: The destructive stage consists of yellowish-white, legless larvae that grow up to 1/4 inch long.
: 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
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