The elm leaf beetle, Pyrrhalta luteola (Muller), is a serious pest of elm trees. Wilson elms are resistant but the Chinese elms are especially susceptible to attach by the beetle. Urban trees are also more likely to be host to the elm leaf beetle than are elms in a more rural setting.
Damage by the elm leaf beetle is distinctive. Feeding by the larvae results in loss of leaf tissue in areas in between the leaf veins leaving a "skeletonized" pattern. Severely affected leaves often drop from the tree into a mass of brown leaf skeletons. By mid-summer the tree can be entirely defoliated. This will not kill the tree out-right but does leave the tree weak and susceptible to attack by other insects and diseases. However, repeated defoliations can eventually cause the death of the elm.
Elm leaf beetles overwinter as 1/4-inch adults that are yellowish to olive green with a black stripe along the outer edge of each wing cover. In the spring, about mid-May, adult beetles emerge, fly to nearby elms, mate and begin laying orange-yellow, spindle-shaped eggs on the undersides of leaves. Adults will also feed on the leaves, leaving small circular holes in them.
The 600 to 800 eggs, in groups of 5 to 25, hatch into small black grub-like larvae that feed on the underside of the leaves for 3 weeks until they are mature larvae. Full-grown larvae are about 1/2 inch in length and are dull yellow with two black stripes down the back. At the end of the feeding period, the larvae move to the base of the tree to pupate in crevices or cracks in the bark. Pupation lasts for 7 to14 days, after which adults emerge. There are one to two generations per year.
Hibernating adults often find their way into homes and garages. They do not cause any damage but can be a nuisance. Most can be kept out of homes by screening all openings.
