The adult sawfly resembles a black wasp, is about ½ inch in length with a thick waist and has brown to orangish markings on the abdomen.

Larvae have black heads, gray-green bodies with white undersides, and are about 15–18 mm long when full grown.

In the spring when larch trees are leafed out and the new growth is expanding, the female sawfly can be seen depositing eggs in new shoots near the branch tips.

Adults emerge in late spring and females lay eggs in elongating shoots.

The larch sawfly is native to Europe, however introduced into North America in the mid to late 19th century.