   Elm Leaf Beetles - April 6, 2005 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Elm leaf beetles (Xanthogaleruca luteola) feed on elm tree foliage across most of Arizona. Adult beetles are olive green with black longitudinal stripes along the margin and center of the back. Curiously, I usually receive more calls about them when they become a nuisance after entering homes and buildings through torn screens, under doors, and other small openings with the onset of cool weather in the fall. The damage they inflict is substantial. However, they often go unnoticed because the primary elm species in our area, Siberian elm (Ulmus pumila), is a weedy species and is seldom planted. Hence, we tend not to pay much attention to them. Female elm leaf beetles lay their yellowish to gray eggs in double rows of about 5 to 25 on the underside of leaves. The eggs hatch into small black larvae that resemble caterpillars. These larvae feed on the elm leaves and go through three stages (called instars). As the larvae feed, they skeletonize the leaf leaving behind the tougher vascular tissues and turning it brownish gray. After feeding in the canopy for several weeks, mature larvae crawl down the tree trunk to the ground, become curled, inactive prepupae and develop into yellowish pupae. After about 10 days, adult beetles emerge from pupae around the tree base and fly to the canopy to feed, mate, and lay more eggs during spring and summer. Adult feeding creates small holes that go completely through the leaf rather than skeletonizing it. In central Arizona, elm leaf beetle can have three generations per growing season. As cold weather approaches, adults seek sheltered locations in bark crevices, litter, woodpiles, or in buildings. The following spring, the cycle starts over again. Aside from reducing the amount of shade under the elm tree, the physiological effect on the tree is to reduce leaf area. This consequently reduces the amount of light energy that can be collected and converted into sugar that can be used for growth and storage reserves. Over the course of several years, repeated defoliation causes an overall decline in tree vigor that may result in dead limbs and poor growth. Elm leaf beetles rarely kill trees. Our summer rains also tend to partially rejuvenate affected elm trees. While there are some natural enemies of elm leaf beetles, they are not highly effective at reducing their populations. Even so, pesticide applications can be in such a way as to preserve natural enemies while effectively controlling elm leaf beetles. One method is to apply carbaryl (Sevin) to the trunk so that mature larvae crawling down the trunk to pupate travel through the sprayed area. The application should focus on the area around the first main branch crotch and the rate should be the same as for elm bark beetles (a 2% solution). The insecticide kills the larvae when they crawl down to pupate around the tree base after feeding in the canopy. I feel this is the most appropriate pesticide for the homeowner to employ due to the simple equipment needed to apply the material. Systemic insecticides are also effective at managing elm leaf beetle populations. These products include acephate (Orthene) and imidacloprid (Bayer Advanced Tree & Shrub Insect Control, Imicide, Merit). Some formulations of these materials can be sprayed onto the tree foliage, but soil applications minimize environmental contamination and may be more effective than foliar sprays. Soil-applied systemic products should be applied in early spring just before leaves emerge and irrigated if rainfall is insufficient to allow uptake by the tree. A major disadvantage of this method is that you are applying a pesticide before you see evidence of the pest. For this reason, systemic insecticides should only be used where elm leaf beetles have been a continuous and significant problem. Azadirachtin (Azatin), a Neem product, is also effective when applied on first and second instars as a foliar spray. However, this type of application requires a large spray rig and likely will need to be done by a professional. Siberian elm is one of my least favored trees, but I know it is often the only shade tree that grows in drier areas. I hope you think carefully before applying pesticides to control elm leaf beetles. Tolerate some level of activity and only apply them after they have caused significant damage. Naming of companies or products is neither meant to imply endorsement by the author nor criticism of similar companies or products not mentioned. Always read product labels and MSDS. The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |