Divisi
Larger grain borer
Prostephanus truncatus · Storage
Photo: Sarah McCaffrey, Museum Victoria · CC BY 3.0 au
The larger grain borer (Prostephanus truncatus) is a small, dark, cylindrical beetle — barely the length of a grain of rice — that attacks stored maize and dried cassava. Adults bore neat round tunnels straight into the kernels and into whole cobs and lay their eggs inside; the grubs and adults then feed within, grinding good grain into powder. Because it bores far more than it eats, it leaves tell-tale mounds of floury dust and can turn a full store into shells and flour within a matter of weeks. Accidentally brought into Africa, it is now one of the continent's most destructive storage pests.
division, CSIRO · CC BY 3.0 
CSIRO · CC BY 3.0
Why it matters
The larger grain borer attacks your maize and dried cassava after harvest — the food you have already worked a whole season to grow, that you are counting on to feed your family or to sell. Left alone in a warm store it can destroy up to 40% of your grain in just a few months, grinding good kernels into worthless flour and empty shells. It hides and breeds deep inside the grain and the cobs, so by the time the dust is piling up the damage is already done. And because the beetles fly in from old grain and nearby stores and settle in fast, the battle is won by keeping them out and catching the very first beetle — not by fighting them after they multiply.
When to act
any live borers or fresh boring dust seen in the store within the first weeks of storage
This guidance is general, compiled from published agronomic references including FAO and CABI. Thresholds and timing shift with your crop, your variety and the season.
In much of the region a spray is simply called mushonga — medicine. But a chemical group you cannot name is one you cannot rotate, and a product you did not write down has no pre-harvest interval you can observe. Mushonga, and other farming words →
The full larger grain borer field guide
- Identify it
- Scout for it
- Control it
13 field-guide sections in the app
The full guide to larger grain borer — every symptom to check, how and when to walk your field looking for it, and the controls that actually work, ranked. Built into your crop plan, so you are told to look before it costs you a harvest.
Crops it affects
Frequently asked questions
How do I identify larger grain borer?
The dust is the first thing you notice. 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.
When should I take action against larger grain borer?
any live borers or fresh boring dust seen in the store within the first weeks of storage
How do I control larger grain borer?
Prevent it — start with a clean, empty store. 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.
Which crops does larger grain borer affect?
Among the crops covered here: Cassava, Maize.
Catch it before it costs you a harvest
Divisi builds scouting into your crop plan — it tells you when to walk your field, what to look for at that stage, and what to do when you find it. Free for farmers, works offline.
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