A new exotic beetle from Asia was discovered feeding on ash (Fraxinus spp.) trees in southeastern Michigan. It was identified in July 2002 as Agrilus planipennis Fairmaire (Coleoptera:Buprestidae). Larvae feed in the phloem and outer sapwood, producing galleries that eventually girdle and kill branches and entire trees. Evidence suggests that A. planipennis has been established in Michigan for at least five years. Surveys to determine the extent of the infested area are underway.
Adults are larger and a brighter green than any of the native North American species of Agrilus. The slender, elongate adults are 7.5 to 13.5 mm long, and females are larger than males. The adult body is brassy or golden green overall, with darker, metallic, emerald green wing covers, or elytra. The top of the abdomen under the elytra is metallic coppery red (seen only when the wings are spread). The prothorax, to which the first pair of legs is attached, is slightly wider than the head but the same width as the base of the elytra.
The Emerald Ash Borer appears to have a one-year life cycle in southern Michigan but could require two years to complete a generation in colder regions. Adult emergence begins in mid- to late-May, peaks in early- to mid-June and continues into late June. The adults are active during the day, particularly when conditions are warm and sunny. Most beetles remain in protected locations in bark crevices or on foliage during rain, heavy cloud cover, high winds or temperatures above 90°F. Chinese literature indicates that beetles usually fly within 2 meters of the ground. The likelihood of long distance flights is not known. Adults, which are present into August, feed on up to 0.45 cm2 of foliage per day, leaving irregularly-shaped patches of leaf tissue with jagged edges.
Infestations of emerald ash borer can be difficult to detect until canopy dieback begins. Evidence of infestation includes D-shaped exit holes on branches and the trunk. Callus tissue produced by the tree in response to larval feeding may cause vertical splits 5 to 10 cm in length to occur in the bark above the gallery. Infested branches in the canopy die when they are girdled by the serpentine tunnels excavated by feeding larvae. Many trees appear to lose about 30 to 50 percent of the canopy in one year and the tree is often killed after 2 to 3 years of infestation.
Stress likely contributes to vulnerability of ash trees and recent summer droughts may have contributed to high emerald ash borer populations in southeastern Michigan. However, emerald ash borer attacked and killed apparently vigorous trees in woodlots and urban trees under regular irrigation and fertilization regimes.