Elm leaf beetle feeds as both a larva and an adult. During feeding, the larvae skeletonize the leaf, leaving the upper surface and veins intact. This gives the foliage a net-like appearance. Adults, on the other hand, will chew small, irregularly shaped holes in the expanding leaves. When high populations of the beetle are present, trees may lose their leaves prematurely. They often develop a second set, only to have them consumed when the next generation is produced. Most trees will not be killed outright as a result of this damage, unless it is nearly complete and is repeated for two to three consecutive years. The larval feeding, however, does weaken host trees and therefore makes them susceptible to diseases and attacks by other insects.

The elm leaf beetle overwinters as adults in protected sites. In early spring, the adults will emerge from these protected sites. Adults are about ¼ inch long, oval-shaped, yellowish to olive green, with a black stripe along each wing cover and three to four dark spots on the segment right behind the head. Often confused with other beetles, such as the western corn rootworm and the striped cucumber beetle, the elm leaf beetle can be positively identified by the distinctive black spots on the body segment behind the head, and by their emergence earlier in the season than their look-alikes. The adults will feed on new leaves and lay eggs. The yellow, spindle-shaped eggs are laid in clusters of 5 to 25 on the undersides of leaves.

About one week later, the eggs will hatch to produce tiny larvae that begin immediately to feed on the undersides of the foliage. These larvae are black or black and yellow and can reach up to ½ inch in length. Their feeding can last two to three weeks, at which time the larvae will migrate to the lower parts of the elm tree and pupate in cracks and crevices in the bark, or drop to the ground and pupate at the base of the tree. In about two more weeks the adults will emerge and return to the foliage of the same or adjacent elms to produce a second generation, and the cycle of leaf-feeding begins again. One female may produce as many as 600 to 800 eggs over the course of the season. As the days of late summer begin to shorten to less than 14 hours, egg production will stop and the adult will feed for a brief period before leaving the tree in search of its winter habitat

Chemical:

While there are pesticides available, their usefulness is contingent upon an understanding of the elm leaf beetle’s life cycle. When using insecticide sprays, the most effective time for control is when the larvae or adults are on the tree. Unfortunately, it is often only after extensive damage has been done that the homeowner is aware of the elm leaf beetle’s presence, at which point any insecticide application will be of little benefit. If spray application is attempted, a thorough coverage of the foliage, especially the undersides, is critical. Such coverage may be difficult to achieve, especially on tall trees, and special care should be taken to avoid application on windy days when spray may drift to other ornamental plants or food crops nearby.

Systemic insecticides can also be used to manage the elm leaf beetle. Since these products can be slow to move through the tree and reach the leaves on which the beetle is feeding, they need to be applied early in the season before extensive feeding begins.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).