Fenusa ulmi

The elm leafminer is a European species that was introduced into the United States prior to 1898. This insect is a leafmining sawfly whose adults are approximately 3 mm. in length and look like tiny black wasps. In New England, the adult is said to be active in May at which point it lays eggs in host plant leaf tissues through slits cut in the leaf using a saw-like ovipositor (egg laying structure of the females). It takes the eggs approximately 1 week to hatch, and tiny larvae begin to mine blotches in host plant leaves. Mines may appear as tiny, whitish spots confined between leaf lateral veins. Tissues between the two layers of epidermis are eaten. When several mines coalesce, large blotches are formed. By late spring, the larvae have completed their development and will cut their way out of the leaf, drop to the ground, and prepare to pupate in a papery cocoon buried in the soil beneath the host plant. They are thought to remain as pre-pupa through the summer, fall, and winter only to pupate and emerge as adults the following spring. One generation is thought to occur per year.