Boxwood leafminer
Boxwood leafminer is a small orange fly, a gall midge, whose maggots feed between the upper and lower surfaces of boxwood leaves. The leaves develop blisters on the underside, which may or may not discolour, and a heavily infested leaf can swell all over. Heavily infested leaves drop early, and the plant turns thin and ragged. University of Maryland calls it the most significant insect pest of boxwood there. There is one generation a year. Adults emerge in late April or May and lay eggs in the new leaves. The maggots feed inside from June into autumn and spend the winter there. Pruning out infested growth, treating at the right moment and choosing less susceptible boxwoods are the tools. No biological control is sold for it.
Boxwood leaves blistered by Monarthropalpus flavus larvae. Photo: pending.
Identification
Adults are small, gnat-like flies about 2.5–3 mm long, yellow to orange-red. They emerge in spring. The larvae are whitish to lemon-yellow maggots about 3 mm long, living inside the leaf.
The damage is a blotch-shaped mine, not a winding trail: a blister, most visible on the underside of the leaf, that may or may not discolour. A heavily infested leaf may hold several blisters and swell all over.
Easily confused with
- Boxwood psyllidAlso a pestCups the leaves at the branch tips instead of raising blisters; boxwood leafminer makes blotch mines that look blistered from late summer to spring.
- Boxwood spider miteAlso a pestLeaves a fine stippling early in the season and then a dull, dingy look, not blistered mines inside the leaf.
Damage symptoms
The blisters are most visible from late summer through spring. Penn State notes that heavily infested leaves may carry several blisters and swell all over, drop early, and leave the plant looking ragged, with the odd dead twig.
- Blisters on the underside of leaves, from late summer to spring
- Blotch-shaped mines; blisters that may discolour, and swollen leaves
- Early leaf drop, and a thin, ragged look
- Occasional dead twigs
- Small orange flies on the plant in May
Lifecycle & degree days
One generation a year in Pennsylvania and Maryland. Larvae overwinter in the leaf blisters. In spring a translucent 'window' forms in the blister and the larvae pupate. Adults emerge in late April or May. After mating, each female lays about 30 eggs into the new leaves. The eggs hatch in two to three weeks, and the larvae feed inside the leaves from June into autumn. The pest was first reported in the US in 1910 and is now found across the country.
- Generations / yr
- 1
- Adults fly
- Late April–May
- Eggs per female
- About 29 (PSU) to 30 (UMD)
- Overwinters
- Larvae inside the leaf
Host crops
Penn State says most boxwood varieties are susceptible. University of Maryland adds that many cultivars of Buxus sempervirens and of B. microphylla var. japonica are relatively resistant. Neither source names specific cultivars, so check with your local extension office or nursery before replanting a hedge.
Monitoring & action thresholds
No numeric threshold is published. Treat when blistering is widespread and the plant is thinning, and time it to the adult flight or the young larvae.
From late summer to spring, look at the undersides of leaves for blisters and open a few to find the maggots. In spring, watch for the translucent windows that form before the adults emerge. From late April, University of Maryland suggests shaking the branches to see whether the orange adults are flying; Penn State times treatment for May, when weigela is in bloom.
Biological control
There is nothing to buy or release. No biological control for boxwood leafminer appears in the Penn State or Maryland advice, and the larvae are sealed inside the leaf for most of the year. Ohio State notes that birds such as titmice are the main predators, and their pecking for larvae can itself damage the leaves.
Cultural controls
On a home hedge, pruning and timing do most of the work.
- While the larvae are in the leaves, prune an infested plant back by about a third to remove most of them, and bin the clippings. UMD says this should remove the need for an insecticide
- Replace badly affected plants with less susceptible boxwoods; UMD notes that many B. sempervirens and B. microphylla var. japonica cultivars are relatively resistant
- If treating, time it. Spray spinosad when adults are flying from late April (UMD; shake the branches to check), or use a systemic against young larvae from late June, ideally in June before serious damage. Ohio State advises applying systemics after the boxwood has flowered, to protect pollinators, and some leafminer insecticides can flare boxwood spider mites. Follow the label, and check state rules: in Maryland some systemics are restricted to certified applicators
Chemical, last resort
Sources
Penn State Extension, Boxwood Leafminer (G.A. Hoover, 2001)
University of Maryland Extension, Boxwood: Identify and Manage Common Problems (rev. 2020)
University of Maryland Extension, Leafminers on Trees and Shrubs
Ohio State University BYGL, Bad Looking Boxwoods (J. Boggs, 2023)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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