My friend sent me a photo of an insect to identify. The photo was of the common, prolific pests of elm trees, elm leaf beetles. If you have elm trees, most likely you have these beetles. They go together like ash trees and aphids.
Earlier in the summer, elm trees started looking dry and raggedy. That was the result of the larvae, the grub-like pre-adult stage of the actual beetles, which chew on the underside of leaves skeletonizing them. With no green tissue remaining, except for the leaf veins, the leaves turn lace-like and brown and fall off the trees.
While the damage is primarily aesthetic and although it diminishes the amount of shade available, most healthy trees survive. However, repeated defoliation can weaken a stressed tree causing eventual decline.
The adults chew ragged holes in leaves rather than rasping the tissue away as the larvae do. The adults are starting to look for overwintering sites from now into fall. They overwinter in bark crevices, woodpiles, loose mulch, eaves, attics and cracks in buildings. While they are really annoying when they end up in buildings in huge numbers, they don’t eat anything within a building.
Come spring, these critters are ready for action and move into elm trees as the leaves emerge. They lay masses of yellow eggs on the undersides of leaves a few weeks after they emerge.
During their first stage out of the egg, the larvae are tiny black things. They then grow into smallish “caterpillars” that seem to cover a tree’s leaves. In approximately four weeks from the egg stage, these insects pupate for about two weeks, generally at the base of the tree. Then, new adults emerge and produce a second generation, which is probably what we are seeing now.
Long winters and late spring freezes can reduce the numbers of overwintering beetles. Natural predators can’t control large numbers of beetles. Insecticides can help in managing these pests, but whether the insecticides are used as a systemic soil drench, which travels through the roots into the leaves killing feeding larvae or adults; a foliar spray that kills on direct contact; or as a trunk band that kills larvae as they climb down the tree to pupate, timing of application is important.
For detailed information on chemical application see https://extension.colostate.edu/topic-areas/insects/elm-leaf-beetles-5-521-2/.
For guidelines on how much damage can be tolerated and when to take action, go to: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7403.html.
JoAnne Skelly is associate professor and Extension educator emerita of University of Nevada Cooperative Extension.
