Walliser: Sawfly larva knows what it likes
Question: I planted a couple perennials, including two columbines. Last summer, one of the columbines disappeared. The leaves were completely eaten.
When it started to happen to the second one, I investigated and found small green worms on the underside of the leaves. I mixed some dish detergent with water and sprayed the plant. The next day, the leaves were gone. It had no effect.
I don't want this to happen to the other plants there. My perennials are starting to look great, and I'd hate to lose them. I love columbines and would like to plant more in the gardens.
Answer: Your columbines were the victim of the larvae of a species of sawfly (Pristophora aquiligae). This particular species of sawfly targets only columbines, so there is no need to worry about damage to your other plants.
The small, green worms you describe are the larvae of a tiny species of non-stinging wasps. There are many species of sawflies found in the garden, with most targeting only one specific species of plant. Other commonly affected plants are roses, pines and perennial hibiscus, though the sawfly species that attack those plants are not the same as the one feeding on your columbines.
Larval sawflies look much like little, green caterpillars. However, because they are not the larvae of a butterfly or moth, they are not true caterpillars. Larval sawflies start off very small and eventually mature to a half-inch or more in length. Their brown heads are another distinctive feature.
Columbine sawflies can quickly skeletonize the leaves of host plants, leaving only the mid-rib behind. Severe infestations can kill a plant, but this is rare. Even when completely defoliated, most plants bounce back. Columbine sawflies have only one generation per year, and they're active primarily during the months of May and June.
To protect your columbines from sawfly larvae, search the plants for the tiny, green, caterpillar-like creatures early and often, starting the first week of May. They're most often found on the undersides of the leaves. Hand-pick any that you find, squishing them or dropping them into a cup of soapy water.
Hanging a wren or chickadee house near the plant will also be helpful. The mother birds love to feed sawfly larvae to their growing young.
As a last resort, organic pesticides based on the active ingredient of Spinosad are very effective against all types of sawfly larvae. Common brand names found at many local garden centers and nurseries include Captain Jack's Dead Bug Brew, Monterey Garden Insect Control, and Conserve Naturalyte.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is www.jessicawalliser.com.
Send your gardening or landscaping questions to email@example.com or The Good Earth, 503 Martindale St., 3rd Floor, D.L. Clark Building, Pittsburgh, PA 15212.