Boxwood leafminer (family Cecidomyiidae) is a gall midge that as larvae feeds and mines in the foliage of boxwoods, Buxus species.
The adults are small and the eggs, larvae, and pupae occur hidden in boxwood leaves. Therefore, the damage they cause (described below) is generally noticed first.
Adults are delicate mosquitolike flies about 1/10 inch (2.5 mm) long. They have long slender antennae and legs and a body that is reddish to yellowish orange. When abundant the adults are readily visible around and on boxwoods for about 2 weeks in the spring.
Larvae are green, orange, whitish, or yellow. They are legless maggots up to 1/8 inch (3 mm) long. Pupae are dark yellow, orange, or whitish and occur within mined leaves or in spring protruding from the underside of boxwood leaves. When the adult gall midge emerges, its translucent to whitish pupal skin about 1/8 inch (3 mm) long can be observed protruding from a small hole in the mined leaf.
Boxwood leafminer develops through 4 life stages: egg, larva, pupa, and adult. During spring (generally in April), the adults are present, and females lay up to about 20 eggs per leaf into the underside of recently expanded leaves. Eggs hatch in 2 to 3 weeks and the larvae initially feed individually in tissue they chew and mine. As larvae grow through 3 increasingly larger instars, their mines merge and they feed in groups. Larvae feed through the summer then overwinter and later pupate inside their mine. In spring as temperatures warm and boxwoods flush new leaves, the pupa chews a small hole in the leaf through which it protrudes, and the adult gall midge emerges. Boxwood leafminer has only 1 generation per year.
Leaf mines initially appear as pale green or yellow spots on the upper leaf surface. Irregular brownish to green, swollen blisters appear on the lower surface of the leaf where internal portions of the foliage have separated due to the larval feeding. Once larvae are fully grown, translucent to transparent circular spots appear on the under surface of the leaf. Infested leaves appear more yellow and are smaller than healthy leaves.
When boxwood leafminers are abundant they cause leaves to turn brown and drop prematurely. Severely infested plants develop branch dieback and become more susceptible to environmental stresses and plant pathogens.
Provide boxwoods optimal cultural care to keep them growing vigorously and make them more tolerant of pest damage. Appropriate soil moisture during the dry season is especially important. Rake and dispose of dropped boxwood leaves in the fall and winter because some of these may contain overwintering leafminer pupae. If relatively few leaves on the plant are mined, these can be clipped or picked off and disposed of.
Consider replacing susceptible species with resistant plants. Boxwoods resistant to the leafminer include:
- Buxus harlandi ‘Richard’
- Buxus microphylla ‘Green pillow’ and ‘Grace Hendrick Phillips’
- Buxus microphylla var. japonica
- Buxus microphylla var. sinica ‘Franklin’s Gem’
- Buxus sempervirens ‘Argenteo-varigata’, ‘Handworthiensis’, ‘Justin Brouwers’, ‘Pendula’, ‘Pyramidalis’, ‘Suffruticosa’, and ‘Vardar Valley’
- Buxus sinica var. insularis ‘Nana’
Where valued boxwood are being intolerably damaged by this pest, dinotefuran or imidacloprid can be drenched onto soil around the trunks of boxwoods in late winter as directed on product labels. In spring, spinosad can be sprayed onto foliage where it will move short distances into plant tissue. Adding horticultural oil to the spray mix can increase the efficacy and persistence of spinosad. Note that dinotefuran, imidacloprid, and spinosad are toxic to bees and certain natural enemies of pests. Because the neonicotinoids dinotefuran and imidacloprid move systemically in plants to contaminate nectar and pollen and spinosad persists some on plant tissue, wait until boxwoods have completed their spring flowering before making the application. Apply spinosad in the evening when honey bees are not active.
Adapted from Common Name: Boxwood Leafminer Scientific Name: Monarthropalpus flavus (Schrank) (Insecta: Diptera: Cecidomyiidae) from the University of Florida and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Severe feeding damage of boxwood leafminer larvae.
Mines of boxwood leafminer larvae.
Larvae of boxwood leafminer exposed in their leaf mines.
Adult boxwood leafminer resting beneath a boxwood leaf.