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written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published – SEPTEMBER 5, 2014


Have you been wondering why so many local elm trees are looking so sick? It is because large numbers 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 1/4 inch long, olive-green, with two dark longitudinal stripes down its back. The larvae are yellowish green with black stripes and spots.

Both the adults and larvae of elm leaf beetles 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 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 resistant. Siberian elm is a fast growing, large tree that reaches a height of 50 to 70 ft.

Dr. Michael Dirr, renown tree and shrub expert, says that 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.

Add to long list of Siberian elm’s negative traits, the defoliation caused by elm leaf beetles. 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 attacked repeatedly.

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 if beetles will a problem or not) to be absorbed by the roots and moved systemically to the top of the tree. This movement can take four to six weeks or more and is dependent on water being applied to the soil regularly after application.

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 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.

Published: 9/5/2014 12:32 PM



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