Birch leafminers, Fenusa pusilla (Lepeletier), are among the most common insects affecting gray, paper, river, and European white birches. The birch leafminer is not a native insect but one accidentally introduced from Europe. The leafminers feed inside the leaves, forming what is known as blotch mines. Partial or whole areas inside the leaf are consumed in blotch mines, which makes them different than serpentine leaf mines, which form meandering lines through the leaves.
Normally a healthy tree can lose part or nearly all the current crop of leaves without being seriously weakened; but repeated losses, year after year, will weaken the tree, resulting in death, or will make it susceptible to the attacks of other insects, especially the bronze birch borer, which will hasten the death of the tree.
Source: The University of Minnesota Extension, University of Connecticut Extension