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How to identify it
- Look for small brown, tube- or cigar-shaped cases, rarely more than 1/4 inch long, made of leaf tissue and silk and hanging from the undersides of the leaves.[1][2][3]
- The leaves show small, angular brown blotches between the veins where the larvae have mined; in heavy infestations the blotches merge and whole leaves turn brown.[1][2]
- Adults are small, dull yellowish-beige moths with gray markings and a wingspan of about 1/2 inch.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Elm leafminer | Its translucent white, legless larvae feed inside the leaves in late spring and build no cases; Siberian and red elm are most often hit.[3] |
Where it lives
A native North American moth that UMass and the University of Minnesota both cover as a pest of elms.[1][2]
Biology and life cycle
Moths mate in late July and lay eggs on elm leaves; the young larvae mine inside the leaf, then cut out a case and feed from it on the leaf underside, enlarging it as they grow.[1]
In fall the larvae carry their cases to the twigs and overwinter there, then move back to the new leaves in spring to finish feeding; they pupate in time for the adults to emerge in July, with one generation a year.[1]
Damage and signs
- A single larva does little harm, and UMass considers the casebearer a minor pest with a low risk of killing a tree; heavy infestations can brown and kill whole leaves.[1][2]
- UMass lists American, Scotch and slippery elm as hosts, and the University of Minnesota names American, red and slippery elm as preferred.[1][2]
Health risk
It feeds only on elm leaves; neither source reports any harm to people or pets.[1][2]
Control
Prevention
Scout for brown patches between the veins in July and August and for the small cases under the leaves in late summer.[1]
Sanitation
Raking fallen leaves in fall doesn't help much, because the larvae move to the twigs with their cases to overwinter.[1]
Biological control
Parasitic wasps, including Apanteles species, have been collected from elm casebearers in Vermont and Massachusetts.[1]
Pesticides
Treatment isn't needed unless most leaves on the tree carry more than one larva; follow the label of any product used.[1]
Common questions
- What are the tiny brown cases under my elm leaves?
- Small cigar-shaped cases of leaf tissue and silk hanging from the undersides of elm leaves hold elm casebearer caterpillars.[1][2]
- Is the elm casebearer harmful to my tree?
- Rarely; it is a minor pest with a low risk of killing a tree, though heavy infestations can brown many leaves.[1][2]
Sources
- UMass Extension Landscape, Nursery and Urban Forestry Program (Landscape Insect & Mite Guide), “Coleophora ulmifoliella (Elm Casebearer).” www.umass.edu/agriculture-food-environment/landscape/publications-resources/insect-mite-guide/coleophora-ulmifoliella (accessed 2026-10-17)
- University of Minnesota Extension (What's wrong with my plant?), “Elm: unusual projections on leaves.” apps.extension.umn.edu/garden/diagnose/plant/deciduous/elm/leavesprojections.html (accessed 2026-10-17)
- University of Minnesota Extension (What's wrong with my plant?), “Elm: dots, spots or blotches on leaves.” apps.extension.umn.edu/garden/diagnose/plant/deciduous/elm/leavesspots.html (accessed 2026-10-17)
- iNaturalist, “Coleophora taxon page (order and family).” www.inaturalist.org/taxa/87566 (accessed 2026-10-06)