The adult birch leaf miner is a small, black sawfly. The larvae and black fecal matter are easily seen in the mines when infested leaves are held up to the light. Young larvae have four characteristic black marks on the underside of the body. These marks disappear when full growth is attained. The adults are very short-lived and are rarely seen except during periods of egg laying at which time they fly and crawl over the leaf surfaces.
Females begin egg laying as soon as the leaves unfold from the bud.
The eggs are inserted singly inside the soft, newly expanding leaves. After a week or ten days the eggs hatch and the tiny larvae begin to feed on the tissue between the upper and lower leaf surfaces. As the larvae feed, the individual blotches or blisters become larger. The mines are often united to form a single large area in which the larvae then live together. After a period of one to two weeks the larvae mature and stop feeding.