I have received several calls in the past week dealing with roses.
Concerned homeowners have noticed several of their Knockout roses with "brown spots" on them, usually paired with a comment about how they thought Knockouts were supposed to be indestructible. While these roses are known for holding up to insect and disease pressure, they, like every plant, are still susceptible to damage.
After seeing samples of the affected plants, the damage was clearly from an insect, not a disease. Something had chewed away at the underside of the leaves, leaving small spots where the leaves had been "skeletonized."
In many places, all of the green tissue had been eaten away, leaving only a thin layer of clear or brown plant tissue. This damage was caused by the larvae of the rose sawfly, also called a rose slug.
Rose sawflies are small, primitive wasps. Adults have small saw-like structures on their abdomen for egg laying. Adults emerge from the soil in spring and fly to nearby roses. They use their saw-like appendages to insert eggs into newly-forming leaves. The larvae hatch out and begin skeletonizing the rose leaves. Mature larvae can eat all the way through a leaf.
Three different species, the rose slug, bristly rose slug and the curled rose sawfly, all feed on roses. Larvae resemble caterpillars, but differ in the amount of fleshly pro-legs (five or more) and their susceptibility to certain pesticides.
Larval bodies are green and often translucent, slimy, or covered in bristles. Their heads are hard and colored yellow or orange. Mature larvae can reach .5 to .75 inches in length.
Control of rose sawfly larvae begins with visual inspection. As soon as you begin to notice slight feeding damage in spring, treatment should be started. Larvae often feed at night or in the early morning, so initial detection can be difficult.
Light infestations can be treated by simply picking off larvae by hand and throwing them on the ground or into a bucket of soapy water. They can also be sprayed off the leaves with a directed shot of hose water.
Chemical controls for rose slugs are also available. Insecticidal soap can be used to "suffocate" rose slug larvae. Unfortunately, because they are not true caterpillars, B.t. products are not useful against sawflies.
Conventional synthetic insecticides, such as carbaryl, acephate, malathion and cyfluthrin are all effective. Spinosad, a product derived from a natural bacterium, can also be used for rose slug control.
Both sides of the leaves need to be coated for good control. Most of the chemicals listed above will also kill bees and other pollinators, so use caution and try to avoid direct spray contact with the flowers.
Clark MacAllister is the Dawson County extension agent. For more information, call (706)265-2442.