Sunflower head moth

The clearest sign is a dirty mat of silk webbing spun across the open face of the flower head, tangled with sticky brown droppings called frass — as if a spider had fouled the flower. Peel the webbing back and the seeds beneath are hollow, chewed and tunnelled; a badly webbed head often turns dark and starts to rot.

Part the webbing and florets and you'll find small caterpillars up to about 1.5 cm long — the length of your little fingernail — greyish-green with darker stripes running down the body and a dark head. They stay tucked among the flowers and never crawl out in the open.

It is a small, plain grey moth, narrow and dart-shaped with its wings folded, about as long as a single sunflower seed. You'll spot them resting on or fluttering over the open heads in the evening — almost never in the heat of the day.

— a bigger, fatter caterpillar (up to 4 cm), coloured green, brown or pink with pale side-stripes, that chews large open holes in the head but spins no silk mat. The rule is simple: if the head is webbed together with fine silk and frass, it's the head moth; big holes and no webbing means the bollworm.

warm, dry weather during flowering (around 20–32°C), and late or staggered planting that keeps fresh heads opening while the moths are still on the wing.

The danger runs right through flowering and into early seed-fill — the whole time the heads stand open and the disc flowers show. Walk the field every 2–3 days through this window, and be extra watchful in warm, dry weather, which is exactly when the moths swarm.

They rest and lay their eggs on the heads in the evening, so a midday walk will miss them. Cross the field in a big "W" shape so you cover all four corners and the middle — not just the row nearest the path.

spread along your "W". At each one, look on and around the open face of the head for small grey moths, then gently part the florets and check for tiny eggs, fresh silk, and young caterpillars among the flowers.

Do not wait for webbing and rot to build up — by then the caterpillars are already deep inside the seeds, where no spray can reach them. At flowering, the moths and eggs are your signal to act, not the damage.

The moment you reach the scouting threshold, spray Bt (Bacillus thuringiensis, a natural caterpillar-killing bacteria that is harmless to people and bees) over the opening heads to kill the young caterpillars while they still feed on the surface, before they tunnel into the seeds. Bt only works on small caterpillars, so timing it to first egg-hatch is everything — and spraying in the cool of the evening spares the bees working the flowers.

If moths and eggs are heavy and Bt is not holding them, use an approved pyrethroid. It is a contact insecticide — meaning it only kills the eggs and caterpillars it physically lands on and cannot touch any already tunnelled inside the seeds — so it too must go on at first egg-hatch. Before you spray, cover your skin, eyes, nose and mouth; spray in the cool of the evening with the wind at your back; and keep the chemical well clear of food, drinking water, children, animals and bees. Sunflowers are alive with bees at flowering, so spraying only at dusk — after the bees have gone home — protects them. Respect the pre-harvest interval (the least number of days the label says you must wait between the last spray and harvest, so no poison stays on the seed), and wash yourself and your clothes well afterwards.

If you have to spray more than once, switch to a different chemical group each time (ask for a product with a different active ingredient) so the head moth cannot build up resistance — where the few survivors breed and the spray slowly stops working. Never spray the same product again and again, and never mix a dose stronger than the label allows.

When webbing and rot are spreading across many heads despite your sprays, the caterpillars are already inside the seeds and no spray will save those heads. Cut your losses: harvest the least-damaged heads a little early and dry them hard and fast before mould takes hold, cut and burn or bury the badly webbed, rotten heads so the caterpillars cannot finish feeding and drop into the soil to breed, and set up to win next season with an armoured variety, earlier planting, and rotation.