Comprehensive DescriptionRead full entry
General DescriptionAdults: Females only, males not recorded. Small black sawflies approx. 4 mm long. Middle tibia whitish, all tarsi black to whitish but variable. Can be distinguished from Fenusa pumila by length of antennae; are longer than thorax in F. dohrnii (shorter than thorax in F pumila). All sawflies of this group posses an ovipositor modified to be used as a saw to facilitate the deposition of eggs under the surface of leaves. Serrula (teeth) of lancet (saw) low and flat but more obvious than those on the saw of F. pumila. Larvae: Creamy white with a black band on the ventral surface of the thorax, more visible in the later feeding stages. The body is covered in small dark spines and with fewer teeth on the mandibles than Fenusa ulmi or F. pumila. There are 6 instars. The three North American Fenusa species complete development inside leaves of the host plant. Therefore, the larval host is an important diagnostic character. Fenusa dohrnii feeds in Alder, F. pumila on Birch and F. ulmi occurs in eastern North America on Elm. Adapted from Smith 1971.