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Two elm leaf insects are causing concern in Longmont and along Front Range

The elm leaf beetle and elm leaf miner eat Siberian elm leaves, causing damage and loss of leaves

Two insects that feed on the leaves of Siberian elm trees — the elm leaf miner and elm leaf beetle — are generating concerns for residents in Longmont and along the Front Range, according to a news update issued Thursday by the city of Longmont Public Works & Natural Resources department.

The update described long-term health risks to the trees as “minimal,” per Colorado State University Extension, and stated while the city “has no plans to treat city-owned and maintained Siberian elm trees,” options for treating private trees are available. Also noted was the nuisance potential for the elm leaf beetle to overwinter inside homes.

“Generally speaking, the trees aren’t entirely defoliated, but it can look quite devastating. The canopy that remains can have a yellowish tinge, or a brown tinge, and then leaves can also fall from the trees – the leaves that have been heavily chewed on can desiccate (dry out). They’re not really providing much resource to the trees, so they drop the leaves prematurely,” said Brett Stadsvold, city forester with the city of Longmont.

The Colorado State University Extension webpages for elm leaf beetles and leaf-mining insects, including elm leaf miner, offer information about treatment options. A list of tree contractors licensed by the city of Longmont is available here.

Wide-scale treatment not advised, however repeated infestations can cause harm

The damage from these elm leaf insects is generally considered aesthetic, according to Stadsvold and Jacqueline Buenrostro, Colorado State University graduate student in entomology and Longmont resident. 

Entomology is the scientific study of insects.

Private consulting arborists, Stadsvold said, “have agreed that treatment isn’t a great option for the scale at which we manage trees, and we manage over 20,000 trees in the city ... It’s not like emerald ash borers, to give you a comparison, which damages the conductive tissues of trees, they feed under the bark and kill the trees that way. These insects generally do not kill trees,” he said.

However, repeated leaf damage can negatively impact elm tree health over time, Buenrostro said.

“If the trees keep getting slammed year after year, eventually that tree is going to die much earlier than it would normally. But it’s not going to outright kill the tree the first year that it happens,” she said.

Buenrostro expressed some concern about the use of chemical controls in particular.

“There’s a lot of research about these insecticides that they typically use — how they just kind of cascade in the ecosystem and are not good for pollinators — and all that typical stuff that can come about as a byproduct of using insecticides,” she said.

Double damage: elm leaf miner and elm leaf beetle work in tandem

The insects work in tandem, often feeding on the same leaves and causing browning and premature defoliation, Stadsvold said. But because they feed on leaves in different ways, each insect causes its own telltale damage as well.

“The elm leaf miner is a small larva, in its juvenile phase, that feeds on the inner parts of the leaves between the veins and makes them look lacy. They turn brown and they can prematurely fall from the tree,” Stadsvold said. "It’s quite amazing that the larva of the elm leaf miner is so small that it’s actually between the surfaces of the leaves feeding. So, it eats the leaf parts from the inside out, essentially.”

Although they’re tiny, it is still possible to see elm leaf miner larvae with the naked eye, Jacqueline Buenrostro, Colorado State University entomology graduate student and Longmont resident, said by email.

“The miner larvae are visible. If one holds a leaf with mining damage up to the light, the larvae should be visible inside the leaf. Late May and early June are the best times to look. After this, the miner larvae will drop from the leaves to the soil,” Buenrostro said by email.

“Once the larvae drop from the leaves, the leaves themselves will continue to dry out and eventually drop to the ground through June and July, which is why we are experiencing all the leaf drop now,” she said.

In contrast to the lacy pattern left behind by the elm leaf miner, the elm leaf beetle chews small holes.

“The elm leaf beetle is a larger larva that feeds externally on the leaf. They can leave these shot holes — if you took a shotgun and shot it at a tree — it would be these small quarter inch holes in the leaves. That’s more characteristic of the elm leaf beetle in both the larva and the beetle phase,” Statsvold said.

Risk to other types of elm trees

While the Siberian elm is the primary concern, the elm leaf miner may pose some potential concern to other types of elm trees as well.

“Based on the research, I don’t believe the elm leaf beetle poses much risk to American or hybrid elm, at least it hasn’t historically. The research is more mixed with the miner. We are seeing severe damage to Siberian elm here in Colorado, but other states experience damage to their hybrid elms or other native elm species,” Buenrostro wrote.    

Elm leaf beetles invade homes in colder weather

The city of Longmont’s news release noted that elm leaf beetles commonly seek shelter inside homes during the winter.

“As we go throughout the year these beetles overwinter in their beetle phase. They look for protection in warm places and can start to invade homes,” Statsvold said. “The way to try to keep them out is looking at all those windows and doors in your house, and insulating better, just sealing up those cracks, because they’ll make their way into your house.”

While the beetles are a nuisance, according to the City of Longmont news update, they don’t do any harm.

“They’re totally benign once they’re in your house. They’re just looking for a warm place to overwinter. They won’t eat anything and won’t really move around much. They just wait it out until winter’s over, and then they’ll leave the house, start feeding on leaves again, reproduce and go through two life cycles a year,” he said.