Elm leaf beetle adult and damage. William M. Ciesla, Forest Health Management International, Bugwood.org
Elm leaf beetle larvae. Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org
Skeletonized leaves on an elm (Ulmus) caused by feeding of elm leaf beetles (Coleoptera). Note interveinal feeding pattern. J. Solomon, USFS, Bugwood.org
Eggs and newly hatched black larvae of elm leaf beetles on elm (Ulmlus). G. Csoka, Hungary Forest Research Institute, Bugwood.org
Overview
Introduced from Europe in 1938, the elm leaf beetle is the most serious elm defoliator in the United States. Although the damage does not usually kill the tree, it does make it more susceptible to disease and other pests, especially the elm bark beetle, the vector for Dutch elm disease. Elm leaf beetles may enter homes and other buildings looking for an overwintering site.
Symptoms and Diagnosis
All species of elm (Ulmus sp.) are susceptible to the elm leaf beetle. Adult beetles feed on elm leaves by chewing small round holes which may create a shothole effect. The larvae, the beetle’s most destructive stage, consume the leaf tissue from the underside, skeletonizing the leaf and leaving the upper tissue intact. Damaged leaves turn brown, dry up, and fall, resulting in partial or complete defoliation of the affected tree. Repeated defoliation will cause the tree’s health to decline. Beetles overwintering in homes do not cause damage but if present in large numbers, may become a nuisance.
Life Cycle
Adult elm leaf beetles are ¼ inch long and are yellow to olive-green with a black stripe on each side and 4 black spots near the head. The adults overwinter in protected locations such as under the bark of trees, in woodpiles, and in buildings. When elm trees begin to leaf out in spring, the adults emerge and the females lay orange spindle-shaped eggs on the underside of elm leaves. The larvae hatch in about a week, feed on the elm leaves for 2 to 3 weeks, pupate, and emerge as adults in about 10 days. Newly hatched larvae are black; full-grown larvae are dull yellow with two black stripes and are about ½ inch long. The pupae are ¼ inch long, orange to bright yellow, and are found at the base of elm trees. In Missouri, there are three to four generations per year.
Integrated Pest Management Strategies
1. Do nothing, especially if the tree was not affected the previous year. Healthy elm trees can tolerate some defoliation, especially if it occurs near the end of the growing season. Adult beetles can fly from untreated trees so controls may only be effective if all elm trees in the area are treated.
2. Practice good cultural techniques to keep plants healthy and free of drought, and nutritional or injury-induced stress.
3. Close openings where elm leaf beetles can enter buildings. Dispose of beetles in the home by vacuuming them or spraying with a pyrethrin-based insecticide.
4. Spray trees with an organic insecticide such as insecticidal soap, horticultural oil, Neem, Bacillus thuringiensis or pyrethrin. For best results, spray when the elm leaves are ¾ grown and again three weeks later, being sure to get good coverage on the underside of the leaf. Large trees may require a certified arborist.
5. Use conventional insecticides such as carbaryl (Sevin) or imidacloprid (Merit). Follow the package instructions and observe all safety procedures. If timed when larvae are climbing down the tree to pupate, trunk banding may be as effective as spraying the entire tree. Large trees may require a certified arborist.
6. Plant resistant trees. Japanese zelkova (Zelkova serrata) is in the same family as elms and looks similar but it is not susceptible to the elm leaf beetle.
Organic Strategies
Strategies 1 - 4 and 6 are strictly organic approaches. In Strategy 5, trunk banding is an organic approach.