Ugly elm trees are not dying
September 19, 2007
Elm trees around the area look burned, with brown crispy leaves. Although they look like they’re dying, they’re not.
These Siberian elms are hardy trees, suited for this desert environment. Unfortunately, they are prone to elm leaf beetles, which eat the elm leaves and make them look bad.
The larvae of this beetle – when it looks like a caterpillar – has rasping mouth parts that it uses to scrape off the green tissue between the veins, a process called skeletonizing. This gives the leaves an appearance of brown lace, which, from a distance, makes the tree look like it’s either dying or dead. The adult beetle chews holes in the leaves, which is actually a less obvious problem.
Carson City certified arborist Molly Sinnott called me to say she is getting requests from people to cut down their elm trees, because they think the trees are dying. A hardy, drought-tolerant, full-grown shade tree is a terrible thing to lose in this desert environment, particularly when the trees are actually healthy, even though they don’t look it. On the other hand, having beetles or larvae drop on your head and looking at sick-looking trees isn’t much fun either.
What can be done? An application of a nonspecific insecticide by a certified pesticide applicator in the spring can reduce larval damage. I recommend hiring a professional applicator because the spraying has to reach throughout the tree. Timing is critical to the success of this approach. A problem with this method is that the chemical used can also kill beneficial insects, including bees.
Another option is to use Bacillus thuringiensis (Bt), variety tenebrionis or San Diego. This bacteria is a biological control specific to killing beetle larvae, but doesn’t kill bees and other beneficial insects.
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However, complete foliage coverage is required and Bt only lasts a few days, so several applications will be needed. In addition, be sure to use a Bt product labeled for control of elm leaf beetle, as Bt products labeled for use against moth and butterfly larvae are not effective.
The option I prefer is to use a systemic containing the active ingredient imidochloprid. I apply it in a watering can around the base of the tree, following label directions. Product manufacturers recommend applying this in the fall, so the tree absorbs it and moves it through the entire plant before the larvae attack in the spring. It’s easy and target-specific. Although the product is not cheap, one November application lasts for one year.
Whatever control method you choose, don’t cut down your elm trees for their poor appearance. Select a care option that reduces the beetle populations, and occasionally water and fertilize your trees. Then, sit back next summer and enjoy the shade without the crispy look.
For more information, e-mail email@example.com or call me at 887-2252. You can “Ask a Master Gardener” by e-mailing firstname.lastname@example.org or call your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at http://www.unce.unr.edu.
• JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension.