Where there are elm trees, there are likely elm leaf beetles. Elm leaf beetles aren’t as popular as they once were, but nonetheless, they still cause damage. The elm leaf beetle lives throughout the United States but is especially problematic among elm trees in the southern part of Indiana. Elm leaf beetles cause harm to the trees in a number of ways: they chew through leaves, which reduces shade in the summer, compromises the tree’s appearance, and causes leaves to drop. Over time, this defoliation weakens the trees, contributing to their decline.
Elm Leaf Beetle Identification
Adult elm leaf beetles are about ¼ inches long with an elongated shape. They start out yellow and darken to an olive-green color as they get older. They also have a black stripe on the outside of the wing covers, but this becomes less noticeable as the beetles gets older. The thorax features four black dots.
The eggs of elm leaf beetles are bright yellow and in the shape of spindles. They are laid in groups of five to 25 on the bottom side of elm leaves. Brand-new larvae are slug-like and black in appearance. Once full grown, they are about half an inch long and light yellow with a black head. They have legs, hairs, and a two of black stripes on the back.
Elm Leaf Beetle Habitat
Elm leaf beetles reside—where else?—in elm trees. All elm trees are susceptible to hosting the critters, though the beetle prefers “Chinese” elm. Trees in urban areas typically have a greater infestation of elf leaf beetles than those in wooded areas. One kind of elm—Wilson—are safe. They are not subject to attack by the beetles.
Life Cycle of Elm Leaf Beetle
After wintering in protected environments, these insects fly to elm trees in the spring. They eat the new leaves, and deposit five to 25 yellow eggs on the bottom of leaves. With a week or so, small black larvae emerge and eat on the bottom of the leaves. Larvae grow through a few stages over the course of about three weeks. Mature larvae then make their way down the tree and pupate in the soil at the bottom. It takes about two weeks for adult beetles to emerge. Three or more generations can transpire within a year’s timeframe.
Elm Leaf Beetle and Humans
These beetles do not cause any harm to humans or animals, but because they feed on elm tree leaves, they cause damage and early defoliation to the leaves, which is unpleasant to the eye. Humans find elm leaf beetles a nuisance because of their musty smell and the significant number of them. The adult elm leaf beetle survives the winter months by dwelling in protected areas, including houses. Adult beetles become most objectionable when they enter homes in the fall and leave in the spring.
Elm Leaf Beetle Control
The best way to control the elm leaf beetle is to take action while the beetles are still on the tree. This can be done using sprays of insecticidal soap, Astro, Lawn and Garden Multi-Insect Killer, Orthene, Sevin, Azatin, or light horticultural oils.
In addition, the following insecticides are effective for control when sprayed on leaves:
permethrin (Astro, etc.)
bifenthrin (Talstar, etc.)
cyfluthrin (Tempo, etc.)
neem (Azatin, etc.)
For best results, one should treat the area when leaves are first out in the spring and once again in July.
A good way to keep beetles out of the house is by caulking or screening any openings, thresholds or door sweeps. It is possible to further increase control by adding a spot insecticide treatment to the sealing. Treatments of pyrethroid insecticides with bifenthrin, permethrin, and similar mixtures can be found at nurseries or other stores. Regular vacuuming is also helpful. Dead beetles can be vacuumed up to avoid a recurrence of infestation. Even live beetles that make their way inside can be vacuumed from under carpets or other areas. They can also be eliminated with short residual contact insecticides like pyrethrins.
Elm Leaf Beetle Entomology
Detailed Development and Reproductive Life Cycle
Around mid-May, the eggs of elm leaf beetles are laid in clusters along the leaves’ midribs near the bottom of the tree. This occurs in early summer. The larvae hatch about 10 to 14 days later. They then start skeletonizing host greenery. They molt three times and then mature and make their way down the tree to pupate. This usually occurs in the top layer of soil around the tree; sometimes they will situate themselves in bark crevices. Within 10 to 15 days, the adults surface, spread out, and mate. Once again they feed on the elm leaves and lay eggs for a second generation. The adults from this generation then hibernate. Damage of the tree is typically done by beetles from the first generation. The trees that have lost leaves may grow new ones during the same season, but of course the new leaves may be attacked by the beetles from the second generation.
Nest and Home Construction
When the elm leaf beetle isn’t making itself at home in an elm tree, it finds other comfortable shelter during the winter months. It will make a home for itself in homes, particularly attics; in buildings; under bark or other woodpiles; and in cracks. During these winter months, the insects basically go into hibernation. They are in suspended development; they don’t eat or reproduce. On warm days though, watch out, they may become active and move around within homes. While doing no harm, they may be a nuisance to homeowners.
Elm leaf beetles severely defoliate elms because of their chewing habits. Larvae reduce the leaves through skeletonizing, and adults further the damage by chewing all the way through the greenery, sometimes in a shot hole-like image. Leaves that are seriously damaged turn brown and sometimes drop too soon. Quite a few years of this defoliation could lead to dieback or even death for the tree.