Stored-grain pests and their management
The insects, moulds and rodents that attack stored grain, why moisture and temperature decide the risk, and how drying, sealed bags and hygiene protect it.
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A harvest is not safe until it is eaten. Between the field and the plate, grain and pulses are attacked by insects, moulds and rodents, and lose weight, quality, seed viability and safety. FAO estimates that around 14 per cent of the world's food is lost between harvest and retail, and storage is one of the main points of loss, especially where grain is kept on farms in simple structures for months.
Stored-product protection is a standard part of entomology, post-harvest technology and agricultural engineering courses. Its logic is simple: keep the grain dry, cool, clean and sealed, and most pests cannot thrive.
The main insect pests
Stored-grain insects are mostly small beetles and moths, many of them now found worldwide because they travel with traded grain. They are usually grouped by how they feed.
Primary (internal) feeders can attack sound, whole grains. The female lays an egg inside a kernel, and the larva develops hidden inside it.
| Pest | Main hosts | Notes |
|---|---|---|
| Maize weevil (Sitophilus zeamais) and rice weevil (S. oryzae) | maize, rice, wheat, sorghum | snout-nosed weevils; among the most destructive; infest in the field and in store |
| Granary weevil (S. granarius) | wheat, barley | flightless; cooler climates |
| Lesser grain borer (Rhyzopertha dominica) | wheat, rice, maize | bores into kernels, leaves floury dust |
| Larger grain borer (Prostephanus truncatus) | stored maize on the cob, dried cassava | invasive in Africa from the Americas since the late 1970s; very destructive |
| Angoumois grain moth (Sitotroga cerealella) | maize, wheat, rice, sorghum | larva inside the grain; small exit holes with "windows" |
| Pulse beetles or bruchids (Callosobruchus species) | cowpea, chickpea, other pulses | larvae develop inside seeds; round exit holes |
Secondary (external) feeders attack broken grain, flour, dust and grain already damaged by primary pests:
- red and confused flour beetles (Tribolium species),
- sawtoothed grain beetle (Oryzaephilus surinamensis),
- flat grain beetles (Cryptolestes),
- the Indian meal moth (Plodia interpunctella) and other moths, whose larvae web the grain surface.
The khapra beetle (Trogoderma granarium) deserves special mention: very hardy, able to survive long periods without food in cracks, and a quarantine pest that many countries work hard to keep out.
Why moisture and temperature decide everything
Grain is alive: it respires, slowly in dry, cool conditions and fast in warm, moist ones. So do the insects and fungi in it.
- Moisture: moulds grow when the air between the grains is humid, roughly above 65 to 70 per cent relative humidity, which corresponds to moisture contents of about 14 per cent and above in most cereals. Dry grain, about 12 to 13 per cent for long storage in warm climates, keeps moulds in check.
- Temperature: most stored-grain insects develop fastest between about 25 and 35 °C, slowly below about 20 °C, and barely at all below about 15 °C; above about 45 to 50 °C they die.
- Hot spots: insects and moulds produce heat and moisture as they respire. A pocket of infestation warms the grain around it, moisture moves and condenses, and spoilage spreads. Monitoring grain temperature catches problems early.
Moulds also bring mycotoxins, such as aflatoxins in maize and groundnuts stored warm and moist, which are a serious health hazard even when grain looks acceptable.
How much water to remove when drying grain
A farmer harvests 1,000 kg of maize at 18 per cent moisture (wet basis) and must dry it to 13 per cent for storage. The dry matter does not change:
- Dry matter = 1,000 × (100 − 18) ⁄ 100 = 820 kg.
- Final weight at 13 per cent = 820 ⁄ (1 − 0.13) = 820 ⁄ 0.87 ≈ 942.5 kg.
- Water to remove = 1,000 − 942.5 ≈ 57.5 kg per tonne.
A useful shortcut: final weight = initial weight × (100 − initial moisture) ⁄ (100 − final moisture). Selling wet grain by weight and storing dried grain by weight are not comparable, which is why grain is traded with moisture adjustments.
Protecting stored grain
Before storage
- Harvest at maturity and dry promptly, by sun drying on clean surfaces or with mechanical dryers, to a safe moisture for the crop and climate.
