JoAnne Skelly: Management of pesky Elm Leaf Beetles
November 1, 2018
We cut down a large Siberian elm tree yesterday. Although these trees are renowned for elm leaf beetle infestations, in the early years of its life, it didn't have elm leaf beetles. However, as it aged, the beetles and their larvae became prevalent. The larvae skeletonize leaf tissue until only a lacy brown residue remains, giving the tree a scorched look by the end of summer. The adults chew irregularly round holes in leaves.
Elm leaf beetles are mainly annoying because of their high numbers, particularly if they get inside a home to overwinter. I once went on a visit to house in Carson where the upstairs was infested with beetles. Elm leaf beetles start out as eggs, change into larvae, then pupae and finally become adults. It's the adults that overwinter in bark crevices, woodpiles or buildings. In the spring females become active and lay eggs on leaves. A female can lay 400 to 800 eggs through spring and summer. Each egg hatches in about a week becoming a caterpillar-looking larva.
Although the damage looks terrible, elm trees are usually strong enough to survive defoliation, especially if it occurs late in the season. However, if you're worried about your elm trees' health, irrigate them during the summer. Vigorous trees can withstand an attack better than drought-stressed trees. Elm leaf beetles do not transmit Dutch elm disease.
As far as management strategies go, tolerating the damage is a suitable course of action in most situations. Natural predators include some true bugs, earwigs, lacewing larvae and predaceous ground beetles. Long winters or a late spring freeze may kill large populations of overwintering beetles. Biological controls such as Bacillus thuringiensis (Bt), a bacterium, and botanical insecticides made from neem tree seeds are options. To maximize benefits from biological controls, minimize the application of long-lasting insecticides. Chemical controls include bark banding in the spring, which involves spraying an insecticide near the base of the tree up to the first set of branches. Larvae are killed when they crawl down the tree to pupate at the base of the tree. This technique kills fewer natural enemies than spraying the entire tree. Systemic insecticides that are absorbed through leaves can be sprayed on the tree, killing the insects when they feed. Some systemics are applied to the soil as drenches. Broad-spectrum insecticides are the least desirable because of their damage to natural enemies.
For information see Cooperative Extension's publication "Elm Leaf Beetle" at http://www.unce.unr.edu/publications/files/nr/2004/FS0474.pdf. Or UC Davis' publication at http://ipm.ucanr.edu/PMG/PESTNOTES/pn7403.html.
JoAnne Skelly is associate professor and Extension educator emerita at University of Nevada Cooperative Extension. Email email@example.com.