May. The female inserts her eggs only into new emerging leaves. Larvae do not develop in mature leaves.
Initial injury appears as small, blistered, translucent spots on the leaves.
As the larvae feed, the spots enlarge and turn brown. The whole inside of the leaf may be engulfed. When fully grown, the larva exits the leaf and drops to the ground. In the following three weeks, it transforms into an adult sawfly. There are three to four generations each season in Wisconsin.