The emerald ash borer (EAB), Agrilus planipennis, is an invasive beetle that has caused widespread destruction of ash trees across North America. Originally from northeastern Asia, this wood-boring insect poses a serious threat to various ash species, leading to significant ecological and economic impacts. Identifying this pest and its signs is important for homeowners and land managers to protect trees and manage infestations. Early and accurate identification can help in implementing control measures.
The Emerald Ash Borer Beetle
The adult emerald ash borer is a small, slender beetle with metallic emerald-green coloration. It measures 3/8 to 1/2 inch (8.5 to 13.5 mm) long, is about 1/16 inch wide, and has a bullet-shaped body with a flattened back. When its wing covers are spread, a coppery red or purplish abdomen can be seen underneath. Adults emerge from infested trees from late May through September, with peak activity during warmer months.
EAB larvae cause the most significant damage to ash trees. These creamy white, legless larvae have a flattened, segmented body. As they grow, larvae can reach lengths of about 1 to 1.5 inches (26 to 32 mm). They tunnel beneath the bark, feeding on the tree’s phloem and outer sapwood.
Signs of Infestation on Ash Trees
D-shaped exit holes on the bark are a key sign of EAB infestation. These small holes, about 1/8 inch (3-4 mm) in diameter, are created by emerging adult beetles. The flat side of the “D” can be oriented in any direction. While these holes are a strong indicator, they may not appear on the lower trunk until the infestation is advanced.
Beneath the bark, EAB larvae create distinctive S-shaped feeding galleries as they tunnel and feed on the phloem tissue. These tunnels, about 1/8 inch wide, disrupt the tree’s ability to transport water and nutrients, leading to decline. The galleries are often packed with frass, a fine sawdust-like insect excrement.
Infested ash trees often exhibit canopy thinning and dieback, typically starting from the top and progressing downward. This occurs as the larval galleries damage the vascular system. Epicormic sprouting is another common symptom, where new shoots grow from the main trunk or branches, often at the base, as a stress response.
Increased woodpecker activity on an ash tree indicates EAB infestation. Woodpeckers feed on larvae beneath the bark, often causing noticeable damage to the outer bark, sometimes called “flecking” or “blonding.” Vertical splits or cracks in the bark can also form over larval galleries as the tree attempts to create callus tissue in response to internal damage.
Common Look-Alikes
Many insects are mistaken for the emerald ash borer due to similar metallic coloration or body shape. Native metallic wood-boring beetles, like the bronze birch borer or honeylocust borer, can appear similar. However, these typically infest different tree species or have subtle differences in their appearance, like a black rather than green body, or slightly different body proportions.
Other green insects sometimes confused with EAB include the six-spotted tiger beetle and cuckoo wasps. The six-spotted tiger beetle, while metallic green, is typically larger than EAB, measuring about 5/8 inch long, and has a more pinched-in waistline. Cuckoo wasps, also metallic blue-green, have clearly separated body regions and membranous wings, unlike the compact, bullet-shaped EAB. Japanese beetles, also metallic green with copper hues, are readily identified by the series of white dots along the side of their bodies and their broader, more oval shape.
Identifying Ash Trees
Identifying an ash tree (genus Fraxinus) is important, as the emerald ash borer exclusively attacks these trees. Ash trees are deciduous and recognized by their opposite branching pattern, where branches and buds grow directly across from each other on the stem. This is a distinguishing feature, as many other common trees have alternate branching.
Ash leaves are compound, with multiple leaflets attached to a central stem. Depending on the species, each leaf typically has between five and eleven leaflets, arranged in pairs with a single leaflet at the tip. The leaflets can be oval-shaped with smooth, serrated, or toothed edges.
The bark of mature ash trees often displays a distinctive diamond-shaped ridge pattern, which becomes more pronounced with age. Younger ash trees, however, may have smoother, grayish-brown bark. Another identifying feature is the fruit, known as samaras, which are single-winged, canoe-paddle shaped seeds that hang in clusters and typically mature in the fall.