University of Illinois Extension
Why are there holes in my roses?
Roseslugs
 

[June 27, 2024]   Roses are grown in many gardens for their large, showy, often fragrant flowers. If you grow roses, you may have noticed something eating the leaves of your plants, causing them to become unsightly.

While Japanese beetles often get much of the blame during the summer months, they aren’t the only insects that will feed on the foliage of roses. Roseslugs can also frequently be found feeding on and damaging rose leaves.

What are roseslugs?

Roseslugs are caterpillar-like insects. Three different species of roseslugs can be found in Illinois: the European roseslug (Endelomyia aethiops), the curled rose sawfly (Allantus cinctus), and the bristly roseslug (Cladius difformis).

Despite their common name and appearance, they are neither slugs nor caterpillars but rather sawflies. Sawflies are related to bees, ants, and wasps (Hymenoptera) and are often considered pests because they feed on plants. Sawflies get their common name from their saw-like ovipositor (egg-laying structure) that they will use to cut slits into the stems or leaves of plants to lay their eggs.

Life cycle and damage

European roseslug larvae are light green with orange heads and will grow to be about ½ inch long. They feed on the upper surfaces of leaves, leaving behind the veins and lower leaf surface from May through June. When they are done feeding, they will leave the plant and pupate in the ground, where they will remain until the following spring (one generation per year). The adults will emerge in spring, and eggs will be laid in pockets near the edges of leaves.

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Curled rose sawflies are green with white spots and have yellow heads with a dark stripe down the middle. They will grow to be around ¾ inch long. When small, they will skeletonize leaves and, as they grow larger, will eventually eat the entire leaf except for the main leaf vein. In addition to their feeding damage, when they pupate, they will bore into twigs, particularly those that have been pruned, which can further damage plants. They will often curl up when resting, thus their common name. There are two generations per year.

Bristly roseslugs are green, with fine, hairlike spines, and grow to about ½ inch long. When small, they will feed on the undersides, causing them to have windowpane-like damage (the upper leaf surface is all that is left). As the larvae get larger, they eat the entire leaf except for large veins. When they are done feeding, they spin a cocoon in plant debris or on the plant themselves. There can be six or more generations per year, meaning they are capable of causing extensive damage, especially towards the end of the growing season. Eggs are laid in the midribs of leaves.

[SOURCE/WRITER: Ken Johnson, Horticulture Educator, Illinois Extension]