Roses are increasingly popular with gardeners in Central Indiana. The days of hard-to-grow, finicky tea roses are long gone with the advent of new varieties bred for great resistance to powdery mildew and the dreaded black spot disease. ‘Knock Out’ and ‘Oso Easy’ varieties paved the way for a genuine rose revolution of the new millennium.
While these new roses have great resistance to disease, they are still susceptible to Japanese Beetles and a curious pest called “rose slugs”. Although the latter look like caterpillars, they are actually the pale green semi-transparent larva of a type of sawfly called the Roseslug Sawfly a.k.a. Rose Skeletonizer. Luckily, in this area they have only one generation, and usually by the time the damage is noticeable in May, the larva have matured and disappeared. However, there is another similar pest, the Bristly Roseslug Sawfly with multiple generations that end only with the first frost. Expanding generations can result in heavy defoliation by late summer. Only recently has this more serious sawfly become a concern, possibly because of the warming climate.
To control infestations of sawflies, insecticidal soaps, lightweight horticultural oils, and neem oil are effective, but direct contact is necessary. Products containing spinosad are effective, too. Notably, The organic bacterium Bt is unfortunately not effective as these are fly larvae, not those of a moth. Insecticides applied as a soil drench are effective and provide lengthy protection. There are easy-to-use products available specifically formulated for roses that contain insecticide against rose slugs, aphids, and Japanese beetles, fertilizer for strong roots and blooms, and disease controls for black spot, powdery mildew, rust, and blight.
It takes a few weeks for systemics to be absorbed by plants, so it’s important to apply them by late April to prevent damage to your plants.