At this time of year, the extension office receives a lot of calls about elm leaf beetles.
What follows are some of the basic practices that you can take to recognize, treat and manage elm leaf beetles. Just remember that you're aiming for moderate control. It's almost impossible to eradicate all the beetles.
Elm Leaf Beetle
Adults are olive green beetles with black, longitudinal stripes along the margin and center of the back. Females lay their yellowish to gray eggs in double rows of about 5 to 25 on the underside of leaves. Larvae are black when newly hatched. After feeding, they become a dull yellow or green with rows of tiny dark tubercles. Larvae develop through three stages called instars. Third instar larvae have dense rows of dark tubercles that resemble two black stripes down their sides, making them easy to distinguish from first and second instar larvae.
Adults commonly overwinter in bark crevices, litter, woodpiles, or in buildings. They fly to foliage in spring, and feed and lay yellowish eggs, which become grayish before hatching. After feeding in the canopy for several weeks, mature larvae crawl down the tree trunk, become curled, inactive pre-pupae, and then develop into yellowish pupae. After about 10 days, adult beetles emerge from pupae around the tree base and fly to the canopy to feed and (during spring and summer) lay eggs. Elm leaf beetle can have two to three generations per year in Eastern New Mexico.
Elm leaf beetle is a serious defoliator of elms. Larvae skeletonize the leaf surface, while adults chew entirely through the leaf, often in a shothole pattern. Defoliation eliminates summer shade, reduces the aesthetic value of trees, and causes annoying leaf drop. Repeated, extensive defoliation weakens elms, causing trees to decline.
Manage elm leaf beetle with an integrated program that incorporates good cultural practices, conservation of natural enemies, regular monitoring, the use of less toxic insecticides, or systemic insecticides. Recognize that beetle populations fluctuate from year to year and most trees do no require treatment every year. Healthy trees can withstand substantial defoliation. If Elm leaf beetle problems persist use a combination of methods. Remember, you’ll never get rid of all the beetles but aim for moderate control. Keep in mind that beetles fly from tree to tree, so management directed to a single tree or application will give less results in comparison with control to several trees or an area.
When using an insecticide bark band, foliar spray or trunk injection of a systemic insecticide, monitor elm trees to determine the need for treatments and when to apply them. Evaluate beetle populations during spring by inspecting foliage weekly for beetles starting in April. Watch for the appearance of yellowish eggs, which darken before hatching.
Methods for chemical control of elm leaf beetle include bark banding, soil applications, tree injection, or foliar spraying in spring after monitoring beetle abundance to determine treatment need. Avoid treatment unless necessary. Insecticide can have unintended effects, such as contaminating water or killing natural enemies and causing secondary pest out breaks.
Bark Banding – Bark banding is an inexpensive and environmentally sound technique that involves spraying a small area of the tree trunk with an insecticide. Use a hand pump sprayer or low pressure to spray a band of bark several feet wide around the first main branch crotch. Carbaryl (Sevin) is most commonly used and should be applied at the rate labeled for elm bark beetles (2% active ingredient). Pyrethroids also provide control. About one half gallon of dilute material is applied on each large tree. The insecticide kills the larvae when they crawl down to pupate around the tree base after feeding in the canopy. By reducing the number of elm leaf beetles that pupate and emerge as adults, bark banding reduces damage by later beetle generations, especially when done to all nearby elms. Inspect the foliage and spray when mature larvae are first observed, or for more accurate timing, by accumulating degree days and spraying the trunk band when about 700 degree days (above 51.8 degrees F) have accumulated from March 1.
Although it is not scientifically proven, relatively warm and wet winters are believed to reduce the likelihood that beetles will be a problem the following spring. Wet winters can increase overwintering mortality of beetles from insect pathogenic fungi. Warm winters may cause many hibernating beetles to starve to death be causes warmer weather increases the rate at which these insects consume their stored energy increasing the likelihood that beetles will become weakened or starve before elm leaves appear in spring.
Tom Dominguez is an agent with the Quay County Extension, NMSU Extension Service. He can be reached by calling 461-0562 or emailing firstname.lastname@example.org.