Boxwood was brought to the United States from Europe during the 1800s and at some point the boxwood leafminer was introduced along with some of this planting stock. It was first confirmed in the U.S. in 1910 and now occurs throughout the country wherever boxwood is grown.
Boxwood leafminer is one of the most damaging boxwood insect pests. In spring, the larvae that overwintered within the mined leaves become bright orange pupae and emerge as adult flies when the Weigela begins to bloom. The flies are tiny, delicate and yellowish-orange. They look like gnats. Females begin egg laying shortly after emergence and die soon after completing this task. White to clear eggs are laid deep in the leaf tissue. When the yellowish-white larvae hatch they begin feeding on the tissue within the leaves and grow slowly during the summer. They then overwinter within the mines completing the life cycle. There can be many larvae within a single mine or leaf.
Egg-laying punctures can be seen on the leaves as tiny yellow to brown spots. As feeding progresses leaves develop discoloration and blistering. Some defoliation may occur. Severely affected boxwood become unattractive as they defoliate and lose their nice dense appearance.
Some boxwood are more susceptible to this pest than others. Common boxwood (Buxus sempervirens) and littleleaf boxwood (B. microphylla) are generally susceptible but there are some resistant cultivars including ‘Handworthiensis’, ‘Pyramidalis’, ‘Suffruticosa’ and ‘Varder Valley’ (B. sempervirens). B. microphylla var. japonica shows some resistance. If you have had trouble with boxwood leafminer and are considering adding new boxwood to your landscape, look for resistant varieties.
The boxwood leafminer has few natural enemies and no biological control products are currently available. Overwintering larvae and pupae can be removed by pruning before adult emergence or soon after egg-laying to reduce the population. Insecticides are available to control this pest when needed and should be applied while Weigela is in bloom. Weigela is pictured so you’ll know what to look for if you’re unfamiliar with it.
By J. Allen