Garden Tips: Elm leaf beetles hit trees this year

Have you been wondering why so many local elm trees are looking so sick? It is because large numbers of elm leaf beetles have been dining on our elms this summer. Most elm owners do not notice elm leaf beetle damage until they are done feeding for the season. An adult beetle is about a quarter-inch long and olive-green, with two dark longitudinal stripes down its back. The larvae are yellowish green with black stripes and spots.

Elm leaf beetle adults and larvae feed on elm leaves. Adult beetles eat holes in the leaves and the larvae skeletonize them, leaving only the veins and the waxy top layer of the leaf behind. The leaves then turn brown. If the population is large enough, they can defoliate a large tree by the end of the summer.

Some species of elm are resistant to the elm leaf beetle. However, the Siberian elm (Ulmus pumila), the predominant elm found in many area landscapes, is not. Siberian elm is a fast-growing, large tree that reaches 50 to 70 feet high.

Dr. Michael Dirr, renowned tree and shrub expert, said the Siberian elm is "one of, if not, the world's worst trees" because it is so messy and has brittle wood. Siberian elms produce prolific amounts leaf, branch and seed litter, plus the tree is prone to limb breakage in wind and ice storms.

The defoliation caused by elm leaf beetles only adds to the long list of the Siberian elm's negative traits. Because they are such tall trees, insecticide applications to control the beetles must be performed by licensed applicators. This is a costly service but may be worth it if the tree is of high value to the owner and if repeatedly attacked.

A less costly pesticide application is an insecticide drench applied to the soil at the base of the tree. However, this must be applied in late winter or early spring (before knowing whether beetles will a problem) to be absorbed by the roots and moved systemically to the top of the tree. This movement can take four or more weeks and is dependent on water being applied to the soil regularly after putting down the drench.

Elm leaf beetle populations have a tendency to fluctuate from year to year. In fact, it has been a number of years since we have experienced a severe elm leaf beetle outbreak in this area. Many insect populations tend to ebb and flow because of environmental conditions, the availability of food and natural enemies. University of California experts note that more overwintering adult elm leaf beetles tend to die if winter weather is relatively warm or wet.

Just because the beetles are causing damage this year does not mean they will be a problem next year.

At the end of the summer, adult elm leaf beetles look for protected places to overwinter. "Protected places" include wall voids of nearby homes. In the spring, they come out of hibernation and move back outdoors. However, some get lost and find themselves indoors. Vacuuming is the best method of control, along with caulking cracks and wall voids to prevent their entry into the house in late summer.

I am wondering if the elm leaf beetles will be plentiful again next year. Maybe not if we have a mild winter.

-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.

This story was originally published September 4, 2014 at 12:00 AM with the headline "Garden Tips: Elm leaf beetles hit trees this year ."