The adults of birch, elm, and alder leafminers are out in abundance this spring. These closely related sawflies live within the leaf as larvae, feeding between the upper and lower leaf surfaces.
These sawfly leafminers spend most of their life cycle burrowed into the ground. The adults emerge in spring to lay eggs in leaf tissues. After about a week, the eggs hatch and young larvae begin to make mines in the leaves. The mines look like brown elongate or roundish spots between veins in the leaf. Eventually these insects will eat a hole through the leaf epidermis, fall to the ground, and excavate a hole in the soil. Elm leafminer attacks American elm, English elm, and Scotch elm.
A systemic insecticide is necessary to obtain control of the larvae within the leaves. Acepate, sold as Orthene, traditionally has been used (avoid using Orthene on American elm), although imidicloprid, sold as Merit and Marathon, is gaining favor among professional applicators. Unfortunately, to get the best efficacy out of Merit against the first generation of these leafminers, you should apply it in October or November of the previous fall.
(Other contributors to this article: Dave Shetlar, The Ohio State University; Donna Danielson and Karen Jacobs, The Morton Arboretum)