Elm Leaf Pests
Below are the four Elm leaf pests in Denver – three beetles and a sawfly.
- Elm Leaf Beetle, Xanthogaleruca luteola
The Elm Leaf Beetle was accidentally introduced into the eastern United States early in the nineteenth century.
- Elm Leaf Miner, Fenusa ulmiis
The Elm Leaf Miner is a sawfly originally from Eurasia.
- European Elm Flea Weevil, Orchestes alni
The European Elm Flea Weevil is an invasive species from Eurasia.
- Japanese Beetles, Popillia japonica
Japanese Beetles came to Denver in the 1990s. Japanese Beetles love Siberian Elms and other Elm varieties planted here.
Elm Trees In Denver
Elm trees, Ulmus spp., grow well in cities because they tolerate extreme weather and drought. During Colonial times, settlers noticed their size and beauty and started to transplant these Elms from the forest into towns and cities. By the early 1900s, the American Elm was ubiquitous across the urban landscape in the country. The Denver area is lucky because our harsh, dry weather allows us to enjoy the beauty and grace of American Elms lost to the Dutch Elm disease in most areas of the country starting in the 1920s. The tree produces six-inch textured green leaves that turn vibrant yellow in the fall. The Siberian Elm also grows here. It grows to 60 to 70 feet tall with small leaves. Siberian Elms are brittle, with branches prone to breakage. The trees are disease, insect prone, and grow like weeds. Homeowners should be aware that Siberian Elms are considered a trash tree and can no longer be planted in Denver’s public right-of-ways.
Since the Dutch Elm disease outbreak a hundred years ago, cultivators have been busy finding trees to replace the American Elm. According to the Denver Street Tree Guide, these trees grow here.
- Scale Buster American Elm
- Colonial Spirit Elm
- New Horizon Elm
- Princeton American Elm
- New Harmony American Elm
- Prairie Expedition American Elm
- Valley Forge American Elm
- Pioneer Elm
- David Elm
- Discovery Elm
- Freedom Elm
- Northern Empress Japanese Elm
- Commendation Elm
- Accolade Elm
- Danada Charm Elm
- Allee Lacebark Elm
- Central Park Splendor Elm
- Emerald Sunshine Elm
- Vanguard Elm
- Homestead Elm
- Triumph Elm
- Prospector Elm
- Patriot Elm
Elm Leaf Beetle
Elm Leaf Beetles chew the leaves of Elm trees. Their dark grub-like larvae feed on the leaves’ underside’s soft tissue but avoid the leaf veins, skeletonizing the leaves. In large outbreaks, these beetles cause a lot of damage and even defoliate an elm tree. Highly skeletonized or defoliated trees decreases summer shade leaving the tree in danger of sun scorching. Brown or bear trees are unsightly.
Elm Leaf Beetles overwinter in the adult stage. In late summer, they seek out protected sites such as woodpiles, loose mulch and leave piles where they go into a semi-dormant state called diapause and turn a khaki-green color. They stop feeding and reproduction but sometimes become active during warm days in the winter. Occasionally, they enter cracks and openings in buildings, becoming a nuisance. Fortunately, they do not damage homes.
In spring, the surviving Elm Beetles move to trees when leaves emerge. Once they begin to feed again, their color shifts back to a yellow-green. In early spring, female beetles begin laying masses of yellow eggs on the underside of leaves. They hatch, producing tiny, black larvae that chew little holes in the leaves. As they mature, they scatter through the tree, skeletonizing the leaves. Within 3 to 4 weeks, the larvae are fully grown and look for shelter to transform to their next stage.
Their favorite spot to pupate is at the tree’s base, where sometimes they can be found in large numbers. It takes about two weeks for adults to emerge, where they fly back into the branches to feed and mate, producing a second generation in a season. The next batch of larvae chews leaves from mid-July into September. They pupate and overwinter as adults to start the cycle again the following spring.
Elm Leaf Miner
One reason the leaves of Elms in Denver turn yellow is because of the Elm Leaf Miner. The larvae stage of the insect’s lifecycle causes leaf damage. The adult sawfly emerges in the spring and slits an opening to lays eggs on the leaves’ upper surface. Eggs hatch about a week later, producing brown-headed white larvae that tunnel in to the leaves’ soft tissues. Some of these tunnels interconnect, forming large, open, dead pockets. An interesting fact about this sawfly is the adults are primarily parthenogenetic, meaning that they are females capable of producing viable eggs without males. Parthenogenesis or asexual reproduction occurs commonly among lower plants and invertebrate animals but rarely among higher vertebrates.
