The elm (Ulmus) makes an ideal shade tree in Sunset's Climate Zones 2a through 9 and 14 through 21. The elm complements any yard with its showy oval leaves that turn yellow in the fall. Unfortunately, the American elm (U. americana) and related varieties are most susceptible to disease caused by insects, and this should be considered before to planting the tree in your yard.
The woolly elm (Eriosoma amiercanum) and the elm leaf (Myzocallis ulmifolii) are the most common types of aphids that feed on elm trees, with the elm leaf aphid being the most destructive. They feed off plants by sucking the sap, and a great number of them feeding on one tree often leads to wilting or curling leaves and dieback of new growth. The insects secrete a sticky, waste material known as honeydew, which is not so much damaging as unsightly.
Elm Leaf Beetle
Elm leaf beetles (Xanthogleruca luteoa) feed on the leaves of elm trees, leaving irregular-shaped holes in the leaves. Their chewing effect causes the leaves to dry up and die. With repeated infestations, trees weaken and become susceptible to wind damage and dieback. These beetles are the most destructive insects for elm trees. Adult beetles are olive green with black stripes along the wings. Their eggs are a yellow-orange and hatch wormlike black grubs.
There are many species of leafhoppers. While some cause no harm, others cause damage to trees by feeding on their leaves and spreading plant diseases, like elm yellows. The potato leafhopper (Empoasca fabae) is one species that commonly lays eggs and feeds on elm leaves. An infestation causes browning, wilting and curling of leaves, often referred to as hopper burn. Leafhoppers vary in color and may be yellow, brown or pale green. They jump when disturbed. The pests destroy the elm by removing plant fluids and destroying chlorophyll.
Scales infest trees and feed on plant fluids through a strawlike mouth. They are named for the shell-like covering that hides their bodies. European elm scales (Gossyparia spuria) appear only on elm trees. They collect on the bark in the forks of trees and on lower limbs, causing branches to die back. Honeydew may also occur, leading to black, sooty mold. Scales do not look like insects. Instead, they have unrecognizable body parts, and the adult female European elm scale's body is surrounded by white fur. An infestation of these insects looks more like a fungus on the bark.
Keeping trees healthy by giving them adequate amounts of water helps protect them from insects. Environmental controls, such as introducing natural enemies like parasitic wasps, lacewings or lady beetles, can help control the pests, along with removing the insects you see. Spraying insecticides kills the pests but may also kill beneficial insects. For this reason, it is best to try environmental controls first.
- Learn2Grow: Ulmus "Morton" (Accolade)
- University of California Statewide Integrated Pest Management Program: Aphids
- University of California Statewide Integrated Pest Management Program: Elm Leaf Beetle
- USDA Natural Agriculture Library: Variation in Leafhopper Species Abundance and Diversity Among Elm Seedlings and Cultivars
- University of California Statewide Integrated Pest Management Program: Scales
- Hemera Technologies/PhotoObjects.net/Getty Images