Fine Gardening magazine

Pest Patrol: Rose Slugs

First, know your enemy, then take steps to control it

by Jaret Daniels, Ph.D

Rose slug
Rose slugs are the larvae of sawflies. About 1/2 inch long, they are light green with brownish-orange heads.
Rose slug damage
Young rose slugs nibble on leaves, turning them to skeletons; older larvae take bites.
What do rose slugs look like?
Rose slugs look more like caterpillars than slugs. They are the larvae of primitive wasps called sawflies. There are three species: the bristly rose slug (Cladius difformis), the European rose slug (Endelomyia aethiops), and the curled rose slug (Allantus cinctus). The larvae of all three are light green with brownish- orange heads, and they range in size from 1/2 to 3/4 inch long. The bristly rose slug is usually covered with many short hairs, while the other two species are smooth. When fully grown, rose slugs closely resemble butterfly or moth caterpillars.

What kind of damage do rose slugs do?
Young rose slugs turn rose leaves to skeletons, leaving a clear layer of tissue that eventually turns brown. In addition to being unattractive, the damage also interferes with photosynthesis, thus possibly weakening the plant. Older larvae are larger and are able to chew on leaves, taking bites out of them, as a caterpillar does.

What is their life cycle and how many generations will there be in a year?
Female sawflies (the adult life stage of rose slugs) lay individual eggs in slits along the margins of leaves. When the larvae hatch they begin feeding on the leaf. Once they are fully grown, they drop to the ground and pupate in cocoonlike chambers in the soil, then emerge as sawflies. Depending on the species, there can be one to many generations each year -- one for the European rose slug, at least two for the curled rose slug, and six for the bristly rose slug.

How can I prevent and control rose slugs?
First, check your roses regularly for signs of damage. Like all pest problems, rose slugs are easier to control if you catch them early. For small infestations simply remove the affected leaves and squash the larvae with your fingers. With larger infestations you may need to spray the leaves thoroughly on both sides with insecticidal soap, neem, or horticultural oil. Bacillus thuringiensis (Bt), an insecticide commonly used to combat caterpillars, is not effective against rose slugs.

Jaret Daniels, Ph.D, is an insect ecologist in Bluffton, South Carolina.

Photos: Baldo Villegas

From Fine Gardening #86, p. 80
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