Boxwood leafminer

What is boxwood leafminer?

Boxwood leafminer is a small fly that came to the US on boxwood more than a century ago. The adult is a tiny orange gnat. The damage comes from the maggots, which feed between the top and bottom surfaces of the leaf and make blister-like mines.

Mined leaves turn yellow and spotted and often drop early. Over a few years a heavily infested boxwood gets thin and ragged, with some dead twigs. UMass says weakened plants are also more open to other stresses, disease and winter injury.

There is one generation a year, and the adults are only out for a short time in May. That short window is when control is timed. For new plantings, choosing a resistant boxwood cultivar is the easiest fix.

Signs and damage

• Puffy blisters on leaves, best seen as yellowed areas on the underside
• Leaves spotted yellow and swollen, sometimes several blisters per leaf
• In May, tiny orange gnat-like flies swarming around the shrub
• Small empty pupal skins sticking out of leaf undersides after flies emerge
• Early leaf drop, thin and ragged plants, and some dead twigs

Boxwood leafminer identification: what to look for

• Adult: Tiny (about 2.5 mm) yellow to orange-red fly that looks like a gnat. Out in May.
• Larva: Whitish to lemon-yellow maggot about 3 mm long, inside the leaf blister. Several may share one mine.
• Eggs: Inserted into new leaves; about 29 per female. Penn State says they are easiest to spot by holding new leaves up to the light.
• Damage: Blisters (mines) in the leaves, yellow spotting, swelling, early leaf drop, thin plants and twig dieback.
• Timing: Larvae spend winter in the blisters, pupate in early spring, and adults emerge in May in PA. New mines may not be easy to see until fall.

Life cycle

One generation a year. Partly grown maggots spend the winter inside leaf blisters. In spring a see-through 'window' forms on the blister and the maggots pupate by late April. Adults push out through the bottom of the leaf in May, mate right away, and females lay eggs into the new leaves before dying within hours. Eggs hatch in about two to three weeks, and the maggots feed slowly inside the leaf through summer.

When to look: Watch for flying adults in May (shake the shrub from late April on, or use yellow sticky cards). Check the underside of last year's leaves for blisters any time of year.

Boxwood leafminer look-alikes

A few things get mixed up with it:

• Box tree moth (Cydalima perspectalis): Also attacks boxwood, but it is a caterpillar that chews leaves, ties them with silk webbing and leaves frass. Leafminer damage is blisters inside the leaf with no webbing.

How to control boxwood leafminer

Cultural control

• Plant resistant boxwood. UMass lists Buxus sempervirens 'Handsworthiensis' and 'Vardar Valley' as highly resistant, and 'Memorial', 'Pendula', 'Pyramidalis', 'Suffruticosa' (English boxwood) and 'Argenteovariegata' as resistant or least susceptible.
• Monitor adults with yellow sticky cards or by shaking shrubs from late April.

Biological control

• UMass says few natural enemies attack boxwood leafminer on susceptible plants.

Chemical control

Penn State says to apply a registered insecticide in May, when weigela is in bloom, to hit adults before they lay eggs. UMass says larvae are protected inside the mines, so systemic or leaf-penetrating products work best on them; it lists soil-drench systemics such as imidacloprid, dinotefuran and clothianidin and contact options such as horticultural oil, among others. Apply after bloom and when pollinators are less active. Use only products labeled for this use; the label is the law. Pesticide work for hire needs a licensed applicator; see our pesticide applicator license guide. Muthler Landscaping does not spray or apply pesticides.

Use only products labeled for this pest and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Choose resistant boxwood cultivars for new hedges and beds.

When to call a pro

Call a licensed pesticide applicator if a valued boxwood hedge is thinning year after year from leafminer. Contact your Penn State Extension county office for help confirming the pest.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

Questions about boxwood leafminer

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