Issue 7, June 6, 2011
Emerald Ash Borer and Damage
Emerald Ash Borer, Agrilus planipennis, is an exotic, invasive insect in North America that attacks and kills healthy ash trees. All ashes in the genus Fraxinus are attacked, including green, white, blue, and black ash. Its native range includes China, Korea, Japan, Mongolia, the Russian Far East, and Taiwan. It was first identified in the Detroit, Michigan, area in 2002 and was first found in Illinois in 2006. Ash trees are important in our residential landscapes, towns, cities, and forests in Illinois. In many communities, ash trees comprise 10 to 20 percent of the trees.
Adult beetles are 1/3 to 1/2 inch long and elongate, with metallic emerald green wing covers on a bronze body. The upper surface of the abdomen is bright red, which is obvious when they fly. They emerge primarily in late spring through 1/8-inch-wide, D-shaped holes in the bark of ashes. Adult beetles are present through June and July. After mating, the female inserts her eggs, one or two at a time, between bark flakes.
The eggs hatch into larvae that tunnel through the bark into the cambium, where the water-, nutrient-, and sugar-conducting tissues, the xylem and phloem, are located. The larvae are white, elongate, and flattened, growing to about 1-1/2 inches long. The larval body appears as flattened beads, and there are two short, dark brown to black spine-like cerci at the posterior end of the body. After feeding for one or two growing seasons, the larvae tunnel as much as 1/2 inch into the sapwood below the cambium to pupate. Adult beetles emerge the following spring.
The larvae create slender, winding tunnels under the bark. As the tunnels become numerous, they effectively girdle the branch, causing the branch to die due to lack of water and nutrients. Emerald ash borer attacks at the top of the tree first, causing thinning of the canopy. Attacked trees try to compensate for reduced foliage with numerous epicormic branches near the base of scaffold branches. Attack continues down the tree, resulting in the gradual death of branches, and the entire tree eventually dies. Larvae commonly attack the tree for about 4 years before branch dieback becomes evident. Once dieback starts to occur, the tree usually dies in 2 to 3 years. The bark on attacked trees separates from the tree trunk, allowing the larval tunnels to be easily seen. Once the tree dies to the ground, suckers form around the base of the trunk, but they do not grow into strong, attractive trees. These suckers are also attacked and killed by the emerald ash borer. Woodpeckers chip away the bark to reach the larvae, appearing as light areas on the bark, and is a useful means of identifying potentially attacked trees before dieback becomes evident. (Phil Nixon)