Q: My rose leaves have white spots and holes in them. What causes this and how do I treat it? Is there a natural remedy that does not involve powerful chemicals?
Answer: It looks like your rose has symptoms of sawfly damage. Check the undersides of the leaves and look for tiny green larvae that look like caterpillars. These are the juvenile stage of an insect called rose slug sawfly.
Rose slug sawflies are neither slugs nor flies. They belong to the same order of insects as wasps, bees, and ants (Hymenoptera). Adult female sawflies use their unique ovipositor (egg-laying part) to saw a small slit in a leaf or stem where they lay their eggs. When the eggs hatch, the larvae feed on the leaf surfaces and cause an etched appearance. Some rose slug larvae chew through leaves entirely. Damaged foliage turns brown and curls up as the season progresses.
In Maryland, there are three species of rose slug sawflies that cause damage to roses: the bristly rose slug sawfly, the rose slug sawfly, and the curled rose sawfly. Most of the feeding activity on roses in Maryland is seen in May and June, but sawfly larvae can continue to be active until fall. Other insects, such as Japanese beetles, also cause chewing damage on rose foliage (typically in June-July).
The best way to manage rose slug sawflies without chemicals is to monitor your plant(s) for damage symptoms and manually remove any larvae (squish them or toss them). Insecticidal soap, horticultural oil, and spinosad work well against these sawflies. These products are environmentally friendly insecticides listed by the Organic Materials Review Institute (OMRI). As with any pesticide, read and follow the label instructions carefully. Avoid sprays when your roses are in bloom, to protect pollinators and other beneficial insects.
Predatory insects and parasitoids help regulate sawfly populations naturally. Adding more flowering plant diversity to your landscape will provide food and habitat for beneficial insects that in turn will help reduce pest problems.
Rose Slugs on Shrubs | UME Home & Garden Information Center
Rosie Defoliators | Bug of the Week, University of Maryland, Department of Entomology
Rose Insects & Related Pests | Clemson Cooperative Extension
Sawflies | University of Wisconsin-Madison
By Christa K. Carignan, Maryland Certified Professional Horticulturist, Coordinator, University of Maryland Extension Home & Garden Information Center
Have a plant or insect question? University of Maryland Extension’s experts have answers! Send your questions and photos to Ask Extension.