The Voracious Roseslug

By Therese Ciesinski, GardenSMART

Roseslugs are not slugs, nor are they related to them. All they share besides a name is how much gardeners loathe them. Roseslugs are the larval form of certain species of sawfly. They feed on rose leaves, and though only ½” long at most, it’s hard to believe how much damage these tiny green eating machines can do.

In the U.S., the three most common species are the European roseslug sawfly (Endelomyia aethiops), the bristly roseslug sawfly (Cladius difformis), and the curled roseslug sawfly (Allantus cinctus).

Photograph by Whitney Cranshaw, Colorado State University, Bugwood.org

The larvae devour the soft tissue of rose leaves, skeletonizing them and disfiguring the plant. Picture gorgeous flowers surrounded by tattered leaves; roseslugs don’t eat flowers. A bad infestation can defoliate a young shrub.

Leaves aren’t chewed so much as the green between the leaf veins is nibbled away, leaving translucent “windowpanes” of tissue that eventually turn brown, fall off, and leave holes. Though the damage usually isn’t fatal on established shrubs, it can weaken plants to the point that they become vulnerable to other pests and diseases.

Photograph by Whitney Cranshaw, Colorado State University, Bugwood.org

Sawflies appear in early spring. Females lay eggs on the undersides of rose leaves, and in a few weeks, larvae appear and begin eating. Feeding occurs for about a month, and then the larvae drop into the ground to pupate. Depending on the species, there can be multiple generations per year, however in the Northeast I’ve only seen them in late May and June.

The least toxic control is to pay attention, and remove and destroy leaves when you see signs of feeding. I patrol my roses daily, and squish roseslugs whenever I see one. But their green coloration and habit of feeding on the undersides of leaves makes them hard to detect. Another suggestion is to spray the shrub with a stream of water to dislodge the insects.

Photograph by Whitney Cranshaw, Colorado State University, Bugwood.org

Both organic and synthetic insecticides should be used only in extreme circumstances, because they are toxic to bees and other pollinators that visit the flowers at the same time. Insecticides are effective only when the larvae are present on the leaves, and since they’re not caterpillars, Bt doesn’t work.

Roseslugs are part of the food chain; other insects, including wasps and beetles, birds, and some small mammals eat them, so avoiding chemicals and creating a welcoming habitat for wildlife will help keep things in balance.

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