Boxwood leafminer
Monarthropalpus flavus
Blistered, yellow-blotched box leaves with a grub inside each swelling. We carried box blight and not this — the American half of the same problem.
How to know it
Hold a leaf up to the light: a blistered, swollen area with a small orange maggot visible inside it. From above the leaf shows a yellow or brown blotch and a distinct bulge on the underside. In spring the adults — tiny orange gnats like miniature mosquitoes — swarm around the bushes for a few days as the new growth breaks.
What you'll see
- Yellow or brown blotches on box leaves, with a raised blister on the underside.
- Leaves smaller than normal, distorted, and dropping early.
- A small orange maggot inside the blister when the leaf is held to the light or split.
- Clouds of tiny orange gnats around the bushes for a few days in mid-spring.
- A hedge looking thin, off-color and generally unhappy, with the worst of it on the older leaves.
Where it comes from
One generation a year. The adults emerge in spring as the new growth breaks, mate and lay within a day or two, inserting eggs into the new leaves. The maggots mine inside those leaves all summer, autumn and winter, pupate in early spring, and emerge — so for ten months of the year the insect is inside the leaf and out of reach of anything you could spray.
Common throughout the eastern and central United States wherever box is grown, and the most serious insect pest of boxwood there. American box and its cultivars suffer worst; Korean and littleleaf box are far more resistant.
Dealing with it
- Shear the hedge in late spring, immediately after the adults have flown and before the eggs develop — the clippings take most of the year infestation with them.
- Bag and remove the clippings rather than leaving them under the hedge.
- Time any spray to the few days when the adult gnats are swarming; nothing else in the year works.
- Feed and water a heavily mined hedge so it can carry the load.
- Accept light mining; a vigorous box hedge tolerates a good deal of it.
Keeping it out
- Plant resistant cultivars: the Korean and littleleaf boxwoods, and cultivars such as Green Beauty and Winter Gem, are markedly less affected.
- Inspect nursery box for blistered leaves before buying; this is how it arrives.
- Prune annually rather than letting the hedge grow away, which keeps the mined leaves being removed.
- Keep box out of hot dry positions, since stressed plants show the damage worst.
- Watch for box blight at the same time; the two together will finish a hedge.
What it goes for
Boxwood only — American box worst, Korean and littleleaf box far less so.
Who hunts it for you
- Garden birdsPasseriformes Both things are true: birds strip your currants and a brood of blue tits takes several thousand caterpillars out of one fruit tree in a fortnight. Net what you must and welcome them everywhere else — a nest box near the apples is worth more than a spray. Grow to recruit it: Echinacea,Elderberry,English holly,Goldenrod,Rudbeckia,Sunflower
- Parasitic waspsBraconidae & Trichogramma Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled. Grow to recruit it: Cilantro,Dill,Fennel,Lovage,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com