- Boxwood leafminer is the most destructive insect pest of boxwood.
- The larvae of this fly feed on the tissue between the outer surfaces of the leaves. This feeding results in blotch shaped mines in the boxwood leaves. The infested leaves appear blistered from late summer through the following spring.
- New leaves do not show signs of mining until late summer when the larvae are larger.
- By fall, or in early spring, premature leaf-drop may result from heavy infestation.
- Adult leafminers emerge in late April or early May (in Maryland), depending on the weather.
- The adults are small (3mm), orange, mosquito-like flies (see photo above).
- The adult flies emerge over a period of 10-14 days but each fly only lives about 24 hours.
- After mating each female inserts about 30 eggs in the surface of new boxwood leaves.
- The larvae hatch in about 3 weeks and feed within the leaves from June through early fall.
- They spend the winter in the leaves and pupate the following April.
- There is one generation each year.
- Many cultivars of Buxus sempervirens and Buxus microphylla var. Japonica, are relatively resistant to this pest.
Boxwood leaf torn open to show leafminer larvae. Photo: Brian Kunkel, University of Delaware, Bugwood.org
- Pruning boxwood back by about 1/3rd to remove the stems with infested leaves, will help reduce this pest. Dispose of the clippings. This should eliminate the need to use an insecticide.
- It is difficult to control the adult leafminers because of their short adult life stage. Beginning in late April, shake the branches of boxwoods to detect flying adults. When they are present, thoroughly spray the plants with a registered insecticide (spinosad).
- If developing mines are observed in the leaves, larvae can be controlled from late June through the summer by spraying with a registered systemic insecticide. It is best to control larvae in June before serious damage has occurred. Some systemic insecticides may only be applied by certified pesticide applicators, as per Maryland’s Pollinator Protection Act of 2016.