There are three types of insects and mites that attack boxwoods:
Boxwood psyllid: These insects overwinter as eggs inserted in the boxwood’s buds. The nymph, the psyllid’s immature stage, may start feeding through the egg in late fall through late May. As they grow, the nymphs produce a white waxy coating and can be found pressed against the inside area of the cupped leaf. There is only one generation per year.
Boxwood leafminer: The adult is a delicate, light orange-colored midge (fly). It overwinters in the leaf as a pupa. In spring, pupae can be seen sticking partially out of the leaf’s underside around the time native dogwoods are flowering (early May in central North Carolina).
When adults emerge in a few days, they will insert more eggs into the undersides of the afflicted boxwood leaves. The orange larvae maggots feed inside the leaf for the rest of the year. Damage starts with slight blisters, then progresses to yellow discoloration, then to brown. Premature defoliation and twig dieback can occur.
Boxwood spider mites: These mites feed on specific boxwoods and will not move onto other plant species. They prefer to feed in cool weather and will be active in late winter. They have eight generations per year.
Boxwood psyllid: Damage is usually aesthetic. However, large numbers of boxwood psyllid can cause defoliation. Shearing and pruning will remove the injured foliage.
For all three boxwood pests: It’s recommended that a Plant Health Care technician inspect the diseased boxwoods to determine the best strategy. In early spring, one foliar application with a new product may control all three pests. Soil injections of an insecticide with Imidacloprid will help reduce leafminers and psyllids, but not spider mites. Repeated applications may be needed to reduce spider mites with either management technique.