Leaf-eating caterpillar

Where the tiny new caterpillars feed together they scrape the leaf down to a thin, pale, papery skin — hold it up to the light and it looks lacy, like a little window — because only the soft green flesh is eaten and the tough veins are left behind. It shows up on one or two leaves first, right where the egg cluster hatched.

The neat windows turn into ragged holes and chewed edges, eaten from the leaf margin inward, until a badly-hit plant is left as bare stalks and veins. Look on the leaves below and in the leaf joints for dark green or black droppings like coarse pepper — and remember fresh, moist droppings mean a caterpillar is still feeding nearby.

It is a smooth, soft grub — a few millimetres when newly hatched, growing to about 3–4 cm (a finger-joint) long — coloured green, brown or near-black, usually with pale stripes running down each side. Turn the leaves over: they hide on the underside by day, and some drop to the litter at the plant base, doing most of their eating in the cool of early morning, evening and night.

On the underside of a leaf, look for a flat patch of eggs, often covered in a fuzzy buff or brown 'felt' (scales the moth brushes over them for cover). Just after they hatch you will find a tight crowd of tiny caterpillars on that same leaf — the cheapest moment of all to stop the whole family at once.

Grasshoppers and locusts also chew leaves but leave no caterpillar and no droppings, and they jump away as you reach in. Snails and slugs leave a shiny slime trail and no pepper-like droppings. A single fat green tomato hornworm (with a horn on its tail) or a grey cutworm (that curls into a 'C' and cuts young plants off at the soil line) come one at a time — the leaf-eating caterpillar arrives as a crowd from an egg patch and eats the leaves up top.

Start checking at the seedling or just-transplanted stage, and keep it up right through the leafy growing stage and into flowering — a young plant with few leaves can least afford to lose any. Look harder in warm weather with soft, lush growth, and in the days after you notice moths flying at dusk.

Cross the field in the shape of a 'W' so you cover all four corners and the middle — not just the edge by the path where you always glance. Make about 10 stops along the way.

Turn the leaves over to find egg patches, crowds of tiny caterpillars and daytime hiders; look for lacy 'window' leaves, fresh chewing and fresh moist droppings; and part the growing point (the cabbage heart, the top leaves) and poke the litter at the base where they shelter in the heat of the day.

Treat when you find small caterpillars or fresh feeding on 2 in every 10 plants (20%), or when plants have lost roughly 15–20% of their leaves (about one leaf in six) before flowering — whichever comes first. And read the droppings: fresh, moist droppings = a caterpillar still there right now; dry old droppings with no caterpillar is a past attack, so don't spray for that.

One fresh egg patch, or a crowd of newly-hatched caterpillars on a single leaf, is your cheapest early signal — pick and crush that one leaf on the spot, before the crowd spreads across the plant and forces you into a spray.

After harvest, clear away crop residues and leafy weeds, which shelter the moths and their eggs. Plant early and all at once with your neighbours so the crop is sturdy before moth numbers peak, and on vegetable seedbeds lay a light net or cover (a 'floating row cover') over the young plants so the moths cannot land and lay.

Birds, ants and tiny parasitoid wasps — wasps that lay their eggs inside the caterpillars and kill them from within — do a lot of the work for free. So avoid broad, kill-everything sprays that wipe them out and leave you to fight the pest alone.

Because the young caterpillars feed in a tight crowd on one leaf, you can kill dozens at once by picking and crushing that leaf (or the fuzzy egg patch) early each morning. Drop any strays into a tin of soapy water. Do it while they are still small — it costs nothing and it is the difference-maker.

Bt (Bacillus thuringiensis) is a natural soil bacterium that sickens only caterpillars and is harmless to you, to bees and to the helpful insects. It has to be eaten to work, so wet the surfaces they chew — especially the leaf undersides — and spray in the cool of morning or evening, out of the hot sun (strong sunlight breaks it down). It works well on small caterpillars and poorly on big ones, so time it to a fresh hatch and spray again if new crowds appear.

If hand-picking and Bt are not holding the pest at 2 in 10 plants, reach for a selective insecticide (one aimed at caterpillars that spares bees and natural enemies) rather than a broad poison, and aim it at the leaf undersides while the caterpillars are still small. Protect yourself: cover skin, eyes, nose and mouth; spray in the cool of the day with the wind at your back so it blows away from you; keep it off food and away from water, children and bees; then wash yourself and your clothes straight after. Respect the pre-harvest interval — the number of days the label tells you to wait between spraying and picking — which matters most on cabbage and tomato you eat directly. And rotate to a different chemical group each time (never the same one over and over), or the caterpillars build resistance and shrug the poison off.

New caterpillars turning up after a spray usually means fresh moths laying or egg patches you missed — go back over the leaf undersides, remove the patches, switch to a different chemical group, and widen your clean-up to nearby weeds and other host crops (cotton, soybean, other vegetables); ask your local extension officer or agro-dealer for the currently recommended product. When it is truly too late — plants stripped at or after flowering — stop spending on sprays: pull and destroy the worst-hit plants with the caterpillars still on them to break the next generation, harvest anything still usable, and protect your next crop by clearing residues and planting early and clean.