- Clean and grade: remove broken grain, dust, weed seeds and insect-damaged kernels, which harbour secondary pests and moulds.
- Clean the store: remove old grain and residues, repair cracks, and treat the empty structure where recommended; residues from last season are the main source of new infestations.
During storage
- Hermetic (airtight) storage: multi-layer plastic bags, metal or plastic silos and other sealed containers. Insects and the grain itself consume the oxygen; carbon dioxide builds up, and the insects stop feeding and die, without chemicals. Hermetic bags have been adopted widely by smallholders in Africa and Asia for maize and pulses. They work only if the grain is dry and the seal is intact.
- Cooling and aeration: blowing cool ambient air through grain bulks lowers temperature and evens out moisture; grain cooled below about 15 to 18 °C is much less at risk.
- Inert dusts and admixtures: diatomaceous earth and some traditional admixtures (ash, sand, certain plant materials) damage insects' waxy cuticles or hinder movement, used particularly for seed and small quantities.
- Varietal resistance: some maize and pulse varieties resist weevils and bruchids better than others.
- Rodent proofing: raised, sealed stores, metal guards, cleanliness.
Monitoring
- Sampling with grain probes or sieves at regular intervals.
- Traps: probe and pitfall traps in the grain, pheromone traps for moths and some beetles.
- Temperature sensors in bulks.
Fumigation, as a last resort
Where infestations must be killed in large bulks, fumigants are used in sealed structures by trained, licensed operators. Resistance to the main grain fumigant is now widespread in several key pests in many regions, mainly because of fumigation in leaky structures; sealing and correct dose and exposure time are essential. Grain protectant insecticides are registered in some countries for specific uses; follow local registration and label instructions, and maximum residue limits.
Key points
- Primary (internal) feeders such as weevils, the lesser and larger grain borers, the Angoumois grain moth and pulse bruchids attack whole grains; secondary feeders such as flour beetles attack damaged grain and flour.
- The khapra beetle is a major quarantine pest.
- Moulds need humid conditions (cereal moisture of about 14 per cent and above); insects multiply fastest at about 25–35 °C and slowly below about 20 °C.
- Hot spots form where insects and moulds respire; monitor temperature.
- Drying: final weight = initial weight × (100 − initial moisture) ⁄ (100 − final moisture).
- Protect grain by drying, cleaning, hermetic storage, cooling, hygiene and monitoring; fumigation is a last resort needing sealed stores.
What the exams ask
- School-leaving and entrance papers: names and hosts of the main storage pests, primary versus secondary pests, and simple storage practices.
- Degree and research-fellowship entrances: the biology and damage of each pest, the effects of moisture and temperature, hermetic storage principles, and moisture calculations.
- Post-harvest engineering and plant protection examinations: storage structure design, aeration, safe storage periods, fumigation and resistance, quarantine pests, and mycotoxin control.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does grain that was clean at harvest become infested in store?
Some pests, such as maize weevils and the Angoumois grain moth, infest grain in the field before harvest, and their larvae are hidden inside kernels. Others come from old residues in the store or from neighbouring stores.
How do hermetic bags kill insects without chemicals?
Insects and grain use up the oxygen inside the sealed bag and release carbon dioxide. When oxygen falls low enough, insects stop feeding and die. The bags must be sealed properly and the grain must be dry.
Is it safe to store grain that has a little mould?
Mould can produce mycotoxins that are not destroyed by cooking. Badly mouldy grain should not be eaten or fed to animals; sorting out damaged grain and drying properly before storage are the best prevention.
Sources
- FAO. The State of Food and Agriculture 2019: Moving Forward on Food Loss and Waste Reduction. Food and Agriculture Organization of the United Nations, 2019.
- Hagstrum, D. W., Phillips, T. W. and Cuperus, G. (eds) Stored Product Protection. Kansas State University, 2012.
- Rees, D. Insects of Stored Products. CSIRO Publishing, 2004.
- Murdock, L. L., Margam, V., Baoua, I., Balfe, S. and Shade, R. E. "Death by desiccation: effects of hermetic storage on cowpea bruchids." Journal of Stored Products Research 49, 166–170 (2012).
- Navarro, S. and Noyes, R. T. (eds) The Mechanics and Physics of Modern Grain Aeration Management. CRC Press, 2001.
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