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.
Blister beetles can be commonly encountered during the summer months in Wisconsin. These beetles can often be found on flowers or on the ground, but can be found in a variety of other places as well.
Dogbane beetles are metallic-green in color with hints of red and orange. These beetles are occasionally encountered and adults can be found feeding on Dogbane plants.
A large number of ground beetle species exist in Wisconsin. These beetles are excellent runners and can often be seen scurrying on the ground or on tree trunks. Some species can have a metallic green appearance. Some can be up to 1″ long.
Tiger beetles are a type of predatory ground beetle. They are often found on the ground in open sandy areas, where they are active hunters. Most tiger beetles are shiny, metallic colors.
Japanese beetles can be all too common during July, August, and September in Wisconsin. These beetles feed on over 300 plant species, including many ornamental and garden plants, and are seen by many as pests. They are readily identified by their metallic green and copper colors, and a series of white dots along the side of the body.
Several weevils can have a metallic green appearance. These insects can be quite common in parts of the eastern U.S. Weevils, such as this one, can often be readily identified by the snout-like projection on the head.
Linden borers belong to a group called the “long horned” beetles and their long antennae reach nearly to the back of their body. Linden borers are beige in color and attack Linden trees.
The metallic wood boring beetles (Family Buprestidae) includes EAB and its relatives. There are over 100 native species in our area and only a few are considered pests. These insects are known for their colorful, metallic bodies.