Boxwood is a widely used landscaping plant, particularly in historical gardens or to accent Colonial architecture. Varied forms of boxwood permit its use as a screen, a framework for a formal garden, an outline or border and even as a topiary piece. There are two primary flying insect pests of boxwood.
The boxwood leafminer is boxwood's most serious pest. The leafminer is the larva of a small, mosquitolike fly. This adult is orangeish and less than 1/8-inch long. These adults are often visible swarming around the boxwood after they emerge from the leaves where they overwinter. The leafminer adults, or flies, only live a few days, but the larvae, which persist all summer, cause the leaves to develop a blistered appearance.
This pest is a 1/8-inch-long greenish insect with clear wings and strong legs. The psyllid hops or flies away when the boxwood is disturbed. Feeding nymphs cause leaves to cup and stunt leaf growth, but plants generally outgrow psyllid injury.
Although neither of these two pests generally require any treatment, if a plant is suffering from poor culture in addition to the insect, a pesticide may be considered necessary. To mitigate leafminer damage, use a spray at adult emergence or a soil drench nearly anytime, or choose less susceptible varieties. The psyllid can be treated with repeated horticultural oil applications during the crawler stage.