Whitefly

Leaves turn sticky and shiny, as if sprinkled with sugar-water — that is honeydew, the sweet waste the flies drip. A black, sooty mould then grows on the stickiness, blackening leaves and fruit, while the leaves themselves yellow, curl and wilt and the plant looks tired and stunted.

like fine ash or smoke, then quickly settles back under the leaves. Up close each one is pure white and about the size of a grain of sugar (roughly 1 mm) — like a minute white moth holding its wings roof-like over its back.

Stuck to the underside you'll find the young (called nymphs): tiny, flat, oval scales, pale yellow-green and see-through, glued in place and not moving — so still they look like part of the leaf. Adults rest and lay their eggs here too, which is why sprays aimed at the top of the leaf miss.

On cassava, leaves show a yellow-and-green mosaic mottle and become crumpled and small; on tomato and beans, the top leaves curl upward, cup and stunt. These virus diseases are the real killers — the flies bring them in.

with aphids (small, pear-shaped, mostly wingless lice — green, black or grey — that cluster on soft shoots and stems and do NOT fly up when disturbed) or with mealybugs (bigger, cottony-white fluffy blobs that sit still in one spot). Only whitefly rises as a white cloud when you touch the plant.

— that is when whitefly builds up and does the most harm. Look more often during warm, dry spells (about 20–32°C), the weather whitefly loves best.

Twice a week, walk a big "W" across the field so you pass through all four corners and the middle — not just the row nearest the path. Whitefly starts at the field edges and in warm, sheltered spots, so spread your checks out. Make about 10 stops.

At each stop, gently shake or brush the top of a plant and watch: does a cloud of white flies rise? Then turn over two or three of the youngest top leaves and look for adults resting there and the flat, pale scales (the young) stuck underneath. Check in the cool of early morning, when the flies are slow and easier to catch before they scatter.

Hang a few yellow sticky traps just above the crop — a yellow board or sheet smeared with grease or oil. Whitefly is strongly drawn to yellow and gets stuck, so the traps show you when numbers are climbing and which edge they are coming from.

on young plants. On virus-catching crops — cassava, tomato, beans, cotton — do NOT wait for a cloud: act at the very first few flies, because it takes only a handful to spread the virus.

Plant only clean, virus-free seedlings and cuttings: for cassava, take cuttings only from healthy-looking mother plants; for tomato and beans use fresh, healthy seedlings. One infected plant carries the virus straight into your field, so never plant material taken from sick-looking plants.

As soon as you harvest, pull out and burn or bury the old crop remains, and keep the weeds around the field cleared — whitefly lives on both and crosses to your next crop. Don't plant a new susceptible crop right beside an old, infested one (for example tomato after tomato, or cassava next to cotton), so the flies have no green bridge to walk across.

Spreading silver or white plastic mulch (shiny side up) on the ground bounces light upward and makes it hard for whitefly to find and settle on your plants. And never spray broad poisons that kill every insect — that wipes out the flies' natural enemies (below) and whitefly, which breeds faster, roars back worse than before.

Tiny helpful wasps called parasitoids (harmless insects that lay their eggs inside the whitefly young and kill them from within), such as Encarsia and Eretmocerus, together with ladybirds and lacewings, quietly hold whitefly down at no cost. Keep them alive by avoiding broad poisons. For the young stages, spray insecticidal soap or neem (an extract from the neem tree) in the cool of the day, aiming the spray up under the leaves where the flies actually live.

If flies keep rising past the threshold on a virus-prone crop, use a selective insecticide (one chosen to spare natural enemies) and direct it under the leaves where whitefly hides. Safety, every time: cover your skin, eyes, nose and mouth; spray in the cool of morning or evening with the wind at your back; keep it away from food, drinking water, children and bees; obey the pre-harvest interval (the set number of days you must wait between spraying and picking, so no poison is left on the crop); and wash yourself and your clothes afterwards. To stop the flies becoming immune, rotate — change to a different chemical group each spray rather than using the same one over and over, because whitefly builds resistance very quickly.

immediately pull out and destroy the worst virus-sick plants (mosaic, curled leaves) — one bad plant keeps feeding both the flies and the virus to the rest. When a field is heavily virus-struck, cut your losses: harvest what you can, clear and burn the crop remains, then leave a break with no host crop to starve the flies before you replant with clean, virus-free material. Chasing a virus-riddled field with more and more spray only wastes your money.