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Dusky Birch Sawfly

Nematus latitarsus (formerly Croesus latitarsus)[3]

The dusky birch sawfly is a sawfly whose spotted larvae feed in groups along the edges of birch leaves.[1][2]

Group
Insect
Order
Hymenoptera
Family
Tenthredinidae
US records
15 states
Sources
3 cited
Updated
2026-10-17
Dusky birch sawfly larvae with black spots feeding along a birch leaf edge
Dusky birch sawfly larvae with black spots feeding along a birch leaf edge. (c) M. Whitson, some rights reserved (CC BY) — via iNaturalist, CC BY 4.0
On this page
  1. Identification
  2. Where it lives
  3. Biology
  4. Damage and signs
  5. Health risk
  6. Control
  7. Related profiles
  8. Common questions
  9. Sources

How to identify it

  • Larvae have shiny black heads and rows of black spots; young larvae are dark gray, turning yellowish or yellowish-green and up to about 1 inch long when grown.[1][2]
  • Disturbed or resting larvae hold an S-shaped pose with the rear end raised.[1]
  • Adults are about 1/2 inch long and look like small black wasps.[1]

Where it lives

Found on native and non-native birches in Massachusetts and Minnesota.[1][2]

US map: public records from 15 states since 1990 (GBIF).

Biology and life cycle

Prepupae overwinter in cocoons in the soil, pupate in spring and emerge as adults in May to lay eggs on the leaves.[1][2]

First-generation larvae feed from about May to early July; a second, overlapping generation can feed from mid-July through September.[1][2]

Damage and signs

  • Gray birch is the favorite host, but black, yellow, paper, river and other birches are also eaten.[1][2]
  • Young larvae eat all but the main veins, older ones eat whole leaves, and groups feeding from the edges can defoliate small trees.[1][2]

Health risk

None to people or pets.[1][2]

Control

Traps and physical control

Drop larvae into a can of soapy water or prune out and destroy the branch section where a group is feeding.[1]

Pesticides

Numbers rarely reach levels that need chemical treatment; check birches in spring and again in midsummer.[1][2]

Common questions

What are the black-spotted larvae lined up on my birch leaves?
Most likely dusky birch sawfly larvae, which feed in groups from the leaf edges.[1][2]
Will Bt control dusky birch sawflies?
No; Bacillus thuringiensis kurstaki kills moth caterpillars, not sawfly larvae, so UMass recommends hand removal first.[1]

Sources

  1. UMass Extension Landscape, Nursery and Urban Forestry Program (Landscape Insect & Mite Guide), “Croesus latitarsus (Dusky Birch Sawfly).” www.umass.edu/agriculture-food-environment/landscape/publications-resources/insect-mite-guide/croesus-latitarsus (accessed 2026-10-16)
  2. University of Minnesota Extension, “Sawflies (Yard and Garden Insects).” extension.umn.edu/yard-and-garden-insects/sawflies (accessed 2026-10-16)
  3. iNaturalist, “Nematus latitarsus taxon page (order and family).” www.inaturalist.org/taxa/1354151 (accessed 2026-10-06)

Updated 2026-10-17. Built from sourced records. Spot an error? Tell us.