How to identify and deal with Cabbage white caterpillars

Cabbage white caterpillars

Pieris brassicae & Pieris rapae

also called Cabbage worm, Cabbage looper

Green or mottled caterpillars of the white butterflies that skeletonize brassica leaves in summer. Net crops with fine mesh before the butterflies lay and pick off caterpillars and yellow eggs.

Insect Leaf-eater Serious SpringSummerAutumn

How to know it

Two culprits: the large white lays yellow eggs in batches and its hairy yellow-and-black caterpillars feed in gangs on the outer leaves; the small white lays single eggs and its plain velvety-green caterpillar burrows quietly into the heart, where it is far harder to find.

What you'll see

Large ragged holes eaten between the veins until the outer leaves are lace.
Batches of upright yellow eggs, or single pale ones, on the leaf undersides.
Heaps of green-black droppings caught in the leaf axils and in the heart of the plant.
Caterpillars tunnelled deep into cabbage and sprout hearts, fouling what they do not eat.
White butterflies quartering the bed on any warm day from May onward.

Where it comes from

Adults fly from May to September, with two or three overlapping generations; eggs hatch in one to two weeks and the caterpillars feed for three to four before pupating on walls, fences and stems. The large white overwinters as a chrysalis in the garden, and migrants top the population up from the continent each summer.

Any warm sunny summer, and worst on unnetted brassicas in an open, sunny bed — which is exactly where brassicas are meant to grow. Damage accelerates through August.

Dealing with it

Rub off the egg batches whenever you pass; a minute a week in June saves the crop in August.
Pick the caterpillars off by hand — check leaf undersides and dig into the hearts where the small white hides.
Cover the crop with fine insect mesh, held clear of the foliage on hoops so the butterflies cannot lay through it.
Spray Bacillus thuringiensis onto young caterpillars if numbers get away from you; it affects nothing but caterpillars.
Leave the parasitized ones — a caterpillar carrying white wasp cocoons is already working for you.

Keeping it out

Net from the day the plants go out, not from the day you first see damage.
Bury or firm the mesh edges: a butterfly walks in under a loose net without hesitation.
Keep the mesh from touching the leaves, or eggs get laid through it.
Grow nasturtiums as a sacrificial trap crop away from the brassica bed.
Clear brassica stumps and old leaves at the end of the season rather than leaving a nursery standing.

What it goes for

Leaves

Every brassica — cabbage, kale, sprouts, broccoli, cauliflower, swede, turnip — and nasturtiums, which they love as much.

and 8 more in the catalog

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