Fenusa ulmi

Elm leafminer. Photo: Michael W. Palmer, Bugguide.
Scientific Name: Fenusa ulmi
Common Name: Elm Leafminer
Growing Degree Days (GDD's): 263–530 GDD's, Base 50F, March 1st Start Date. (Source: Cornell Cooperative Extension.)
Host Plant(s) Common Name (Scientific Name)
American elm (Ulmus americana)
Camperdown elm (Ulmus glabra 'Camperdownii')
English elm (Ulmus procera)
Scotch elm (Ulmus glabra)
Siberian elm (Ulmus pumila)
Insect Description:

The elm leafminer is a European species that was introduced into the United States prior to 1898. This insect is a leafmining sawfly whose adults are approximately 3 mm. in length and look like tiny black wasps. In New England, the adult is said to be active in May at which point it lays eggs in host plant leaf tissues through slits cut in the leaf using a saw-like ovipositor (egg laying structure of the females). It takes the eggs approximately 1 week to hatch, and tiny larvae begin to mine blotches in host plant leaves. Mines may appear as tiny, whitish spots confined between leaf lateral veins. Tissues between the two layers of epidermis are eaten. When several mines coalesce, large blotches are formed. By late spring, the larvae have completed their development and will cut their way out of the leaf, drop to the ground, and prepare to pupate in a papery cocoon buried in the soil beneath the host plant. They are thought to remain as pre-pupa through the summer, fall, and winter only to pupate and emerge as adults the following spring. One generation is thought to occur per year.
Damage to Host:

Damaged leaves may remain on host plants throughout the season. Blotch-like mines occur between lateral leaf veins and may run together to form a large mine. Injured leaves may turn brown and the mined areas might drop out of the leaf over time, leaving behind holes. Aesthetic damage may occur, and in heavy infestations healthy leaf loss may be an issue.
Monitoring:

Look for mines beginning at the end of May and in June. Large infestations may result in defoliation.
Cultural Management:

Leaves found with sawfly caterpillars within can be removed by hand and destroyed, particularly on small trees with light infestations.
Natural Enemies & Biological Control:

No significant natural enemies are currently reported.
Chemical Management:

Abamectin (NL)

Acephate (NL)

Acetamiprid (L)

Azadirachtin (NL)

Bifenthrin (NL)

Clothianidin (NL)

Cyantraniliprole (NL)

Diflubenzuron (N)

Dinotefuran (NL)

Emamectin benzoate (L)

Fenpropathrin (NL)

Flonicamid+cyclaniliprole (N)

Gamma-cyhalothrin (L)

Horticultural oil (L)

Imidacloprid (L)

Insecticidal soap (NL)

Lambda-cyhalothrin (L)

Malathion (L)

Neem oil (NL)

Permethrin (L)

Pyrethrin+sulfur (NL)

Spinosad (NL)
Notes:

If this insect has been a significant issue in an area in the past, applications to manage the adults may be made prior to egg laying.

Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), azadirachtin (injection, soil drench), clothianidin (soil drench), cyantraniliprole (soil drench, soil injection), diflubenzuron (soil drench), dinotefuran (soil drench), emamectin benzoate (injection), imidacloprid (soil drench), and neem oil (soil drench).

Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.

Note: Beginning July 1, 2022 neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.

Read and follow all label instructions for safety and proper use. If this guide contradicts language on the label, follow the most up-to-date instructions on the product label. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Read the full disclaimer. Active ingredients labeled "L" indicate some products containing the active ingredient are labeled for landscape uses on trees or shrubs. Active ingredients labeled "N" indicate some products containing the active ingredient are labeled for use in nurseries. Always confirm allowable uses on product labels. This active ingredient list is based on what was registered for use in Massachusetts at the time of publication. This information changes rapidly and may not be up to date. If you are viewing this information from another state, check with your local Extension Service and State Pesticide Program for local uses and regulations. Active ingredient lists were last updated: September 27, 2024. To check current product registrations in Massachusetts, please visit the MA Department of Agricultural Resources Pesticide Product Registration page and click on "Search Pesticide Products Registered in Massachusetts - Kelly Solutions".