Pest description and crop damage Adult elm leaf beetles are approximately 0.25 inch long and yellow to greenish with two dark stripes. Older adults are darker. The 0.25 inch larvae are greenish or yellow with black lateral stripes and spots. Elm leaf beetles feed on the leaves both as larvae and as adults. They skeletonize leaves, feeding on the underside and eating all parts except the veins and upper cuticle, which turn brown. Adult feeding causes small holes in the leaves. Severe infestations may result in severe premature leaf loss. Trees may releaf in late summer. Repeated infestations may weaken or kill trees. Weakened trees are also more susceptible to attack by elm bark beetles, which can carry the Dutch elm disease fungus.
Biology and life history The insect overwinters as adults in protected places both indoors and outdoors. In spring, the adults fly to trees as the leaves are expanding and chew circular holes in them. Eggs are laid in clusters on the leaves, and the larvae appear in late spring (typically May-June). The larvae emerge and feed on the undersides of leaves. They feed for a period, then migrate to the lower parts of the tree and pupate on the ground or in crevices near the base of the tree. The second generation emerges 1 or 2 weeks later. There are two generations per year.
- Bacillus thuringiensis tenebrionus (Btt) (Novodor)
Plant resistant cultivars of American elm (Ulmus americana). These include 'Dynasty,' 'Princeton,' 'Prospector,' 'Frontier,' and 'Pathfinder.' (Note: Many American elm cultivars are highly susceptible to Dutch elm disease). Other elms reported to be somewhat resistant to elm leaf beetle feeding include Chinese elm (U. parvifolia) and the elm-like zelkovas (Zelkova spp.).
Provide proper culture to maintain plant health. Healthy plants are more tolerant of insect damage. Prune dead and dying branches in late fall or winter.
Chemical Control of Nursery Pests
Apply when first larval damage appears. Do not use acephate on American elm.
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p. 222).