Mealybugs

the easiest sign to spot is leaves that go shiny and sticky, then turn black with a soot-like film, along with little tufts of white 'cotton wool' wedged into leaf joints, growing tips and on the fruit. Under attack the tips bunch up, curl and yellow, growth stunts, and young leaves and fruit drop early. On cassava the growing tip crowds into a tight, stunted 'bunchy top' shaped like a candlestick.

part the white fluff and you find soft, oval, slow-moving bodies smaller than a grain of rice (about 2–4 mm) under a dusting of white mealy powder, often fringed with short waxy threads around the edge. They cannot fly — they sit still in tight clusters or crawl slowly, tucked into the sheltered inner canopy: leaf axils (the notch where a leaf meets the stem), leaf undersides, growing points, shoots and packed fruit. Squash one and it smears pink, orange or yellow. A line of ants marching up the stem leads you straight to a colony.

unlike whiteflies, which puff up in a little white cloud when you shake the plant, mealybugs stay put. Unlike hard, glued-down scale insects, mealybugs are soft and powdery and can crawl. And the black film on the leaves is sooty mould growing on their sweet droppings (honeydew) — it wipes off with your finger and is a sign of the bugs, not a leaf disease to spray with fungicide.

start scouting once the crop is in active growth (the development stage) and keep it up right through mid- and late-season fruiting — the whole time there are tender tips and fruit on the plant for them to feed on. Look hardest in warm, humid weather (around 22–32°C), and on any day you notice ants running up the stems or blackened, sticky leaves.

cross it in a big 'W' so you cover all four corners and the middle, not just the row nearest the path. Make about 10 stops, and at each stop check 5 plants up close — around 50 plants in all.

part the leaves and look into the leaf joints, growing tips, leaf undersides, along the shoots and in the fruit clusters for white, cottony colonies. Follow any ant trail — it almost always ends at a hidden colony. Search the crowded, shaded inside of the plant, not just the outer leaves.

treat now the moment you find white waxy colonies on more than 1 plant in every 10 you check (a handful of your 50), or on the growing tip of any plant, or with ants actively farming them. Don't wait — one colony on a tip today becomes a covered plant in a few weeks, so hit them while they are still small and few.

never take cuttings or seedlings from an infested plant, and check every one for white fluff before you plant — mealybugs travel on planting material more than any other way. Go easy on nitrogen fertiliser, because the soft, lush growth it forces is exactly what they feed on. And don't crowd your plants: an open, airy canopy leaves them fewer sheltered places to hide.

ants defend mealybugs from their natural enemies and ferry them to clean plants, so breaking that partnership is the single most useful thing you can do. Dig out or destroy nearby ant nests and smear a sticky or greasy band around the stem or trunk so ants cannot climb up. With the ants gone, natural predators can move in and do the work for you.

tiny parasitic wasps and the mealybug ladybird hunt down and eat mealybugs, and on cassava they have brought huge outbreaks across Africa under control. Protect them by not spraying broad poisons and by keeping the ants down. One caution: the mealybug ladybird's young look like big, fuzzy white mealybugs — learn to know them so you don't kill your own helper.

on just a few plants, wipe the colonies off with a cloth or brush dipped in soapy water, or blast them off with a strong jet of water. Prune out and burn badly infested tips and shoots — don't just drop them in the field, where the bugs crawl straight back onto the crop.

the waxy coat sheds ordinary contact sprays, so reach for a systemic insecticide (one the plant draws up into its sap, so it reaches the bugs even under their wax and deep in the joints), or add a wetting agent (a spreader that breaks the waxy film so the spray can grip). Spot-treat only the colonies and growing tips, never the whole field. Stay safe: cover your skin, eyes, nose and mouth; spray in the cool of the day with the wind at your back; keep it well away from food, water, children and bees; respect the pre-harvest interval (the set number of days you must wait between spraying and picking, so no poison is left on the fruit); and wash yourself and your clothes afterwards. Rotate to a different chemical group each time, so the bugs don't build resistance (stop being killed by it).

cut out and burn the worst-hit plants before the colonies overrun the rest of the field, and next season's cuttings with them. Never save seed, cuttings or planting material from an infested field. If a crop is overwhelmed too late to save, pull and burn it, clear away all crop residues and weeds the bugs shelter in, keep the ants down, and start the next season with clean planting material and the predators protected — a clean start beats chasing a lost crop.