How to know it

A pale green caterpillar only 10–12 mm long, tapering at both ends, that wriggles violently and drops on a silk thread when touched. The feeding is distinctive: it grazes the underside and leaves the upper skin intact, making a translucent window rather than a hole. The moth is small and gray, with a row of pale diamonds down the back when the wings are folded.

What you'll see

• Small translucent windows in the leaf where the underside has been grazed away.
• Windows tearing through into ragged holes as the leaf grows.
• Very small green caterpillars dropping on silk threads when the plant is shaken.
• Papery net-like cocoons the size of a grain of rice on the leaf underside.
• Damage concentrated on the growing point of young transplants, which can stall them altogether.

Where it comes from

It cannot overwinter where the ground freezes hard and recolonizes the north each summer on the wind. A generation takes as little as two weeks in warm weather, and there may be six or more in a season, which is why it is the most insecticide-resistant crop pest in the world. Eggs are laid singly on leaf undersides.

Hot dry summers speed it up dramatically; heavy rain kills the small caterpillars outright. Brassicas grown in succession through the summer, and any garden where the cabbages are never covered, take the worst.

What it goes for

All brassicas — cabbage, kale, broccoli, cauliflower, Brussels sprouts, collards, radish, turnip and rocket.

• Broccoli
• Brussels sprout
• Cabbage
• Cauliflower
• Collards
• Kale
• Radish
• Turnip

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

• Cabbage white caterpillars
• Cabbage looper
• Flea beetle
• Caterpillars