found under ash tree bark is a good indicator that the tree is infested by EAB. The part of the tree where larvae feed (inner bark (phloem) and outer sapwood) is where nutrients and water are transported through the tree, so when the larvae consume enough of it (this usually takes a year or more) the tree will start to decline and eventually die.

Serpentine emerald ash borer gallery. Image: Troy Kimoto, CFIA

After feeding for several months over the summer, the larvae bore into the sapwood of the tree to spend the winter and to pupate. The pupae are the same shape as the adult beetle but are white when newly formed and then start to look like adult EAB as they develop. After the pupal stage is completed, the adult EAB emerges in the spring, boring through the bark to leave the tree. This leaves a distinctive D-shape exit hole in the bark (approx. 3.6 mm by 2.8 mm), that can be seen on the surface of the tree.

Emerald ash borer pupae and adults showing the progression of maturation. Image: Debbie Miller, USDA Forest Service, Bugwood.org

Adult EAB only live 2-3 weeks which is why insecticides targeting the adult stage are not effective. The entire EAB lifespan described here usually lasts one year, but some EAB need two years to develop.

Typical D-shaped exit hole made by emerald ash borer adult emerging from the tree. Image: Kathleen Ryan, Silv-Econ Ltd.

Emerald ash borer is a proficient flyer and can disperse to new areas naturally, but it is also moved longer distances in infested firewood wood.

See a more detailed version of EAB biology.