Normally, a healthy tree can lose part or nearly all of the current crop of leaves without being seriously injured. Repeated leaf losses year after year may kill or seriously weaken the tree, risking attack by fungi or by serious pests such as the bronze birch borer. This borer will kill birch trees. 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.