Until recently, our major horticulture concern has been controlling Japanese beetles. Now, unfortunately, more serious insects — emerald ash borers (EAB) — have entered the picture.

In early July, two adult EAB beetles were found on the southwest side of Bloomington in one of the purple survey traps recently installed by the Department of Agriculture. This catch represents the southernmost documented penetration of this pest in Illinois. The Department of Agriculture is now surveying ash trees in the capture area to determine how many trees might be infested. EAB are very destructive insects. Since 2002, more than 30 million ash trees have been killed.

If you have ash trees on your property, be on the alert for beetles with bright metallic emerald green wing covers on a bronze, somewhat bullet-shaped, body with a flattened back. When they fly, you can see the upper surface of their abdomen is bright reddish purple. Adult beetles are small, measuring only one-half inch long. They emerge from tree trunks and branches in late spring through one-eighth inch, distinctively “D” shaped exit holes. EAB beetles are typically present May through July.

Adult beetles feed on ash foliage, but cause little damage. Their larvae, however, are very destructive. Female beetles lay their eggs in the bark of ash trees. The eggs hatch into larvae that tunnel into important parts of the tree containing tissues which transport water and nutrients. The legless, one and one-half inch long larvae are white with flattened, bell-shaped segments. After feeding for one or two growing seasons, the larvae tunnel further into ash trees to pupate. The larvae create thin, serpentine tunnels. When numerous, the tunnels can girdle a branch causing it to die from lack of water and nutrients.

EAB normally attack the top of ash trees first. They then gradually move down the tree until the entire tree dies. Trees have usually been infested for two years or more before dieback is evident. Once dieback begins, the tree typically dies in two or three years. Bark on infested trees frequently separates from the trunk, revealing the larvae tunnels. Since woodpeckers like to feed on EAB larva, heavy woodpecker damage may be a sign of an infestation. Suckers may also form around the base of the trunk. Removal is the only certain control for EAB, and infested trees are usually removed immediately. Some treatments have had limited success. Treatments are usually recommended for trees within 15 miles of documented infestations.

Homeowners wishing to take preventative action can spray the trunk and foliage of ash trees with Bayer Advanced Multi-Insect Killer (cyfluthrin) in June and July. Sprays are primarily aimed at controlling adults and reducing the spread of EAB rather than protecting attacked trees. Imidacloprid (Bayer Advanced Tree and Shrub Insect Control) has shown effectiveness in controlling EAB enough to prevent tree death for a few years. It can be applied once a year in April or May as a soil drench within two feet of the trunk of small trees. Since imidacloprid takes longer to move through large trees, it should be applied around them in late summer or early fall. Systemic products, such as imidacloprid, reportedly offer more useful control. Always follow product label directions carefully. Some controls can only be applied by licensed arborists or landscapers. One of these is a new product called emamectin benzoate, sold as Tree-age, which is injected into tree trunks.

Since recommended controls are usually applied annually for at least two years, especially on larger trees, some thought needs to be given to the value of the tree being treated in comparison to the cost of treatment and/or removal and replacement. Treatments are more effective on trees less than 10 inches in diameter and healthy trees, so it is important to fertilize your ash trees in the fall or spring and water them regularly.

Phil Nixon, a University of Illinois entomologist, advises there are several insects that can often be mistaken for the EAB. If you find an insect you believe to be the EAB, call the University of Illinois Extension Office (309) 663-8306 or the Illinois Department of Agriculture (800) 641-3934. To learn more about EAB, visit www.illinoiseab.com.
Smith is a University of Illinois Extension Master Gardener in McLean County. For horticulture questions or information about the Master Gardener program, please call (309) 663-8306 or visit www.mcleanextension.org.