Adults:

Emerald ash borer adults are very small, metallic green beetles. They are about the size of a cooked grain of rice: only 3/8 – 1/2 inch long and 1/16 inch wide.

Adult emerald ash borers emerge from beneath the bark of ash trees late May through mid-July. They create a D-shaped exit hole as they chew their way out of the tree. The adult beetles are most active during warm and sunny days. EAB adults are not able to travel long distances and most will travel less than one mile in search of a mate. After mating, female beetles will lay 60 – 90 eggs, one at a time, in the crevices of ash tree bark. Adult beetles will feed lightly on ash tree leaves, but cause little harm to the trees. EAB adults live a total of three to six weeks.

Eggs:

Emerald ash borer eggs are very small (1 mm), difficult to find and are rarely seen. Female beetles deposit them in bark crevices and as larvae hatch from the egg, they immediately chew their way into the tree.

• Larva (immature EAB):

  This is the most destructive life stage of this insect. Larvae are cream-colored, with bell-shaped body segments. Their size varies as they feed and grow under the ash tree’s bark, but reach a length of 1.5 inches when fully grown. Larvae emerge from the eggs, and tunnel into the cambial layer of the tree (just beneath the bark).  Larvae wind back and forth as they feed under the bark, and create distinct, S-shaped tunnels or patterns (galleries).  Larvae will feed under the bark for one year (potentially two years in cold areas) before transforming to the pupal stage.  The larvae can survive in green wood, such as firewood or logs, as long as the bark is attached.

• Pupa:

  In autumn, after one or two years of feeding under the bark, larvae will create a chamber for themselves in the tree’s sapwood. They stay in this chamber over winter and pupate in the spring, turning into adults. The new adult beetles emerge from the tree, completing the life cycle. The pupae, like the larvae, cannot be seen unless bark is pulled away from the tree.

An overview of EAB’s life cycle is illustrated below:

A helpful video illustrating how to identify the emerald ash borer can be found here.

Other Ash Borers and EAB “Look-Alikes”

Several insects that attack stressed ash trees are native to the state.  A review of these insects can be found in this publication from Michigan State.

Several EAB look-alikes exist in Wisconsin.  Several of the commonest examples are shown below.  Guides showing additional examples of look-alikes are available from Michigan State University and the University of Minnesota.