The birch leafminer, Fenusa pusilla (Lepeletier), is an invasive sawfly that was first detected in the United States in 1923. It has since become a major pest of birch in the Northeast as well as the Northwestern states. A sawfly is a wasp with a saw-like organ for laying eggs called an ovipositor. Female birch leafminers use their ovipositor to cut eggs into the leaves. Once hatched, leafminer larvae consume the tissues between upper and lower leaf surfaces causing irregular-shaped, brown “blotch mines.” Healthy trees can withstand occasional attacks by birch leafminer. Repeated attacks can result in the general decline and death of the tree over time.