Leaf miners
Agromyzidae & others
Larvae that tunnel pale wandering trails or blotches inside the leaf itself. Pick off and destroy mined leaves; damage is usually cosmetic.
How to know it
You never see the insect, only the mine: a pale winding trail widening as it goes, or a dry brown blotch, sandwiched between the two skins of the leaf. Hold the leaf to the light and the grub — or its black frass trail — is visible inside. The adults are small flies, moths or sawflies depending on the species.
What you'll see
- White or pale-green wandering trails inside the leaf that cannot be rubbed off.
- Dry brown blotches within the leaf, with the surfaces intact above and below.
- A tiny grub visible inside the mine when the leaf is held up to the light.
- Beet, chard and spinach leaves blistered and browned, spoiling the crop.
- Holly, celery, columbine and chrysanthemum leaves scribbled over in summer.
Where it comes from
Eggs are laid into the leaf tissue and the larvae feed between the leaf surfaces, protected from weather, predators and contact sprays alike. They pupate in the mine or drop into the soil; several generations run through the summer, with the last overwintering as pupae in soil or leaf litter.
Everywhere, every summer, at a low level. Numbers build on crops grown in the same ground year after year, and under glass where generations never pause.
Dealing with it
- Pick off and bin mined leaves as soon as you see them — this removes the larva with the leaf and is the whole treatment.
- Squash the grub inside the mine between finger and thumb if the leaf is worth keeping.
- Accept it on ornamentals: the mines are disfiguring, and the plant is not troubled.
- Grow a fresh sowing of beet or chard rather than nursing a mined one through.
- Do not spray — nothing contact reaches a larva sealed inside a leaf.
Keeping it out
- Cover beet, chard and spinach with fine insect mesh in early summer, when the flies are laying.
- Rotate leaf crops and clear old leaves and debris at the end of the season.
- Encourage parasitic wasps, which parasitize leaf-miner larvae inside their mines.
- Cultivate lightly under affected plants in winter to expose the pupae.
- Sow a little thickly and thin out the mined seedlings.
What it goes for
Beet, chard, spinach, celery, leeks, tomatoes, holly, columbine, chrysanthemums and lilac.
- Amaranth
- Beet
- Calamondin
- Celery
- Chestnut
- Citrangequat
- English holly
- False Acacia
- Good King Henry
- Hardenbergia
- Hawthorn
- Hornbeam
- Horse Chestnut
- Laburnum
- Leek
- Leucospermum
- Lilac
- Lovage
- Mandarine
- Orache
- Pomelo
- Quinoa
- Samphire
- Sorrel
and 4 more in the catalog
Who hunts it for you
- 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