Some elm trees may look like they are dying, having brown and crispy leaves. Despite this somewhat dramatic appearance, they are not. Elm leaf beetles and their caterpillar-like larvae are the culprits. Larvae eat the green tissue from between the veins of the leaves. A lacy-looking framework, called skeletonization, is left behind. With no green tissue left to continue to produce food for the plant through photosynthesis, the leaves turn brown. Adult beetles chew holes in the leaves.
Adult beetles are olive-green with black stripes running lengthwise from their heads to their abdomens. Newly hatched larvae are black, and mature larvae are dull yellow or greenish with dark stripes down their backs.
Elms are hardy, so gardeners could ignore the beetles and do nothing, and the trees would be fine. However, some folks don't like larvae and beetles dropping from their trees all summer.
Historically, spraying with chemicals throughout the season was the main method used to control elm leaf beetles. Now, a "systemic" chemical can be watered in at the base of the tree. A systemic chemical is one that moves throughout a tree. The active ingredient is imidachloprid, a neuro-active insecticide patented by Bayer Crop Science. It is sold as Merit, Gaucho, Admire, Advantage and other trade names. This product must be applied in late winter to early spring, before the beetles are active, to be effective. Bayer Tree and Shrub is the product that is most readily available to homeowners. It is applied to the ground around the trunk, with no aerial spraying. As with any pesticide, always read the label before you buy it, and read and follow all label directions when you apply it.
Bark banding is another elm leaf beetle management technique. This requires spraying a persistent insecticide on several feet of the trunk of the tree around the first main branch crotch. As larvae crawl down the trunk to pupate at the base of the tree, they come in contact with the chemical and die. Apply the insecticide weekly from May to July.
Summer-weight horticulture oils are another option. They smother the insects and the larvae when sprayed on foliage, branches and trunks.
A bacterial disease, Bacillus thuringiensis (Bt) variety tenebrionis or san diego, can also be applied to kill young larvae. Bt labeled for moth or butterfly caterpillars, or mosquito larvae, will not be effective against elm leaf beetles. The foliage must be thoroughly sprayed when the larvae are actively feeding, and several applications are required through the season.
For more information on gardening, contact me, (775) 887-2252 or skellyj@unce.unr.edu, or your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at www.unce.unr.edu. "Ask a Master Gardener" at mastergardeners@unce.unr.edu.
Brand names are used for illustration purposes only and do not constitute an endorsement by Cooperative Extension or the University of Nevada, Reno.
n JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension.
