Columbine sawfly Pristophora aquiligae
IDENTIFICATION — These larvae of a non-stinging wasp have green heads and bodies without stripes or spots. You’ll often see them on the tops or undersides of columbine leaves in late spring.
DAMAGE — Larvae strip leaves by chewing along the edges toward the inner leaf, eating all but the midveins. Their green color blends in well with the leaves, so you may not notice them until they’ve done a lot of damage.
CONTROL — Many birds eat sawflies, so grow bird-attractors like berry and seedhead plants in your garden. Handpick, or spray insecticidal soap on the tops and bottoms of leaves every few days while you see the caterpillars. If most of the leaves are gone, cut the plant down to the ground and it’ll produce fresh foliage in a few weeks or come back the next year ready to bloom.
If you want, spray spinosad, an insecticide made from bacteria, on the leaves before or after you see the larvae (it works both when the caterpillars eat it or come in contact with it).