Photo: Jeff Hahn
If you grow columbine in your garden, watch out for a defoliating insect known as columbine sawfly, Pristophora aquiligae. Sawflies are similar in appearance to caterpillars but grow up to be non-stinging wasps. You can distinguish between them as caterpillars have 2–5 pairs of prolegs, fleshy false legs located on their abdomen while sawflies have 6–10 pairs of prolegs. Columbine sawflies are green with greenish heads and lack stripes or spots on their body.
Columbine sawfly larvae are active in May feeding on the leaves of columbine. They start along the edge of leaves and feed until only the midveins remain. A badly defoliated plant will look like a stem with thin sticks protruding out. Because of their green color and the fact that they are often on the underside of leaves during the day, it is easy to miss these sawflies until defoliation becomes severe and obvious.
If you have had a problem with columbine sawfly in the past or want to protect your plants against these insects, check your columbine frequently to detect the larvae when they are first active. The younger the sawflies are when you find them, the less damage they have inflicted on your plants. Once you find them, you can determine what the best course of action is. If you are dealing with a small number of sawflies, an easy solution is to handpick them. Just throw them into a bucket of soapy water to be sure they die.
If physical removal isn't practical, there are insecticides that are effective. Insecticidal soap is good low impact product. When using insecticidal soap, it is important to hit the larvae directly with the spray. There isn't any residual activity so any sawflies that walk onto treated leaves later will not be affected by it. Another low impact product that should be effective is spinosad (e.g. Conserve).
There are also any number of residual garden insecticides that would work against sawflies, such as esfenvalerate, bifenthrin, and permethrin. Columbine sawflies are thought to have one generation so after they are gone in June, you should be done with them for the season.