The river birch “caterpillars” are not caterpillars at all. Action: The dusky birch sawfly’s primary host is River birch, but sometimes grey, black, red paper and yellow birch are also attacked. It is found throughout the United States and has two generations per year in NC. They are sawfly larvae. Thank you for reviewing this information. As its name suggests, the dusky birch sawfly feeds primarily on birch trees, most commonly river birch in North Carolina landscapes. They LOOK like caterpillars to the uneducated eye but they are not. No intervention is required because the infestations are short-lived. The dusky birch sawfly is an occasional pest of birch trees, particularly river birch. The birch sawfly has only one generation per year. Sawflies are stingless wasps whose larvae are plant feeders. A sawfly is a primitive wasp-like insect. In Quebec, infestations have been very localized and sporadic. This species has at times caused serious defoliation of birch in Nova Scotia and in British Columbia. Right now they are in the larval stage of their second generation. Insecticides can control the pest or small populations can be removed by hand and destroyed.