Damage starts to appear about midsummer as yellow spots. Winding tracks (mines) through the leaves are at first whitish, turning brown as they age. Leaves may fall off early. In addition, heavy infestation can cause plants to look thinned out; even killing branches. Heavy infestation make the plants more prone to winter kill and diseases.
These miners over winter as a partially grown larvae in the boxwood leaves. Warm weather in the spring helps the larva finish growing and become a pupa. A few days before the adult emerges, the pupa wiggles out of the mine to the surface of the leaf. A fly emerges from the pupa case. The flies are about the size of a gnat. After mating, females lay slightly more than two-dozen eggs inside the upper tissue of new leaves. Several weeks later, the larvae emerge and begin feeding. It is not uncommon to find multiple larvae in the same mine. Larvae grow slowly through the summer.
There are few natural predators but there are boxwoods known to be resistant to this insect.