Bruchids (bean and cowpea weevils)

neat, round holes like tiny windows or portholes bored through the beans, and grain hollowed out and full of fine, whitish powder. Infested beans feel light and may rattle; scoop a handful and you'll see powder sifting to the bottom of the tin or sack. This grain fetches little at market, and if you plant it, it won't sprout.

very small — about the size of a rice grain or smaller (2–3 mm) — reddish-brown and rounded, with no long snout or 'nose'. Its pointed tail-end sticks out past the wing covers, often marked with two dark spots. Look for them crawling over the grain and up the sacks in the warm part of the day; they love it hot and humid.

tiny clear, pearly domes glued flat onto the seed coat — like little dried water-drops stuck on the bean — and sometimes a faint pale round 'window' just under the skin, where the next beetle is about to chew its way out.

with the maize or rice weevil, which has an obvious long snout (nose) and attacks cereal grains like maize and rice. On beans and cowpea, a small, snout-less, rounded beetle leaving round window-holes is a bruchid.

Beetles fly in and lay eggs on the pods as they dry and ripen late in the season, before you even harvest, so check the crop at drying, harvest and shelling. Then check the store every 2–4 weeks right through storage — and more often in hot, humid weather (they breed fastest at about 25–35°C).

Scoop grain from the top, middle and bottom of each drum or bag, and from several bags across the stack (walk a 'W' over your store, not one spot). Pour a cupful onto a plate or cloth in good light and spread it out thin.

live beetles crawling, round window exit-holes, light, hollow beans, whitish powder at the bottom, and tiny eggs stuck on the seed coats. Put your ear to a full tin: a faint rustling means a heavy load hidden inside. (Quick test: drop beans in water — badly bored, hollow ones tend to float.)

Even one live beetle, one exit-hole, or eggs on the grain means treat now. What shows on the surface is only a fraction of what is growing hidden inside the seeds, so never wait for 'a few more' — by then it has already multiplied.

Harvest as soon as the pods are ripe; don't leave beans standing in the field to dry, where beetles fly in and lay their eggs. Dry the grain hard (a well-dried bean can't be dented with a fingernail or a tooth). Before storing, scrub out the store and containers and burn last season's leftover grain and pod trash where beetles hide. Never tip new grain onto old — store this season's crop on its own, and store only whole, unbroken beans.

Pack the dry beans into an airtight (hermetic) container — a metal drum or plastic barrel with a tight, sealing lid, or triple-layer airtight storage bags (known as PICS bags). Fill it right up and close it tight: any beetles trapped inside use up the air and suffocate, and no new ones can get in. For a smallholder store this works better than any spray.

Mix the dry beans through with an inert dust — clean wood ash, or diatomaceous earth (a fine, chalk-like mineral powder that is safe on food) — coating every seed; the sharp, dry dust kills the beetles by scratching and drying them out. For seed you'll keep for planting, or small amounts to eat, you can instead rub the beans with a little cooking oil (about a spoonful per few kilos): the oil smothers the eggs so they never hatch.

Spread infested grain in a thin layer inside a black plastic bag or sheet and leave it in full, hot sun for a few hours (an afternoon under clear plastic works too). The heat cooks the eggs, grubs and beetles hidden inside the seeds. Let it cool, then store it airtight so it can't be re-infested.

If you must keep a large amount for a long time and the steps above aren't enough, mix through a grain-protectant dust (a dust made and registered for stored grain), spread evenly at the rate on the label. It is a poison: cover your nose, mouth, skin and eyes, mix it outdoors with the wind behind you, and keep it away from food you're about to eat, from water, children, animals and bees; wash well afterwards. Respect the withholding period — the number of days the label says you must wait after treating before the grain is safe to eat or sell — so only treat grain going into long storage. And don't use the same dust every season: the beetles build resistance (the chemical stops working on them), so lean on airtight storage first and switch to a different chemical group if you ever treat again.

Still seeing them after you've sealed and treated? The seeds were already infested inside when you stored them — tip the grain out, sort out and throw away every holed, hollow or powdery bean, sun-heat or re-treat the good grain, clean the container, and re-seal. If a bag is mostly holes and powder, it's a write-off — don't keep it 'just in case', because it will re-infest your good store. Cut your losses: grind it for animal feed or destroy it straight away, and get the sound grain into airtight storage now.