Be on the lookout now for two leaf-mining insects, the birch leafminer (Fenusa pusilla) and the hawthorn leafminer (Profenusa canadensis). They don’t necessarily cause severe plant damage, but they can ruin the aesthetic quality of landscape plants.
Hosts of the birch leafminer include gray, European white, and paper birch. Hawthorn leafminer primarily attacks Crataegus crusgalli. Both leafminers are generally attracted to healthy, vigorously growing trees that can tolerate damage.
These leafminer adults are small (3-mm-long), black sawflies. Adults are generally present when leaves begin to unfold. Females lay eggs singly in the upper epidermal tissue near the base of a leaf. Eggs are normally laid in the young leaves, almost never in older leaves. The eggs hatch into larvae that feed between the top and bottom leaf layers within the parenchyma cells. The larvae are yellowish orange and grow to 6 to 7 mm long. They generally feed toward the leaf tip and stay close to the leaf margin, creating brown, irregularly shaped, blotched mines. Heavily infested trees appear scorched, appearing as if hit with a blowtorch. There may be three to four generations a year in Illinois. Throughout most of the state, the damage caused by leafminers is not significant enough to warrant control–except in extreme northern Illinois, where they can be a severe problem.
Pest management of leafminers includes planting resistant or tolerant varieties of plants. For example, river birch (Betula nigra) and Dahurian birch (Betula davurica) are less susceptible to attack by the birch leafminer. Several species of Crataegus are tolerant of hawthorn leafminer.
Recommended pest-control materials for managing both leafminers include abamectin (Avid), acephate (Orthene), chlorpyrifos (Dursban), spinosad (Conserve), dimethoate (Cygon), and imidacloprid (Merit, Imicide, and Pointer).