JoAnne Skelly: More pesky bugs — elm leaf beetles

Last week I wrote about root weevils eating leaves. This week’s pests are the elm leaf beetles. While elms are hardy drought- tolerant trees for arid Nevada, they are plagued by elm leaf beetles every year. By this time of year, the trees look half-dead with white to brown ragged leaves.

The problem is the leaves have been skeletonized by the larvae of the beetles. These caterpillar-like creatures scrape off all the green tissue between the veins and leave behind tan-colored remains giving a crisp appearance to the leaves. Once the larvae become adults, they chew holes in the leaves. The larvae are black initially, changing to yellowish-green as they feed. The adults are olive-green with black stripes about one-quarter inch long. Females lay yellowish eggs on the underside of leaves. Before hatching, the eggs turn gray.

Although elm leaf beetle feeding ruins the look of a tree, a healthy tree usually survives attack. According to University of California at Davis (UC Davis), “Insecticide application is unlikely to be warranted if: beetles and damage were low during the late summer the previous year; or systemic insecticide was properly applied the previous growing season.”

However, people often can’t stand having so many beetles everywhere, dropping on them or onto food and into drinks when dining outside. Some people will cut elm trees down rather than deal with beetles year after year.

If you can tolerate these annoying pests and accept the poor appearance of the trees, the simplest course of action is to keep trees healthy with regular watering during dry periods. Also, protect trunks and roots from injury from string cutters, lawn mowers or construction activities. To avoid soil compaction that can damage roots, don’t park underneath the trees.

If you feel your trees cannot survive the extent of the beetle infestation, there are insecticides available to spray on leaves, to band the trunk or to use systemically. Insecticides include neem oil, narrow-range horticulture oils, pyrethrins, spinosad, carbaryl, dinotefuran and imidacloprid. These are active ingredients, not product names. For detailed information on insecticides, go to UC Davis’s website http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7403.html.

Before you buy an insecticide, read the label to see if it lists elm leaf beetle as one of the pests it controls. Read the label again before application and follow the directions carefully, particularly concerning safety equipment and required protective clothing.

JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.