Later in the spring, the larvae mature and drop to the ground to pupate. Sometimes with heavy infestations, it looks it is raining larvae when the wind blows. The larvae tunnel down into the soil about one inch and pupate within a papery cocoon. They remain in the pupal stage throughout the summer, fall, and winter, emerging in the spring as adults, and the cycle repeats itself.
European Elm Flea Weevil
In 2006, a new beetle arrived in Colorado that feeds on Elms—the European Elm Flea Weevil. These tiny reddish insects, with black spots, have elongated snouts with small mouthparts at the end, which they use to feed on tree leaves. They create pinhead-sized, round holes in the foliage. As they eat, these holes enlarge, forming more noticeable holes. These holes show this weevil’s presence. They get their name when disturbed since they jump like fleas.
Female Flea Weevils mine holes partway through the leaf tissue to lay eggs. These hatch into legless, white larvae that tunnel through soft leaf tissue at the leaf tips turning the tips white, then eventually brown. The larvae pupate within the mine and emerge as adults in August. The adults look for protected locations to overwinter and appear in the spring to feed on the new foliage and lay eggs.
The Japanese Beetle was first introduced into Colorado in the early 1990s from nursery stock purchased in the mid-western United States. It was originally thought, that Japanese Beetles would not thrive here because of our dry climate, but the wholesale adoption of lawn and garden irrigation makes Denver a perfect home to this insect.
Japanese Beetles have a one-year life cycle. Adults begin to emerge from the soil in early June, and within weeks they become ubiquitous to neighborhoods across the city. Adult Japanese Beetles are voracious feeders and attack about 300 species of plants ranging from roses to poison ivy. Their favorites seem to be Virginia Creeper, roses, Siberian Elm, and other Elm varieties found in Denver. Large infestations skeletonize the leaves of whole trees, eventually defoliating them.
During the summer, mated females dig 2-3 inches down in turfgrass to lay a cluster of eggs among the plant roots. They crawl out to resume feeding, then return to grassy areas to lay more eggs. Each Japanese Beetle female lays 40-60 eggs during her life span. Upon hatching, the larvae seek out nearby plant roots and feed. The rapid larval development during late summer can cause large brown spots in lawns.
Larvae continue to feed until soil temperatures drop to about 60°F, at which time the larvae dig deeper in the soil where they overwinter. Larvae activity resumes as soils warm in spring and, after a feeding period of about 30 days, the larvae form an earthen cell and pupate. The adults emerge to start the cycle again.
Control of Elm Leaf Pests
Like most evasive species, these beetles have no natural predators in Colorado to keep their numbers down. Stink bugs and other plant insects may feed on various stages of the three elm pests discussed above. The weather probably is the most crucial factor that affects elm leaf pests populations. Long winters or a late spring freeze kill large numbers of overwintering of these insects. Wind storms knock off larvae off trees.
If there are yellow spots on your Elm trees’ leaves in the summer, it is too late for control measures, so plan for early spring treatment the following year. One option is to spray right before the adults lay eggs on the leaves, but it is hard to estimate the right treatment timing. The other problem with spraying large Elm trees, whose canopies spread over houses, streets, playgrounds, parking areas, and other high-use areas, is product drift.
Systemic Product Control of Elm Leaf Pests
Ross Tree, a leading innovator in tree insect control, recommends systemic products to control Elm leaf pests. We were the first tree service company in Denver to offer tree injections, a practicable alternative to spraying trees. Trunk injections and root drenches ensure that only the tree gets the product, not the surrounding environment. There is no need to close windows, cover furniture, or keep pets indoors when injecting trees to control tree pests.
To prevent Elm leaf pests, we inject the systemic product into the root zone of the tree. The solution is taken up by the roots and eventually brought to the leaves by the vascular system, where it stays active for a year. When present in the leaf tissue, it kills any feeding larvae. Systemic Elm Leaf Pest control avoids product drift, and the chemical does not affect lady beetles, bees, and other beneficial tree insects.