Columbus – A researcher recently discovered a population of invasive elm squash fly infesting elm trees at the US Department of Agriculture’s Forest Service Northern Research Station laboratory in Delaware and in northern Franklin County.

The Ohio Department of Natural Resources (ODNR) Forest Service and Division of Forestry reported that subsequent surveys also located the species on nearby properties.

“The curled elm fly is an invasive insect native to Asia that was found for the first time in Canada in 2020 and in several eastern US states in 2021 and 2022,” said Tom Macy, director of the Forest Health Program at ODNR. “This is the first discovery of this species in Ohio.”

Curly elm fly cocoon. ODNR Division of Forestry photo by Tom Macy.

Despite its ability to significantly defoliate elm trees, the overall impact of this species on forest and urban landscapes is not fully known and is an active research area.

There are no reports of the ash midge causing death in ash trees. Management options for this species are still being studied.

ODNR’s Forestry Division will continue to survey the elm midge throughout the state.

Larva of the winding elm fly is up to half an inch long, caterpillar-like, and light green in colour. The larvae feed exclusively on the leaves of elm trees, including both native and introduced elm species.

The caterpillars create a distinctive zigzag pattern through the leaves as they feed. Before pupating, the larvae spin loosely, in mesh cocoons attached to leaves. In the fall, they form cocoons with more rigid walls to overwinter on the ground in leaf litter or soil.

Adults are observed less commonly but are small (0.25 inches long), glossy black, and winged.

“The Northern Research Station has conducted important research on Dutch elm disease-tolerant Dutch elm in a Delaware lab for several decades,” said Kathleen Knight, a Northern Research Station ecologist. “Because we manage multiple acres of elm plantations and keep a close eye on them, we have been able to detect an infestation of the squiggly elm fly.

“NRT scientists worked with the appropriate regulatory agencies to confirm the insect’s identity and identify endemic areas.”

The elm curly fly larva. ODNR Division of Forestry photo by Tom Macy.

A few winding elm fly larvae were observed on trees in the nursery in early July and were officially confirmed by the USDA Animal and Plant Health Inspection Service on July 21.

Subsequent surveys were soon undertaken to understand the extent of the infestation, which led to the identification of the insect in neighboring properties.

The presence of the elm curlew fly in Franklin County was confirmed on August 18. The Forest Service and ODNR have prioritized careful assessment of the situation and sharing knowledge as quickly as possible.

If you find a suspected limping elm fly or signs of infestation, report a sighting using the Great Lakes Early Detection Network’s free mobile app or try to get a good picture or collect a specimen and notify the ODNR’s Forestry Division at 614-265-6694.

The focus of the Northern Research Station extends from Maine to Minnesota and from Missouri to Maryland.

The station manages 22 of the 80 experimental forests that are part of the Forest Service’s experimental forest network.

Agency scientists conduct scientific research in-house, apply research findings to national forests and grasslands, and communicate these findings to others across the country.

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