Elm Leaf Beetle (Xanthogaleruca luteola)
The elm leaf beetle is often considered as much of an indoor nuisance as a garden pest since large numbers of adults migrate into homes seeking a protected place to overwinter. Outdoors, both adults and larvae feed on the emerging leaves of virtually all species of elm trees, leaving skeletonized foliage in their wake. Repeated severe infestations can markedly weaken a host tree, making it susceptible to other insects and diseases. The elm leaf beetle does not transmit the well-known Dutch elm disease, which is carried by another insect, the elm bark beetle.
Adults are about 1/4" long, oval-shaped, yellowish to olive green, with a black stripe along each wing cover and 3 to 4 dark spots on the segment right behind the head. Larvae are black or black and yellow and can reach up to 1/2" in length. Pupae are orange-yellow with black bristles. Yellow eggs are laid in spindle-shaped clusters of 5 to 25 on the undersides of leaves. 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 lookalikes.
In early spring, the adult will leave its protected overwintering site to lay eggs on the undersides of elm leaves. About one week later, the eggs will hatch to produce tiny, grub-like larvae that begin immediately to feed on the undersides of the foliage. This 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 weeks, or mid-summer, 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 600-800 eggs during her lifetime. 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.
During feeding, the larvae skeletonize the leaf, leaving the upper surface and veins intact. This gives the foliage a net-like appearance, and areas around the feeding site will dry up and die, causing the leaf to drop prematurely. Adults, on the other hand, will chew small, irregularly shaped holes in the expanding leaves. Trees that lose their leaves often develop a second set, only to have them consumed when the next generation is produced. Thus, the larval stage is the more destructive part of the elm leaf beetle’s life cycle, and the presence of many egg clusters can give an indication of the extent of the defoliation that will occur later on. Most trees will not be killed outright as a result of this destruction unless it is nearly complete and is repeated for 2-3 consecutive years. However, the larval feeding does weaken host trees and therefore makes them susceptible to diseases and attacks by other insects, most notably the elm bark beetle, which is the carrier of Dutch elm disease.
There are few natural enemies that prey on the elm leaf beetle, although predacious stink bugs and plant bugs have been known to feed on them during various stages of the life cycle.
Weather can also play an important part in the natural control of the beetle, as long winters or late spring freezes are apt to kill large numbers of the overwintering adults. Strong windstorms can help eliminate smaller larvae by blowing them off trees, especially in cases where they have been forced to feed on older, tougher leaves.
While there are chemical controls available, their usefulness is contingent upon an understanding of the elm leaf beetle’s life cycle. The most effective time for control is when the larvae or adults are still 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.
Alternatively, tree trunks banded with an insecticide can be useful if applied when the larvae start to crawl down the tree trunk searching for pupation sites. Usually this activity occurs in mid- to late-June. The bands should be at least 12 inches wide and applied to the trunk just below the junction of the lowest branches. Trunk banding has the advantage of being a more convenient method of insecticide application and also reduces the possibility of spray drift to nearby vegetation. However, since it is designed to prevent the pupation of a second generation, the tree banding system is ineffective at preventing the damage already done by the initial brood of elm leaf beetle larvae. Suppression of future generations and a reduction of the number of beetles that will overwinter is possible via this method.
To prevent the adult from gaining access to homes and other structures, caulk all exposed areas, paying particular attention to moldings and loose boards and shingles.
Refer to the Illinois Urban Pest Management Handbook (University of Illinois Cooperative Extension Service) for a complete listing of chemical recommendations. Use pesticides safely and wisely; read and follow label directions
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.