Rose sawfly
Rose slugs are the larvae of the rose sawfly, Endelomyia aethiops, a small black non-stinging wasp relative. The pale green, slimy-looking larvae graze one surface of the leaf between the veins. They leave see-through windows, which dry white and then brown. The damage looks alarming, but healthy roses usually recover, and the RHS says flowering is not affected. Pick or hose them off, or spray young larvae with insecticidal soap, horticultural oil or neem. Bt does NOT work on them. The caterpillar Bt, Bacillus thuringiensis var. kurstaki, kills only true caterpillars, the larvae of moths and butterflies, and sawflies are not caterpillars.
Endelomyia aethiops larvae on a rose leaf, with translucent grazed patches. Photo: pending.
Identification
The larvae are pale yellowish-green and translucent, often with the dark green gut showing through, and have a light brown to orange-brown head. Full-grown larvae are about 3/8–1/2 inch (10–13 mm; RHS gives both figures, UMN 13 mm). They are fattest just behind the head and taper to the rear, and a slimy coating makes them look like tiny slugs. The adult is a small black sawfly about 3/16 inch (4–5 mm) long with two pairs of smoky, transparent wings, and is rarely noticed.
The test that separates any sawfly larva from a caterpillar is the prolegs, the fleshy stubs under the abdomen. Sawflies have six pairs or more; caterpillars have two to five. On slug sawflies the prolegs are small and easy to miss. Wisconsin Extension names two other sawflies found on roses: the bristly roseslug (Cladius difformis), covered in bristle-like hairs, and the curled rose sawfly (Allantus cinctus), which curls up at rest. The UK also has large rose sawflies (Arge spp.), with spotted larvae that feed in groups.
Easily confused with
- Caterpillars on rosesAlso a pestCount the fleshy prolegs under the abdomen: caterpillars have five pairs or fewer, sawfly larvae six or more. It matters because Bt kills caterpillars but not sawfly larvae.
- Bristly roseslugAlso a pestAnother rose sawfly, but covered in bristle-like hairs rather than slimy and smooth, and it can have several generations a year.
- Large rose sawflyAlso a pestIn the UK: pale green larvae with black spots and yellow blotches that feed together in groups and can defoliate roses, unlike the plain, translucent rose slug.
Damage symptoms
The larvae do not chew through the leaf. They graze away one surface between the veins until only a thin skin is left, a pattern called windowpane feeding. The patches are whitish at first and then turn brown. The University of Minnesota and Wisconsin describe the upper surface being grazed; the RHS finds them mostly underneath, moving to the top only in shade. UC IPM notes that roseslugs as a group, several species, skeletonise the lower surface when young and chew large holes as they mature; the RHS says the rose slug sawfly itself does not make holes. Damage can be severe, but healthy plants regrow, and the RHS says flowering is not affected.
- Translucent windows grazed between the veins, drying white then brown
- Leaves skeletonised to a lace of veins on heavily attacked plants
- Later, larger holes chewed through the leaf by mature roseslugs (UC IPM, several species)
- Small green slimy larvae feeding in the open on either surface, with no leaf rolling
- Heavy defoliation in early summer on otherwise healthy roses
Lifecycle & degree days
In the northern US the rose sawfly has one generation a year. It spends the winter as a prepupa in the soil. Adults emerge in spring and cut pockets in the leaf edge to lay eggs, and larvae feed from mid- to late May through June. Full-grown larvae drop to the soil by the end of June and stay dormant until the next spring. The bristly roseslug is different: it has two to six generations a year, so it keeps going through summer. The curled rose sawfly usually has two. For Britain the RHS page gives two or sometimes three generations between June and October, the second often the most abundant, though its biology section says one a year.
- Generations / yr
- 1 (US); 1–3 (UK, RHS gives both)
- Larvae feed
- Mid-May to end of June (Minnesota, Wisconsin)
- Bristly roseslug
- 2–6 generations a year
- Overwinters
- Prepupa in a cocoon in the soil
Host crops
Roses only. The University of Minnesota lists all wild and cultivated roses as hosts. Other slug sawflies feed on pear, cherry, oak, lime and willow, but they are different species and do not move onto roses.
Monitoring & action thresholds
No extension source sets a threshold. The RHS advises accepting it, as plants usually recover from defoliation with no effect on flowering. The University of Minnesota notes damage can be severe, but healthy deciduous plants regrow leaves. If you do spray, spray young larvae. Once they are full-grown the damage is done.
Start looking in spring. Minnesota advises from early spring, and larvae first appear from mid- to late May. Turn leaves over and check both surfaces as soon as the first see-through patches appear. Check again through summer if the larvae have bristles, since the bristly roseslug has several generations.
Biological control
Nothing to buy. UC IPM notes that roseslugs have many natural enemies, and that birds, parasitic wasps, predatory beetles, small mammals and fungal and viral diseases keep many sawfly populations low. The RHS adds ground beetles. The one product people reach for is Bt, and it does not work: UC IPM, the University of Minnesota and Wisconsin all say Bacillus thuringiensis is not effective against sawfly larvae, because they are not caterpillars.
Cultural controls
Small numbers are easiest dealt with by hand. UC IPM notes that most sawfly larvae feeding in the open are easy to kill with a thorough spray of almost any insecticide, so the gentle ones are enough. Minnesota warns that roses may be in bloom when larvae are present, so protect pollinators and keep to low-impact options.
- Pick larvae off and drop them into a pail of soapy water, or crush them on the leaf
- Knock them off with a strong jet of water from the hose
- Clip off the few leaves that carry most of them
- Spray young larvae with insecticidal soap, horticultural oil or neem, covering both leaf surfaces
- Keep roses well watered and fed so they regrow quickly after an attack
Chemical, last resort
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
UC IPM Pest Notes: Roses: Insects and Mites, UC ANR Publication 7466
UC IPM Home and Landscape, Sawflies
UMN Extension, Sawflies (J. Hahn & J. Lloyd, reviewed 2026)
Univ. of Wisconsin–Madison Extension, Sawflies (P.J. Liesch & S. Mahr)
RHS Advice: Rose slug sawfly or slugworm; Large rose sawfly
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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