The dusky birch sawfly is an occasional pest of birch trees, particularly river birch. Sawflies are stingless wasps whose larvae are plant feeders. In addition to the dusky birch sawfly, there are several common sawflies in Iowa that are bothersome to gardeners; the pearslug
whose larvae feed on the leaves of a variety of fruit trees, the roseslug
on roses, the scarlet oak sawfly
that feeds on oak, the willow sawfly, and the European pine sawfly
Sawfly females lay many eggs in a single area, so usually many sawfly larvae can be found feeding together. Sawfly larvae look like caterpillars (immature moths and butterflies) but they lack crochets on their prolegs (feet attached to the abdomen). Crochets are special hooks that caterpillars have on their prolegs to help them cling to plant material. Since sawflies do not have these hooks they do not cling to leaves tightly and often curl their abdomens up in the air when disturbed.
Sawfly larvae have chewing mouthparts that eat away parts of the leaf tissue in the leaves. Some species of sawfly leave a very characteristic feeding damage called window-paning. Basically they eat the green part of the leaf leaving a fine layer of clear leaf tissue. Older damage browns and can sometimes look like a disease lesion. Other species eat holes in the leaves and then eventually entire leaves.
Dusky birch sawflies will have two generations each summer and overwinter in the soil as pre-pupae. Dusky birch sawfly feeding is not harmful to a healthy tree and no control is necessary.
Sawfly larvae typically feed in clusters. Sawfly larvae react to disturbance by putting their tail end in the air. Sawfly larvae have prolegs on the abdomen but no crochets on the ends of the prolegs.