The adult birch leaf miner is a tiny, black, fly-like insect approximately 1/8 inch long. Unlike true flies, it is a sawfly—a primitive type of stingless wasp.

The birch leaf miner ( Fenusa pusilla ) is a small, invasive sawfly native to Europe that has become a widespread pest of birch trees throughout North America. While primarily an aesthetic concern, chronic infestations can significantly weaken trees, making them vulnerable to more lethal threats like the bronze birch borer. Biology and Identification

Birch leaf miners typically produce two to four generations per year, depending on the climate. Birch Leafminer [fact sheet] - UNH Extension

: Larvae live and feed between the upper and lower surfaces of the leaf, "mining" the soft inner tissue while leaving the outer layers and veins intact. Life Cycle and Damage