WHAT IS IT?
The elm leaf beetle (Xanthogaleruca luteola) is an invasive species belonging to the family Chrysomelidae of the order Coleoptera. The elm leaf beetle is native from Europe to Central Asia, but was accidentally introduced to North America and Australia and it is now widespread.
The elm leaf beetle is a serious pest to many species of elm trees. The insect feeds on the leaves of the tree during multiple stages of its life cycle and has more than one life cycle per year.
The most obvious damage caused by elm leaf beetle is a skeletonizing of the leaves. Skeletonizing is when the leafy green material of a leaf is consumed leaving only the veins. Areas close to this feeding may die and turn brown. In extreme cases, an entire tree will be left defoliated by mid-summer. The damage caused by the elm leaf beetle will rarely kill the tree, however it will significantly weaken the tree and leave it more susceptible to other issues. One of these issues is the potential spread of elm bark beetle, which can spread Dutch Elm Disease.
Elm leaf beetles overwinter as adults and hibernate in places they can hide (even in houses). In the early spring, the beetles come out and lay their eggs on the underside of elm leaves. The first generation of eggs hatch within one week, and the young larvae feed on the underside of the leaves for the next two to three weeks. After this, the elm leaf beetles stop feeding and migrate down to lower sections of the tree to hide in deep crevices of the bark to pupate. The second generation of eggs hatch in mid-summer after a two- to three-week pupation period. This group starts to feed again on the underside of the leaves. However, the first generation of beetles may have caused enough leaf damage that the elm tree could have regenerated a new set of leaves, just in time for the second generation of beetles to feed on them. This takes a lot of energy from the tree, and repeated years of similar damage can be quite draining on the tree’s energy reserves. The second generation of beetles will feed until they sense that the length of daylight in a day has gone below 14 hours. This group then migrates down the tree to look for good places to hibernate for the upcoming winter.
The best treatment methods involve a soil drench with systemic insecticides like imidacloprid. Spraying is generally ineffective for two reasons: first, it is hard to spray very tall trees and control drift, and second, by the time you see the leaf damage, it is too late to spray.
As a Board Certified Master Arborist, Wayne White strives to stay current with the latest research information shared at National Pollinator conferences. Elm trees are wind pollinated, and are not visited by bees as they provide no nectar or digestible pollen. In the future, possible treatments may move to other bee-friendly insecticides, but at this time, there is no industry concern with elm trees and imidacloprid. Wayne will continue to monitor this situation closely.