Larger grain borer

Look for mounds of fine, floury, cream-white grain powder — this is frass (the powder and droppings the beetle leaves behind) — under and among the cobs and grain, and caught in the folds of the bags. The larger grain borer bores far more than it eats, so it makes far more powder than any other store pest. That heavy floury dust is its clearest signature.

neat, round holes bored straight into the kernels, kernels hollowed out and crumbling, and whole cobs eaten down to bare shells and dust. On dried cassava chips and roots you will see the same round tunnels and the same floury powder.

a small, hard, dark brown-to-black beetle shaped like a tiny cylinder or pencil-stub, shorter than a grain of rice (about 3–4 mm long). Its head is tucked under a hood, so you cannot see it from above, and its tail end looks cut off flat. Look for them crawling on the grain and on the bags and boring into the cobs — they are most active when the store is warm.

The weevil has a long, pointed snout, like a tiny elephant's nose, and leaves far less dust. The larger grain borer has no snout (its head hides under the hood), looks like a straight dark cylinder, and leaves those big piles of floury powder — that mountain of dust is the giveaway that it is the borer, not a weevil.

— this is when a new attack takes hold, and it is the easiest time to catch it. After that, look again every month for the whole storage season, and more often in hot weather (around 25–32 °C), when the beetles breed fastest.

Do not just glance at the top of the nearest bag. Take a scoop of grain from about 10 different points — the front, back and corners, the top and the bottom of the store, and from several different bags — because the beetles start in hidden pockets before they spread.

Pour each scoop onto a cloth or through a fine sieve and look for three things: floury dust, round-holed and hollow kernels, and live beetles crawling. Also check the store floor, the bottoms of the bags, and inside a few cobs, where the dust settles first.

You do not wait for a big number. With this pest the first beetle you see is already boring and breeding, so treat that very first sign as your signal to act, especially in the first weeks of storage.

Before you fill it, remove and burn every bit of last season's grain, cobs and dust; old, infested grain is exactly where the new beetles fly in from. Scrub out the cracks in the walls and floor, and keep this year's harvest well away from any old grain.

Dry the grain until a kernel cracks rather than dents when you bite it, shell it off the cob (the borer loves whole cobs), and store it in completely airtight (hermetic) bags, or a sealed metal silo or drum. Sealing starves any trapped beetles of air and smothers them.

If you catch beetles early, spread the grain thinly in the hot sun on a mat or tarp for a day to drive them out and kill them, sieve out all the dust and the bored grain, then re-bag the clean grain airtight. A natural helper — the predatory beetle Teretrius nigrescens, which hunts the borer's eggs and grubs — has been released across much of Africa and keeps wild numbers down; protect it by keeping poisons off, and clean up together with your neighbours, because one clean store beside a dirty one is soon re-invaded.

Mix an approved storage dust — these come in two main chemical groups, organophosphate and pyrethroid, both made for stored grain and not the same as a field spray — evenly through the shelled grain at the exact rate printed on the label before bagging: too little fails, too much leaves poison in your food. Mix in the open air with your nose, mouth, eyes and skin covered; keep children, animals, food, water and bees well away; and wash your hands and change your clothes afterwards. Do not eat or sell the grain until the waiting days on the label have passed (this is the pre-consumption interval — the days you must wait before the grain is safe to eat), and clearly mark the treated bags. Each season switch to the other chemical group so the beetles never build up resistance — the point where the dust stops killing them.

after you have dusted and sealed, empty the store completely, sieve out every bit of dust and powdered grain, feed the worst bored grain to your animals, re-treat and re-seal the rest, then scrub and treat the empty store — and check your neighbours' stores too, because the borer flies back in from nearby grain and even from wooden structures. If the grain is already mostly dust and shells, cut your losses: pick out whatever whole grain is left, use or sell the damaged grain quickly for food or animal feed before it is all lost, and start next season with a clean store, hard-dried shelled grain, and airtight storage from day one.