Bronzing is a normal occurrence on Japanese boxwood in winter. In mild winters, dormant plants that are exposed to direct sun may begin premature cambial activity. Subsequent freezing weather may injure or kill the new tissue, and sometimes cause the bark to freeze and separate from the wood. It is a greater pest in the mountains and piedmont than in the coastal plain. All boxwoods can be infested but slow-growing cultivars are less susceptible.
Infested leaves develop yellowish blotches and are smaller than uninfected leaves.
Leaves first acquire a water-soaked appearance at the feeding site followed by blisters on the lower leaf surface. Leafminers spend the summer and winter in the blisters as larvae. The larvae are small, whitish to lemon-yellow maggots up to 3 mm in length. Larvae pupate in early spring and wiggle through the leaf in May to protrude from the lower leaf surface. In North Carolina, adult flies emerge from the pupae over a two week period in the spring, shortly after boxwoods have begun new growth.