The Elm Leafminer Beetle Is Destroying Trees in Denver: How to Identify, Treat, and Prevent Elm Leaf Beetle Infestation

The Elm leafminer and Elm leaf beetle have been causing damage to Colorado elm trees for several years now. The infestation seems to be getting worse, so it is important for homeowners to learn how to identify these pests and understand the damage they can do.

In this article, we will describe how to identify an Elm leafminer and Elm leaf beetle, list what types of elm trees are being affected in Denver, explain treatment options, and give Colorado homeowners tips on how to protect their trees.

What is the Elm Leafminer?

The Elm leafminer is a small, greenish-yellow fly that lays its eggs on the undersides of elm leaves. The larvae tunnel into the leaves, causing them to turn yellow and eventually brown. The damage from the mines is usually not serious but can make trees more susceptible to other problems.

Here are just a few important Elm leafminer facts to know from the Colorado University Extension:

  • Leafminers are insects that feed within a leaf, producing large blotches or meandering tunnels.
  • Although leafminer injuries are conspicuous, most leafminers produce injuries that have minor direct effects on plant health.
  • Most leafminers have several natural controls and can be further controlled with proactive actions taken by homeowners.
  • Insecticides applied when leafminers lay eggs are useful for the control of many leafminers.

What is the Elm Leaf Beetle?

Elm leaf beetles are slightly larger than a grain of rice. The adult beetle is yellow with black stripes running lengthwise along the body. Underneath the wing covers are two large black spots. Where do they hide out? Adult elm leaf beetles overwinter in protected places like inside homes or beneath tree bark.

These pests do damage to trees by skeletonizing the leaves. This means that they eat the leaf tissue between the veins, leaving only the network of veins behind. Heavily infested elm leaves appear brown and dry and may drop from the tree prematurely. Luckily, Elm leaf beetles can be controlled with insecticides applied when egg masses are first noticed in spring.

Check out these fast facts on the Elm Leaf Beetle from the U.S. Forest Service:

  • Elm leaf beetle feeds upon all elms, particularly Siberian and English, as well as American, rock, slippery, and other native, Asian, and European species of elm.
  • Adults emerge from overwintering sites (usually beneath the bark of elm logs or stumps) in late April or early May and begin feeding on elm leaves.
  • Elm leaf beetle has from one to three generations per year, depending upon weather and location.
  • Elm leaf beetle feeding can greatly affect the appearance of elms, rendering them unsightly, and can reduce growth and weaken them, particularly when large beetle populations persist for several years. 
  • Effective applications against the first generation generally suppress populations sufficiently such that a second application is not necessary. In addition, soil systemic application of appropriate insecticide can also provide effective control.

Being knowledgeable about pervasive elm pests is the first step towards protecting your trees. For more information, contact one of our expert arborists!

The Dangers of Infestation

How serious is the threat to Denver trees? Trees that are heavily infested with Elm leaf beetle or Elm leafminer may be completely defoliated by midsummer. Repeated defoliation can stress trees and make them more susceptible to other pests, diseases, or environmental problems.

Don’t forget that Denver had very dry conditions this winter. Trees that are already stressed from drought, poor drainage, compacted soils, or other factors have difficulty fending off more serious diseases and attacks from other pests.  If a tree is completely defoliated more than once, it may not recover and will likely die.

All serious tree damage begins in small incremental ways. Don’t make the mistake of writing off the Elm leafminer and Elm leaf beetle as harmless. When infestations go ignored or untreated, the real damage is done.

Which Denver Trees Are Threatened By Elm Leafminers and Elm Beetles?

Elm leaf beetles are most commonly found on American elms, but can also be found on Siberian elms. As a homeowner, it is up to you to be knowledgeable about the trees and plants in your landscape. Here are tips on how to easily identify these elm species:

American Elm

This popular Denver elm tree species grows to 60-80’ tall with a spread of 40-60’. It has dark green, alternate leaves that are oblong with double serrated margins.  These leaves turn yellow in a show of beautiful fall color. Also called the White elm or Water elm, it has dark gray, ridged bark.

Revered for its graceful, strong shape, this species has branches that spread like fountains and a trunk that divides into large limbs near the base.

Siberian Elm

This elm tree variety is a fast-growing, deciduous tree that reaches 40-60’ tall with a spread of 35-50’. It has elliptical to oblong leaves that are dark green and have double serrated margins. Due to its ability to tolerate drought and poor soil drainage, the Siberian elm thrives in Denver’s climate.

Siberian elms feature a round crown of slender, spreading branches. Known for their hardiness, these elms also produce petal-less flowers with a greenish-red to brown color. 

Sometimes it takes a trained eye and years of experience to properly identify different Denver trees. That’s why our experienced arborists are ready to help at any time. They can use their expertise to help you know which tree and plant species you have and how to best care for them.

Treatment Options For Elm Leafminer and Elm Beetle Infestation

Are you worried that your tree may have an infestation? Here are a few key warning signs:

  • off-color leaves
  • skeletonized leaves
  • chewed or notched edges on leaves

If you see any of these signs, it’s time to give us a call. Our arborists will come out and take a look at your tree. If we confirm that there is an infestation, we’ll work with you to craft a custom treatment plan.

There are a few different treatment options for Elm leafminer and Elm leaf beetle infestations.  These options include:

  • insecticide treatments
  • trunk injections
  • soil applications
  • foliar sprays

Each treatment option has its own set of benefits and drawbacks. Our arborists will work with you to determine the best course of action for your tree.

According to the U.S. Forest Administration, there are several labeled, registered chemical and biological insecticides that readily control elm leaf beetle populations when sprayed on the foliage.

What Can Denver Homeowners Do to Protect Their Elm Trees?

The best defense against Elm leaf beetles and Elm leafminers is a healthy tree. To prevent elm leaf miner infestation in your Colorado trees in 2022:

  • Select resistant varieties of elm trees when planting.
  • Prune and destroy infected branches as soon as possible after discovery to limit the spread of the disease.
  • Avoid wounding the tree bark as this provides an entry point for the fungus.

Maintain the health of your trees by watering during dry periods, fertilizing as needed, and mulching around the base of the tree. Proper watering and fertilizing is a science. Our arborists can provide a consultation and help curate a custom treatment plan to get your trees in the best shape of their lives!

If you suspect your tree is infested with elm leaf miners, contact a certified arborist for assistance.

When Is The Best Time To Treat Your Elm Trees in Denver?

Trees should be treated for Elm leafminers and Elm leaf beetles in the spring before they emerge from their cocoons. April is the best month to treat your trees in Denver. If you wait until May or June, it will be too late – the elm leaf miners will have already emerged causing damage to your trees.

Don’t miss your window of opportunity. Our team is currently scheduling consultations and treatments for the month of April. The best defense against the oncoming infestation is to strengthen your trees.

Be proactive and get an appointment on the books! Taking action now can enhance the beauty and lifespan of your elm trees for the season and year ahead. You’ll also be positively contributing to your neighborhood. Elm leafminer and Elm leaf beetles can rapidly move from tree to tree, property to property. Stopping the spread and infestation in Denver is truly a community effort.

Ready to get started? Contact our friendly team to schedule your free consultation today!