Now is the time to be on the lookout, especially in southern Illinois, for the elm leaf beetle, Xanthogalerula luteola (say this five times fast!). This pest was introduced into the United States from Europe. All species of elms are susceptible to attack; however, elm leaf beetle tends to prefer Siberian and American elm, with Chinese elm less susceptible.
Adults are approximately 1/4 inch long, slender, and yellowish green, with black stripes running down the elytra (wingcovers). Adults eat small holes in leaves. The females lay yelloworange eggs in clusters on the undersides of leaves from April through May. A female may lay between 600 to 800 eggs in her lifetime. The eggs hatch in 5 to 6 days into green larvae that resemble grubs. The larvae are approximately 3/8 to 1/2 inch long. They are initially black and then develop two black lateral stripes along their sides. Their feeding causes leaves to appear skeletonized because they scrape the leaf tissue from the upper surface or cuticle with their chewing mouthparts, leaving the veins intact. The tissue between the veins eventually turns brown. Larval feeding lasts about 3 weeks, and this is the major source of plant damage. Trees will attempt to put out another flush of new growth, which may be consumed by elm leaf beetles or other insects. In addition, successive years of feeding may kill entire branches or compromise tree health, thus increasing tree susceptibility to other opportunistic insects.
The last larval instar crawls down tree trunks, where they pupate at the bases of trees in and on the ground. They may also pupate in the cracks and crevices of the trunk or in large branches. In 2 weeks, adults emerge and start feeding on plant leaves. They normally fly to the same tree that larvae had fed upon. Adults may be nuisance pests in late summer and early fall when they leave trees and enter homes to overwinter. They will also overwinter in protected places outdoors. There are two generations per year in Illinois.
Pest-control materials may be applied in late May or when Vanhoutte spirea has finished blooming. Applications may be needed again in late July to early August. Pest-control materials recommended for managing elm leaf beetle include acephate (Orthene), carbaryl (Sevin), imidacloprid (Merit), lambda-cyhalothrin (Scimitar), and spinosad (Conserve). Do not use acephate on American elms as this may cause plant injury. Elm leaf beetle is susceptible to a variety of natural enemies. Two parasitoids, Tetrastichus brevistigma and T. gallerueae, attack the eggs and pupae. If present in sufficient numbers, they may provide acceptable control of elm leaf beetle.