–by Dr. Raymond Cloyd
There have been numerous inquiries regarding insects feeding, and completely devouring rose plants. These are sawflies, and there are at least two species that attack roses this time of year: the rose slug (Endelomyia aethiops) and bristly rose slug (Cladius difformis). Rose slugs are the immature or larval stage of sawflies, which are black to yellow-colored wasps.
Rose sawfly females create pockets or slits along the edges of rose leaves with their saw-like ovipositor (egg-laying devise), and insert eggs. Larvae hatch from eggs and resemble small slugs. Larvae are 1.2 cm long when full-grown and yellow-green, with an orange head (Figure 1). Larvae eventually fall onto the soil surface and pupate. Rose slugs overwinter as pupae in earthen cells created by the larvae. Bristly rose slug larvae are pale-green and 1.5 to 2.0 cm in length. The body is covered with numerous bristle-like hairs (Figure 2). There is generally one generation per year in Kansas for both species.
Figure 1. Rose Sawfly Larvae Feeding on Rose Leaf (Author-Raymond Cloyd, Kansas State University)
Rose slug larvae feed on the underside of rose leaves; resulting in the leaves appearing skeletonized (Figures 3 and 4). The larvae eventually they create notches or holes on the leaf margins. Bristly rose slug larvae feed on the underside of rose leaves and also cause leaves to appear skeletonized. However, the larvae may chew larger holes than the rose slug.
Figure 2. Bristly Rose Slug Larva Feeding On Leaf Underside of Rose (Author-Raymond Cloyd, Kansas State University)
Figure 3. Damage on Rose Plant Caused By Rose Slug Larvae Feeding (Author-Raymond Cloyd, Kansas State University)
Figure 4. Damage on Rose Leaf Caused By Rose Slug Larvae Feeding (Author-Raymond Cloyd, Kansas State University)
Small infestations of either the rose sawfly or bristly rose slug can be removed by hand and placed into a container of soapy water. A forceful water spray will quickly dislodge sawfly larvae from rose plants, and they will not crawl back onto rose plants. There are a number of contact insecticides containing various active ingredients that may be effective in suppressing populations of both sawflies. However, the bacterium, Bacillus thuringiensis subsp. kurstaki (Dipel® or Thuricide®) will have no activity on sawflies as this compound only works on caterpillars.