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How to identify it
- Larvae look like tiny slugs: shiny, slimy and unsegmented-looking, widest just behind the head and tapering toward the tail, dark to light green or nearly black, and about 1/2 inch (13 mm) long when grown.[1][2][5]
- The damage is the giveaway: larvae eat the lower surface of the leaf between the veins and leave the thin upper surface, so the leaf looks translucent against the light, then turns brown.[4][2][1]
- Young larvae feed side by side in a row across the leaf underside; older larvae wander and feed singly, and the dried patches eventually fall out, leaving holes between the veins.[3]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Pear sawfly (pear slug) | A related slug sawfly, shiny olive green and about 1/2 inch long, but it windowpanes the upper surface of the leaves of pear, cherry, plum, cotoneaster, mountain-ash and hawthorn, not oak.[1] |
Where it lives
Reported on oaks by extension services from Minnesota and Iowa to New Jersey and North Carolina; Rutgers describes it as mainly a native forest insect, seen mostly in yards next to large wooded areas.[1][2][3][4]
Biology and life cycle
It spends the winter in a cocoon, 2 to 3 inches down in the soil according to UMN and Iowa State, or in the leaf litter according to Rutgers; adults, small fly-like sawflies that don't sting, emerge in May and June.[1][2][3]
Females insert their eggs into the leaves, often along the major veins; the eggs hatch in 1 to 2 weeks and the larvae feed for about four weeks before dropping to the ground to pupate.[1][2][3]
A second generation follows in late July or August in Minnesota and Iowa; Rutgers notes that if three generations occur in New Jersey, larvae can be active from early June through early September.[1][2][3]
Damage and signs
- The damage is mostly cosmetic and not usually fatal to healthy, established trees, and it often isn't noticed until late summer, after the larvae have finished feeding.[2][1][4]
- Repeated defoliation over a few years in a row can stress oaks enough to invite wood borers such as the twolined chestnut borer.[3]
- Sources differ on hosts: UMN lists northern red, northern pin, pin, swamp white, white and bur oak; Iowa State says red and white oaks, most often northern red and pin oak; UMN's diagnostic page names pin and black oak as preferred; and Rutgers adds black and scarlet oak.[1][2][5][3]
Health risk
The adults are non-stinging sawflies and the larvae feed only on oaks; none of the sources reports any harm to people or pets.[2][5][1]
Control
Prevention
Look for larvae in spring and again in midsummer, while they are small; sprays applied after the larvae have dropped from the leaves do no good.[1][2]
Traps and physical control
On small trees with only a few larvae, pick them off or remove the infested leaves.[2][3]
Pesticides
Treatment is rarely needed except on small, newly planted or stressed trees; Rutgers notes young larvae can be suppressed with horticultural oil and that spinosad is labeled against these sawflies. Use a product that lists oaks on its label.[2][3][4]
Common questions
- Why do my oak leaves have see-through patches?
- Translucent patches between the veins, where the lower leaf surface has been eaten away, are typical scarlet oak sawfly damage; look under the leaves for small, slimy green larvae.[4][2][1]
- Do I need to spray for oak sawflies?
- Usually not; the damage is mostly cosmetic and tends to be noticed after the larvae are done, when spraying won't help.[2][4]
Sources
- University of Minnesota Extension, “Sawflies (Yard and Garden Insects).” extension.umn.edu/yard-and-garden-insects/sawflies (accessed 2026-10-17)
- Iowa State University Extension and Outreach, “Oak Sawfly (Yard and Garden encyclopedia).” yardandgarden.extension.iastate.edu/encyclopedia/oak-sawfly (accessed 2026-10-17)
- Rutgers Cooperative Extension (Plant & Pest Advisory), “Summer Pests on Oaks: Oak Sawflies, Oak Spider Mites & Oak Lace Bugs (S. K. Rettke, July 15, 2021).” plant-pest-advisory.rutgers.edu/summer-pests-on-oaks-oak-sawflies-oak-spider-mites-oak-lace-bugs (accessed 2026-10-17)
- NC State Extension (Entomology), “Sawflies Feeding on Oaks (S. D. Frank).” entomology.ces.ncsu.edu/news/sawflies-feeding-on-oaks/ (accessed 2026-10-17)
- University of Minnesota Extension (What's wrong with my plant?), “Oak: pale, white or opaque discoloration of leaves.” apps.extension.umn.edu/garden/diagnose/plant/deciduous/oak/leavesdiscolored.html (accessed 2026-10-17)
- iNaturalist, “Caliroa quercuscoccineae taxon page (order and family).” www.inaturalist.org/taxa/495085 (accessed 2026-10-06)