Cabbage looper
Trichoplusia ni
A smooth green caterpillar that arches its back into a loop as it walks and eats large ragged holes in brassica leaves. Bt sprayed while they are small settles it.
How to know it
A pale green caterpillar up to 40 mm long, smooth, tapering toward the head, with two thin white lines down the back. The tell is the walk: it has no legs in the middle of its body, so it humps into a loop with every step. The adult is a mottled gray-brown night moth with a small silvery figure-of-eight on each forewing.
What you'll see
- Large ragged holes chewed between the veins, rather than the neat rasping of a young cabbageworm.
- Dark green pellets of frass caught in the heart of a cabbage or lettuce.
- Caterpillars sitting still along a midrib on the leaf underside, looping when prodded.
- Papery white cocoons spun into a fold of a lower leaf.
- Damage that runs on well past the first frost in a mild autumn, when the cabbage whites have finished.
Where it comes from
It cannot overwinter where the ground freezes hard, so northern gardens are recolonized each summer by moths drifting up from the south. Eggs are laid singly on leaf undersides, the caterpillar feeds for two to four weeks, and it pupates in a thin cocoon on the plant. Three or more generations run through a long season.
Late summer and autumn are the worst of it, when several generations overlap and the brassicas are big enough to hide a caterpillar. It has a far wider taste than the cabbage white and will move onto lettuce, spinach and tomatoes when the brassicas run out.
Dealing with it
- Spray Bt (Bacillus thuringiensis kurstaki) in the evening, covering the leaf undersides, and repeat weekly while caterpillars are small.
- Hand-pick — they are large, slow and easy to find once you know to look along the midrib.
- Squash the eggs, single pale domes on the leaf underside, on your weekly walk round.
- Leave the white cocoons of parasitic wasps riding a caterpillar; that one is already finished.
- Strip and compost the worst outer leaves rather than trying to save every leaf on a cabbage.
Keeping it out
- Cover brassicas with fine insect mesh from planting out, sealed at the ground.
- Grow nectar plants — alyssum, dill, yarrow — to keep the parasitic wasps and tachinid flies resident.
- Clear brassica stumps and volunteers at the end of the season, along with any lettuce left to bolt.
- Interplant strong-scented herbs and let the flowers open; the moths hunt by scent at dusk.
- Inspect bought brassica plants before they join the bed — a looper often arrives on one.
What it goes for
All brassicas, plus lettuce, spinach, beans, peas and occasionally tomatoes.
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
- Spined soldier bugPodisus maculiventris A predatory stink bug — same shield outline as the pest, but with a sharp spine at each shoulder and a beak it uses to spear caterpillars and beetle grubs. Learn the spines before you squash a shield bug, because this one is on your side. Grow to recruit it: Dill,Fennel,Sweet alyssum,Yarrow
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com