Elm leaf beetle, Xanthogaleruca luteola
Elm leaf beetle is a pest of elm trees and a household nuisance when adults seek shelter during winter months. Larvae feed on the underside of leaves and are historically common in Colorado communities where elm trees are abundant. However, reports of infestations have significantly declined since the mid-1990s.
Adults are 5-6.5 mm (3/16-1/4 inch) in length, are yellow-green and have black stripes when actively feeding during warmer weather. The beetles are khaki green while overwintering in a semi-dormant state.
The eggs are yellow and laid in masses of one to two dozen on the underside of leaves but change to grayish near hatching. Upon hatching, newly emerged larvae are small, black, and tend to feed near the egg laying site. As they grow, larvae appear yellow or green with rows of dark tubercles (projections) and will disperse to other locations on the tree. When fully grown, larvae are about 8.5 mm (1/3 inch) long. Pupae are orange or bright yellow.
- Elm leaf beetle is a pest of elm trees during warmer months and a nuisance in homes during cooler months. Feeding by larvae and adults can result in complete defoliation of elms. While adults can invade homes during cooler months, they are only considered a nuisance pest.
- Infestations of elm leaf beetle have declined in most of Colorado. Recent outbreaks tend to occur in the Arkansas River Valley and occasionally in eastern Colorado.
- Healthy elms can tolerate some feeding by elm leaf beetle. When trees are repeatedly defoliated, or when many adults are found laying eggs, applications with certain insecticides can provide control.
Adult elm leaf beetle. Note the yellow body with black stripes, which is the coloration of adults during periods of warm weather. When overwintering, the beetles are dark green. Both larvae and adults feed on leaves and can injure elms, and adults can invade homes when they seek shelter during winter. Image credit: Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Overwintering elm leaf beetles. Note the dark green coloration. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Larvae of elm leaf beetle. Note the yellow bodies and black spots. Image credit: Phil Sloderbeck, Kansas State University, Bugwood.org
Pupae of elm leaf beetle. Note the yellow-orange coloration. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Egg mass of elm leaf beetle. Note the yellow coloration. Eggs can also be gray when they are close to hatching. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
All beetles have four life stages: egg, larva, pupa, and adult. In the fall, adults begin seeking overwintering shelter in areas such as woodpiles, loose mulch, and piled eaves. They can also shelter behind walls when entering buildings through cracks or other openings. While overwintering, the beetles do not feed or reproduce but are often active during warm days in the late winter.
When temperatures rise in the spring, surviving beetles emerge from overwintering sites and seek out elm trees to feed on new foliage. As the beetle feed, their color changes from green to yellow and females begin to lay masses of one or two dozen eggs on the underside of leaves. Larvae mature within three to four weeks and migrate to sheltered pupation sites, which includes the base of the tree or in bark cracks. Adults emerge from the puparium within two weeks and produce a second generation. The second generation of larvae feed on leaves from mid-July through September, and adults of the second generation will overwinter until the following spring.
Adults often chew shothole patterns in leaves. Larvae cause different feeding injuries depending on the growth stage. Young, newly emerged larvae produce pinhole wounds in leaf tissue, while mature larvae consume the entire leaf except for the main veins, which results in skeletonized leaves with a brown or white appearance. When abundant, this pest can completely defoliate large elms, and repeated defoliation can significantly reduce tree health.
Leaves can be inspected weekly beginning in spring for clusters of yellow or gray eggs and larvae.
Maintaining tree vigor through adequate irrigation is important, especially in areas with summer drought. Dead or dying branches should be removed, and unnecessary pruning should be avoided. When pruning, it is important to make cuts properly on young trees during late fall and winter. Some elms are resistant to attack by elm leaf beetle, including Accolade, Emerald Sunshine, Frontier, Prospector, and most Chinese elms except for Dynasty.
To prevent elm leaf beetles in the household, it is recommended to repair screens, door sweeps, and seal exterior cracks in June or July before beetles attempt to migrate indoors as temperatures drop.
Adequate watering and plant care practices are advised since healthy trees can tolerate some feeding injury. Any beetles discovered in the home can be vacuumed and discarded, which is most effective when beetles are active during warm periods and exposed on windows or walls.
Chemical control is warranted when regular outbreaks occur or when large numbers of eggs are found on leaves. There are three ways to apply insecticides for managing elm leaf beetles: 1) soil drench applications or soil injections of systemic insecticides, 2) foliar sprays applied directly to leaves, and 3) banding trunks with insecticides to kill larvae before they pupate near the base of the tree.
In the home, combining prevention with spot applications of insecticides near potential entry sites can be effective.
Utah State University (n.d.). Elm Leaf Beetle. Utah State University – Extension. Available https://extension.usu.edu/pests/schoolipm/structural-pest-id-guide/elm-leaf-beetle
Elm Leaf Beetle. 2014. Elm Leaf Beetle. University of California – Integrated Pest Management. Available https://ipm.ucanr.edu/PMG/PESTNOTES/pn7403.html