Are your elm tree leaves turning brown and dying? An initial reaction might be to water the tree more because we are in a drought and other trees are also loosing leaves. That is unless you are familiar with the elm leaf beetle. If you observe that the brown leaves on an elm look different from other tree species’ brown leaves and have a “lacey” appearance, then additional watering will not solve the problem.
Adult elm leaf beetles hibernate over winter in wood piles, crevices and leaf litter; emerge in the spring and fly to elm foliage to feed. They lay yellowish eggs on the undersides of leaves in double rows. The eggs hatch and
the larvae go through three growth stages over several weeks during which time they feed on and skeletonize the leaves. This is what causes the leaves to turn brown and take on the “lacey” appearance. Once the larvae mature, they crawl down the tree trunk and pupate at the base of the tree. Roughly ten days later, adult beetles emerge fly to the foliage and begin the cycle again. Adult beetles chew through the leaves and this damage appears similar to shot-hole damage. Multiple generations can occur in one year. In the fall, adults beetles begin looking for places to overwinter and can enter homes through cracks and crevices.
When dealing with elm leaf beetles, it is best to use an integrated pest management (IPM) approach. IPM incorporates a variety of techniques to keep populations of elm leaf beetle to a tolerable level. These techniques involve good cultural practices, conservation of natural enemies, regular monitoring and less-persistent chemical usage. University of Nevada Cooperative Extension and University of California fact sheets are very good references detailing IPM techniques for controlling Elm leaf beetles. Both references incorporate preventive guidelines and cultural, biological, botanical and chemical controls as an integrated approach to dealing with this pest. When choosing a biological control, specifically Bt (Bacillus thuringiensis), be aware that there are varieties of Bt and each is effective against certain insects.
Once a tree is infested with elm leaf beetles, take action to maintain the vigor of the elm tree and reduce other stressing factors. Ensure that the tree is getting sufficient water, don’t compact or disturb the soil underneath the tree and prevent trunk and limb injuries. A healthy elm tree can survive even heavy defoliation if it occurs in late summer and not repeatedly every year.