Elm leaf beetles, again

A few weeks ago you wrote about elm leaf beetles and said that they did not hurt the trees. Here in Ft. Sumner they are eating all the leaves from our trees. Doesn't that hurt the trees? Shouldn't I do something?

In most years the elm leaf beetle does damage which is more cosmetic than damaging to the long term health of the elms. That is because the beetle population is sufficient to cause considerable damage, but not enough to cause defoliation sufficient to permanently harm reasonably healthy trees. This year, in some parts of New Mexico, the elm leaf beetle populations are considerably greater than typical. Damage is more severe than usual and the health of the tree and type of elm are factors which must be considered.

Our common Siberian elms have proven themselves capable of tolerating considerable stress from the environment, insects, and diseases. Other elms, especially the American and English elms, are less weedy and are more likely to be damaged. Elms such as the lacebark and cedar elms have tougher leaves which are usually less damaged by the beetles.

The article you recall is one in which elm leaf beetles were causing a problem in a home. In that case pesticide treatment was not warranted. The trees were not as severely affected as you describe in your situation.

The question is whether or not you should spray. If your trees are Siberian elm, often called Chinese elm in New Mexico, and have been reasonably healthy and growing vigorously, you may choose not to spray. Even if the trees are totally defoliated, healthy trees will resume growth next year. If the trees are old and declining, severely infected with diseases, or had been damaged by previous winters' cold and summers' drought, you may choose to spray. If next winter is unusually severe, even healthy trees may suffer damage following complete defoliation by the beetles. Since the adult beetles can fly, your trees may be reinfested even though your treatment killed most of the larvae on your trees. Several applications may be necessary. However, we are nearing the end of their reproductive phase for this year, so treatment this year may provide little benefit.

If you choose to apply pesticides, you have several options. There are several insecticides labeled for use on ornamental trees to control leaf beetles. Many homeowners apply carbaryl insecticide though some prefer to use the San Diego strain of Bacillus thuringiensis (B.t. San Diego) which is specific for control of beetle larvae and has little other effect on the environment. Carbaryl is available in most places where you can buy garden chemicals. The B.t. San Diego may be more difficult to find; you may have to order from a mail-order supplier.

Because you have so many elm leaf beetles in the area, expect them to invade your home in the late summer and fall as they look for places to overwinter. To reduce the nuisance problems from these insects indoors, make sure weatherstripping around doors and windows is in good condition and seal or cover with screen door material all holes into the house, attic, or crawl space.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!