Cowpea aphid

the growing tips and flower stalks look stunted, twisted and curled and the young leaves pucker; under a heavy attack the plant sheds its flowers and young pods. Everything feels sticky — that shine is honeydew, the sugary liquid the aphids leave behind — and a black sooty mould (a harmless mould that grows on the sticky honeydew) often spreads over the leaves on top of it.

on the newest growth look for dense clusters of tiny, shiny jet-black aphids, each no bigger than a pinhead — smaller than a grain of rice — packed tight on the growing tips, soft stems and flower stalks. The young ones are grey and dusty, the grown ones glossy black, and a few carry wings. Unlike whitefly they sit still and don't scatter when you lean in close.

if you spot ants running busily up and down the stems, stop and search hard for aphids. The ants farm and guard the colonies to feed on the honeydew, so a line of ants is often the first thing you notice from a distance, before you can even see the aphids themselves.

on some plants you will see mosaic — a patchy light-and-dark-green mottling of the leaves, with puckering, distortion and stunted growth. That is the virus the aphids carry from plant to plant; no spray can fix it, and those plants must be pulled out.

whitefly (pure white, and they fly up in a little cloud when you disturb them), thrips (slender and pale, leaving silvery streaks and speckling rather than black clusters), or ordinary dust and soot. Cowpea aphid is shiny black, in dense colonies on the newest growth, with ants and stickiness around it.

start the day your seedlings emerge and check twice a week right through the seedling and vegetative stages, then again at flowering and early podding. The seedling stage is the danger time — that is when colonies explode fastest and when the virus does the most harm — so never skip it.

walk a big "W" shape so you cross all four corners and the middle of the field, not just the row beside the path — the first patches nearly always start on the far side, where you don't normally look.

make about 10 stops along the W and look closely at 10 plants at each stop. Turn over the young leaves and check the growing tips, soft stems and flower stalks for shiny black clusters, stickiness, sooty mould and ant trails.

if 2 or more plants in every 10 (about 20%) carry a dense black colony on the growing tip → act now. Lead with that count — you don't need to count individual aphids, only how many plants out of ten are colonised.

if cowpea mosaic has struck your fields before, or you can already see mottled plants, do not wait for dense colonies. Because just a few aphids can spread incurable virus, treat the very first aphids you find on the seedlings.

only if 2 in 10 seedlings carry dense colonies (or virus risk is high and aphids are present) reach for an insecticide — ideally a selective aphicide, one that targets aphids while sparing bees and your helper insects. A systemic product (one taken up into the plant's sap, so the sap-sucking aphids are poisoned from inside) reaches aphids hidden in the curled tips far better than a contact spray (which only kills what it directly wets).

cover your skin, eyes, nose and mouth; spray in the cool of early morning or evening with the wind at your back so it blows away from you. Cowpea flowers draw in bees, so spray in the evening once the bees have left and never wet open flowers in the heat of the day. Keep the spray off food, drinking water, children and animals; wash yourself and your clothes straight afterwards; and respect the pre-harvest interval — the number of days the label tells you to wait between spraying and picking or selling the pods, so no poison is left on them.

check again 3–5 days after spraying; if colonies are back, switch to a different chemical group rather than re-dosing the same one, so the aphids never get used to a single poison (that build-up is called resistance) — and make sure you are still protecting the helpers and controlling the ants, because spray on its own rarely holds aphids for long. When plants are already stunted and widely showing mosaic virus, no spray will bring them back: pull out the worst virus plants, protect the healthy ones, and plan to sow early with a resistant variety in a clean field next season.