Cotton aphid

the youngest leaves at the growing tips curl and cup downward and pucker, and the plant seems to stall and stop growing. Soon those leaves feel shiny and sticky to the touch — that stickiness is honeydew, the sugary liquid the aphids squirt out as they feed — and a black, sooty coating (called sooty mould, a harmless mould that grows on the sugar) spreads over it, so the leaves look dirty and dusted with black. Ants running up and down the stems are a dead giveaway: they guard the aphids and feed on the honeydew.

turn over a young leaf or look right at the growing tip and you will find them packed in dense clusters — soft, pear-shaped insects about the size of a pinhead (1–2 mm across), so small you must look close. Their colour varies a lot, from pale yellow-green to dark green, grey or almost black, and each one carries two tiny 'exhaust pipes' (called cornicles) sticking up from its tail end — a feature none of the look-alikes has. Most have no wings, but when a colony gets crowded a few grow wings: winged aphids are a warning the colony is about to fly off and start new ones on fresh plants.

whitefly are tiny white specks that fly up in a little cloud when you shake the plant, while aphids stay put in their clusters and do not fly off. Jassids (leafhoppers) are green and hop or dart sideways the instant you touch the leaf; aphids are slow and sit still. And red spider mite leaves fine pale speckling and faint cobwebby threads but no stickiness — only aphids leave the tell-tale sticky honeydew and black sooty mould behind them.

The danger runs from the young seedling stage, right through squaring (when the plant sets its flower buds) and on into flowering and boll-filling — this whole early-to-mid stretch is when aphids rob the most yield. Watch even harder during warm, dry spells when the crop is putting out soft, lush green growth, because that is exactly when their numbers explode.

Walk a big 'W' shape across the whole field so you cover all four corners and the middle — not just the edge by the path where you always end up looking. Make about 10 stops, and at each stop turn over the youngest top leaves and feel the growing tip of a few plants.

At each plant look for clusters of aphids on the underside of the young leaves, tips that are curling and feel sticky, ants marching up the stems, and any winged aphids. While you are there, look for the aphids' enemies too — ladybirds and their alligator-shaped young, lacewings, hoverfly maggots, and swollen bronze 'mummy' aphids (ones already killed by tiny wasps). If plenty of these helpers are about and the colonies are still small, they may well clear it for you — so do not rush to spray.

carry aphid colonies on the growing tips with curling leaves, or show sticky, sooty-black leaves — that is the point where they start stealing real yield. A few scattered aphids with ladybirds already working on them is not a reason to spray. Near harvest, keep watching the opening bolls: honeydew dripping onto the white lint makes 'sticky cotton' that drops your price, so a late flare-up still matters.

aphids breed fastest on soft, over-fed growth, so go easy on nitrogen fertiliser and never pile on extra 'to push the crop' — that flush of lush green leaf is exactly what they multiply on. Pull out weeds in and around the field, because aphids shelter and breed on them between crops. After harvest, plough in or burn the old cotton stalks and trash so the pest has nothing to carry over on. Where you can, choose a tolerant variety and plant early, together with your neighbours, so the crop is past its most tender stage before aphid numbers peak.

ladybirds, lacewings, hoverfly maggots and tiny parasitic wasps eat huge numbers of aphids. The single biggest mistake is spraying a broad poison early for just a few aphids — it wipes out these helpers, and the aphids then flare back worse than before because nothing is left to eat them. Leave the helpers to do their job.

in a small field you can knock the aphids off with a strong splash of water, and pinch off and destroy the few worst, tightly-curled tips where they cluster — this cuts their numbers by hand without harming the ladybirds. Then give the natural enemies a few days to catch up before you even think about a spray.

when half your plants or more carry building colonies and the natural enemies cannot keep up, reach for a selective aphicide — an insecticide made to kill aphids while sparing ladybirds and bees — rather than a cheap broad-spectrum poison that only triggers a worse flare-up. Aim the spray under the leaves and into the growing tips, where the aphids actually sit; a spray that only wets the top of the leaf misses them completely. Spray in the cool of early morning or evening with the wind at your back so the drift blows away from you; cover your skin, eyes, nose and mouth; keep it well away from food, drinking water, children and bees (spray when bees are not visiting the flowers). Respect the pre-harvest interval — the set number of days you must leave between the last spray and picking so no poison stays on the cotton — and wash yourself and your clothes as soon as you finish.

cotton aphids build resistance fast — meaning a poison you lean on again and again slowly stops killing them — so each time you truly must spray, switch to a different chemical group and never repeat the same product back-to-back. If they keep spreading after a spray, do not just re-spray the same thing: change to another group and check you are really hitting the leaf undersides. When it is too late — the crop is near maturity and most bolls are already open — extra spraying will not pay: stop spending on chemicals, pick the clean cotton promptly, keep any honeydew-stained 'sticky' lint separate so it does not downgrade the rest, and fix the real causes next season (balanced nitrogen, clean fields, a tolerant variety, and early planting).