Rose sawfly (rose slug) — damage and treatment

Severity: medium

What is Rose sawfly (rose slug)

Rose sawfly (rose slug) larvae skeletonize leaves by eating soft tissue between the veins, leaving a lace-like appearance. This is an insect pest, not a fungus or a bacterium — no spores, nothing to spread on the wind, and no fungicide will touch it. Despite the "slug" in the name they are not slugs; they are sawfly larvae, feeding on the undersides of rose leaves. Severity is rated medium: the damage is disfiguring and can be extensive across a bush, but it is leaf damage on an ornamental rather than a plant killer.

Symptoms

The feeding pattern identifies this pest on its own. The larvae graze the soft tissue between the veins and leave the veins standing, so the leaf ends up skeletonized — a fine lace-like network where a leaf blade used to be. Before that stage you see translucent window-like patches: the larva has eaten one layer of the leaf and left the opposite surface intact, so light passes through a papery pane. Those windows are the earliest visible sign and the best moment to act. Turn the leaves and you find the culprit: small green slug-like larvae on the leaf undersides, soft-bodied and closely matched to the leaf colour, which is why the damage is almost always noticed before the insect.

• Early: translucent window-like patches on the leaves
• Early: small green slug-like larvae on the leaf undersides
• Advanced: leaves skeletonized with only the veins remaining, lace-like

Skeletonizing means chewing insects, not disease. If the leaf is spotted or coated but intact, look at black spot, rust or powdery mildew instead.

Causes and conditions

Adult sawflies fly to roses and lay their eggs into the leaf tissue; the larvae hatch and feed on the undersides, which is why the damage is missed until the windows show. Populations arrive on the plant rather than building up in the soil beneath it, so a rose can go from clean to lacework quickly once the adults have visited. Feeding runs through spring and summer, and the soft new foliage takes the brunt.

Treatment

Handpicking and water spray — cultural

Spring through summer. Handpick larvae from leaf undersides. Use a strong water spray to dislodge them. Check regularly in spring. On a garden rose this is often the whole answer — the larvae are soft, slow and sit in plain view once you look underneath, and knocking them off with a jet of water ends the feeding.

Spinosad — chemical

At first signs of larvae. Apply spinosad when larvae are first noticed on leaf undersides. Repeat after 7-10 days if needed. Target the undersides of the leaves — that is where the larvae feed, and a spray that only wets the tops will miss them. Spraying in the evening reduces exposure to foraging pollinators, which matters on a plant in flower. Pesticide registrations vary by country — check local approval before use.

Prevention

• Check the undersides of rose leaves regularly in spring, before the windows become lace.
• Deal with the first larvae you find; small numbers are easy, a colonised bush is not.
• Remove and destroy heavily skeletonized leaves rather than leaving them on the plant.

Frequently asked questions

Is this a disease? Will fungicide help? No. Rose sawfly is an insect pest and the larvae chew the leaf — fungicides have no effect on them. The lace-like leaves are feeding damage, not infection.

Is it a caterpillar or a slug? Neither, despite the look and the name. Rose slug is the larva of a sawfly. Identify it by what it is doing — grazing the leaf underside between the veins — rather than by what it resembles.

When should I treat? At the first signs of larvae, which in practice means at the first translucent windows. Handpick or wash them off then; once whole leaves are skeletonized, that damage is permanent for the season.