Cabbage webworm

look at the centre of the seedling, not the outer leaves. The tell-tale sign is a patch of fine silk webbing over the growing point, matted together with grains of green-brown droppings (frass) — like a tiny dirty spider's web sitting in the heart of the plant.

where the caterpillar has bored in, the central bud goes brown and dies, or the plant grows stunted and crooked. A plant that has lost its bud often sprouts several small side-heads instead of one (a 'blind' or multi-headed plant) — that plant will never make a sellable head.

open the webbing gently and you'll find a small caterpillar only about 1–1.5 cm long (shorter than your thumbnail is wide) — creamy to grey with five faint brownish or purplish stripes running down its back and a dark head. It hides deep in the tunnel, so you may only see its droppings and web at first.

with the diamondback moth caterpillar (small, bright green, wriggles and drops on a silk thread when touched, and chews small windows in the outer leaves — not the heart) or with cutworms (fat, grey, hide in the soil and cut seedlings off at the base). Cabbage webworm is the one that webs and tunnels into the growing point.

watch most closely while plants are at their young seedling and freshly-transplanted stage — this is exactly when the growing point is tender and when this pest does its worst. Pressure is highest in hot, dry weather (around 22–32°C), so step up checks during a dry spell.

This pest often starts in the seedbed and rides into the field on infected transplants. Check nursery beds closely every few days, and inspect every batch of seedlings before you transplant — never move webbed or dead-hearted seedlings into the field.

walk a large "W" shape so you cover the four corners and the middle, not just the edge by the path. Stop at about 10 spots and look closely at the heart / growing point of 10 young plants at each stop.

at each plant, look right into the centre bud for silk webbing, fresh green-brown frass, a browning dead heart, and the small striped caterpillar inside.

the threshold is simple — as soon as you find webbing or larvae at the growing point of your young seedlings or transplants, act now. Even a few affected seedlings at this tender stage is enough to treat, because each caterpillar can destroy the bud of the plant it is in. Don't wait for a big count.

at the first sign of webbing, spray Bt (Bacillus thuringiensis — a natural caterpillar bacteria, harmless to people, bees and helpful insects), aiming it straight into the growing point of the young plants while the caterpillars are still small and feeding near the surface, before they tunnel deep. Bt only works on caterpillars that eat it, so cover the heart well and spray in the cool of the day so it lasts longer.

if webbing is spreading fast and Bt is not holding it, use a recommended insecticide, directing the spray into the heart of the seedlings at threshold, before the larvae bore deep (once they are sealed inside the silk and stem, no spray reaches them). Safety: cover your skin, eyes, nose and mouth, spray in the cool with the wind at your back, keep it away from food, water, children and bees, respect the pre-harvest interval (the wait between spraying and harvest printed on the label — brassica leaves are eaten, so this matters), and wash yourself and your clothes after. Rotate to a different chemical group each time so the pest doesn't build up resistance (stop being killed by it).

re-check that you are hitting the growing point and spraying early — most failures are from spraying too late, after the caterpillar has sealed itself in. Rogue out and destroy the worst dead-hearted plants so they don't shelter caterpillars. Where a nursery or a patch is already badly webbed and blind, cut your losses on those seedlings — pull and burn them rather than transplanting them — and start a fresh, netted nursery on clean ground away from old brassicas.