Now is a good time to scout for boxwood leafminer damage and determine if control is needed on your boxwoods this spring. Boxwood is the only known host for this small fly; American boxwood is the most commonly attacked, European boxwood is less susceptible.
Damage is caused by yellow-orange larvae that feed between the upper and lower layers of leaves. Infested leaves appear blistered on the underside and are often discolored. As the larvae matures and the damage progresses, a chlorotic wrinkling may appear on the upper leaf surface as well. Examination of the lower leaf surface in the fall or spring shows that the epidermis can be easily broken open with your thumbnail to reveal the yellow-orange maggots. As many as 28 larvae may be found in a mined leaf, but usually 2 to 6 will be present. Larvae are within their mines from July until the following spring. Small, orange, gnat-like adults emerge in May, leaving holes and pupal skins. The female punctures small holes in the leaves as she lays eggs. There is just one generation per year.
An unusual type of damage occurs with boxwood leafminers that involves plant defense mechanisms. Many times, the plant will attempt to form a callus around the egg and the young leafminer. If the plant is successful, the leafminer is killed, but the small spot of callus remains. Often, the underside of the leaf is covered with these small spots.
Control can be made at two times during the year. A treatment for the adults in May, when they are seen flying about, will give some control. Systemic insecticides can be applied to kill the larvae once they are inside the leaf. This application should be made when the mines are small in early to late June. In addition, if you are selecting new boxwoods to plant, try to select varieties that are insect and mite resistant.