Birch Leafminer

How to identify it

• Damage appears in early summer as irregular brown 'scorched' blotches; held to light, mines show larvae and black frass.^[1]
• Adults are small black sawflies about 3 mm long; larvae are whitish and flattened, 6 mm when grown.^[1]

Where it lives

Native to Europe, it was first recorded in the US in Connecticut in 1923.^[1]

US map: public records from 24 states since 1990 (GBIF). View the map

Biology and life cycle

Larvae overwinter in soil; adults emerge in May and insert eggs singly into soft new leaves.^[1]

Larvae feed for two to three weeks inside the leaf, then drop to the soil.^[1]

Damage and signs

• Healthy trees tolerate damage, but repeated defoliation can weaken them, inviting fungi and bronze birch borer.^[1]
• Sweet, yellow and European white birch are seldom attacked.^[1]

Health risk

It feeds on plants; Penn State Extension mentions no risk to people.^[1]

Control

Prevention

Himalayan whitebarked, Dahurian and river birch are reported less susceptible.^[1]

Pesticides

If needed, time registered sprays for adult activity in early May and the second generation in mid- to late June.^[1]

More from UMN Extension

Leafminer damage seldom reaches more than 40% of a birch's leaves, and well-watered trees can tolerate even heavier losses.^[2]

Common questions

Why are my birch leaves turning brown in early summer?

Birch leafminer larvae feeding inside the leaves cause brown blotches.^[1]

Which birches resist birch leafminer?

Himalayan whitebarked, Dahurian and river birch are reported less susceptible.^[1]

Sources

1. Penn State Extension, “Birch Leafminer.” extension.psu.edu/birch-leafminer (accessed 2026-10-06)
2. University of Minnesota Extension, “Birch leafminers.” extension.umn.edu/garden-and-home/yard-and-garden/yard-and-garden-insects/birch-leafminers (accessed 2026-10-15)
3. iNaturalist, “Fenusa pusilla taxon page (order and family).” www.inaturalist.org/taxa/1028796 (accessed 2026-10-06)