The Latest Dirt - September 2024

Elm Leaf Beetle Causes Considerable Damage

Client’s question
Hello, I was given a business card and told to contact you. I have attached photos of a bug that’s killing the leaves on my tree. What is it?

Susan Heckly’s response
Thank you for contacting the UC Master Gardener Program Help Desk. And thank you for sending the photo of the insect on your tree—it makes it easier to make recommendations with a positive identification of the problem.

The insect in the photo is a larva of the elm leaf beetle. These are leaf-chewing pests of elm trees. Adult beetles chew through the leaf, often leaving a shothole pattern behind, while the larvae skeletonize the leaf surface, which causes leaves to turn brown.

Adults overwinter in bark cracks, woodpiles, or elsewhere on your property. In the spring, they fly up into the tree and start the damage cycle. Larvae hatch from eggs laid in the tree, do their damage, then crawl down the tree trunk to pupate.

Managing elm leaf beetles requires an integrated program that includes good cultural care, conservation of natural enemies, regular monitoring, and possibly insecticides and bark banding by a licensed tree care company.

Cultural care: Elm trees are native to areas with summer rainfall, so you will need to give them adequate irrigation during our dry months (see paragraph about irrigation below). Pruning should be done just to remove dead branches and avoid unnecessary pruning.

Preserving natural predators: Predators of elm leaf beetles include earwigs, lacewing larvae, some ground beetles, and several others. To preserve these in your landscape, minimize pesticide application on surrounding areas and use less-persistent products. Short-lived contact sprays of oils such as Neem, horticultural, or Spinosad can work, but only if the tree can be thoroughly sprayed. Pesticides can be applied in a band on the trunk and will kill larvae when they crawl down the tree to pupate, thus reducing next year’s problem. While it can be effective, Bark banding may require application by a licensed pest control operator because the chemicals are not available for home use.

Systemic insecticides can also be used to control elm leaf beetles. These pesticides are applied to one area of the tree (bark, roots, etc.) and then moved by the tree to other parts of the tree (leaves). When injected into the tree or into the soil, these pesticides minimize environmental contamination. Because elms flower before leaves come out and are wind pollinated, the risk of harming beneficial pollinator insects is low when using a systemic insecticide on elms.

Irrigation: I don’t know how large your tree is (i.e., the width of branches), but it can take a lot of water appropriately applied to keep an elm healthy. Elms are listed as needing moderate water (in a range of very low to high). They do not tolerate dry conditions. To calculate how much water you should apply each month, go to this website: http://www.waterwonk.us/how-much/ where you can input your city, the size of the tree (how wide the branches spread tip-to-tip) and the month. It will calculate the amount of water needed for optimal care.

When irrigating the tree, the water should be applied under the tree canopy. A long, slow application is best, allowing the water to soak in. You want to ensure the soil is moist at least 12 inches down (check by using a small trowel to dig a hole). A thick layer of coarse mulch on top of the soil in that root zone will help save moisture and make the tree happier.

This link will give you more information about elm leaf beetles and their management: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7403.html.

For more information on elm tree pests and diseases, see http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/elm.html.

I hope this information is helpful. Please let us know if you have more questions.