Leaf Miners on American Boxwood
Adult leaf miner flies are present in early spring soon after the boxwood produces new growth. They appear as orange flies about 1/8" long, or about the size of a mosquito. These flies will swarm around and cling to the affected shrubs. They are most efficiently eliminated at this stage, by applying a contact insecticide to the shrub. Be sure to make contact with the underside of the leaves as well at the top. Once the fly lays it's eggs inside the leaves they will need to be treated with a systemic insecticide, such as Merit. Blisters will form on the underside of the leaves as the eggs hatch. Small white worms will feed and grow inside the leaf, eventually turning to a pale yellow. Effected leaves will be discolored, and usually smaller than healthy leaves. Effected shrubs often appear thin and unhealthy.
These are photos of leaf miners on American boxwood. I separated the leaf tissue in the photo on the right to expose the leaf miner.