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 they lose weight, quality, seed germination and safety.
FAO estimates that around fourteen 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 the farm in simple stores for months.
In this video: the two kinds of storage insect, how to tell the main pests apart by the damage they leave, why temperature decides how fast they multiply, how to inspect a store, and how to protect grain and pulses, from cleaning the store to airtight storage,
and when fumigation is, and is not, the answer. The advice is general and global.
Local rules and the product label always come first.
Storage insects are mostly small beetles and moths,
and many now live worldwide because they travel with traded grain.
They fall into two groups, by how they feed.
Primary, or internal, feeders can attack sound, whole grain. The female lays an egg inside a kernel, and the larva grows hidden inside it, where you cannot see it and contact treatments cannot reach it. Weevils, grain borers, the Angoumois grain moth and pulse beetles are all primary feeders.
Secondary, or external, feeders attack broken grain, flour, dust, and grain already damaged by the primary pests.
The red and confused flour beetles, the sawtoothed and flat grain beetles, and the Indian meal moth, whose larvae web the grain surface, belong here. So if you find flour beetles, look for the primary pest, or the broken grain, that let them in.
One more name to know: the khapra beetle. It is very hardy, it can survive long periods without food in cracks,
and it is a quarantine pest that many countries work hard to keep out.
The maize weevil and the rice weevil are almost identical:
small, two and a half to four millimetres long, dark reddish-brown to black, with a long, narrow snout and four faint orange-red spots on the wing covers.
They attack maize, rice, sorghum and wheat.
The female chews a hole in a grain, lays a single egg, and seals it with a plug.
She lays three hundred to four hundred eggs this way. The white, legless grub eats the kernel from the inside,
pupates there, and the new adult chews its way out through a small round exit hole. At about twenty-seven degrees, a generation takes about five weeks, so a warm store can see six or seven generations a year.
Weevils also fly, and they infest maize in the field when harvest is delayed
and the husk cover is poor.
The signs: small round exit holes, adults walking on grain and sacks, fine dust, and slight warmth in the store.
Later, the grains are hollow and powdery. In the tropics, smallholder maize kept in traditional granaries or woven bags without protection commonly loses ten to thirty per cent of its weight over six to nine months.
The larger grain borer turns stored maize and dried cassava into dust far faster than weevils. It is a cylindrical, dark brown beetle, three to four and a half millimetres long, with no snout. Its head is bent down under a hood-like thorax, so you cannot see it from above, and the rear end is flattened and steep, edged with small bumps.
Native to Central America, it reached East and West Africa around nineteen eighty and spread across much of the continent.
There, losses of thirty to forty per cent of farm-stored maize on the cob were reported within three to six months. It thrives in hot, relatively dry stores, where weevils do less well. The sign is round holes bored through husks and grain, with a pile of fine, flour-like dust under the stored cobs.
Shell the maize and store the grain rather than the cobs where you can.
Pheromone traps around stores detect its flights.
And a predatory beetle from its home range, released in several African countries from the early nineteen nineties, has established and reduced its numbers.
Pulses have their own pest: the pulse beetles, or bruchids, in cowpea, chickpea, mung bean, pigeon pea, lentil and other pulses.
The adults are two to four millimetres long, with short wing covers that leave the tip of the body exposed.
They do not need to feed, and live only one to two weeks.
The giveaway is the eggs: tiny white specks glued onto the seed. The grub bores straight in, grows inside, and the adult cuts a round window and emerges, leaving a round exit hole. A generation takes three to five weeks in warm, humid stores.
Infestation starts in the field, on dry, ripening pods,
and is carried unseen into the store. So unprotected cowpea can lose much or all of its value within three to six months.
In cereals, small moths flying in the store,
and grains with a round window where a moth emerged, point to the Angoumois grain moth.
Grain is alive.
It respires, slowly when dry and cool, and fast when warm and moist,
and so do the insects and moulds in it.
Most storage insects develop fastest between about twenty-five and thirty-five degrees, slowly below about twenty, and barely at all below about fifteen.
Above about forty-five to fifty degrees, they die.
Maize weevils also stop developing in grain drier than about ten to eleven per cent moisture.
Insects and moulds produce heat and moisture as they respire. So a pocket of infestation warms the grain around it, moisture moves and condenses, and the spoilage spreads.
That is a hot spot.
A warm spot in a bulk often marks an infestation,
and it is also where moulds, and their toxins, begin.
There is no safe number of live insects in a farm store:
any live weevils mean the population will grow.
So inspect stored grain every two to four weeks.
Sieve samples taken from several depths, not just the top, and count the live insects.
Look for exit holes, windows, white eggs on pulses, and dust.
Use probe or pitfall traps in bulk grain, and pheromone traps for moths and some beetles.
And measure the grain's moisture and temperature, to catch hot spots early.
At harvest, check pulse seed for eggs from the field,
and sort out insect-damaged maize cobs before they go into the store.
Before storage, harvest promptly at maturity, and dry the grain well: thirteen per cent moisture or less for maize,
and twelve per cent or less for long storage.
Clean and grade it, removing broken grain, dust and damaged kernels.
Clean the store, the bags and the equipment, remove all old grain, and repair the cracks, because last season's residues are the main source of new infestations.
And never mix new grain with old.
During storage, airtight, or hermetic, storage is the most effective tool without chemicals.
In multi-layer plastic bags, sealed metal or plastic silos, drums or well-sealed pots, the insects and the grain use up the oxygen, carbon dioxide builds up, and the insects stop feeding and die.
Smallholders across Africa and Asia now use hermetic bags widely for maize and pulses. But they work only if the grain is dry and the seal is intact.
For small household lots, sieving and sun-heating help: spreading pulses in a thin layer under clear plastic kills eggs and grubs.
Mixing pulses with fine ash, sand, or a little vegetable oil protects small stocks. Cooling grain below about fifteen to eighteen degrees greatly lowers the risk.
And some maize and pulse varieties resist weevils and bruchids better than others.
Grain protectants are registered in some countries for stored food grain, such as IRAC group three A, the pyrethroids, applied to shelled grain at storage.
Use only products registered for stored grain, at the label dose, with the waiting period and residue limits respected.
Fumigation is a last resort.
It is dangerous, and must be done only by trained, licensed operators, in gas-tight conditions, for the full exposure time.
Resistance to the main grain fumigant is now widespread in several key pests in many regions, mainly because of fumigation in leaky structures.
So never fumigate a store that is not sealed.
To recap.
Primary feeders like weevils, grain borers, grain moths and pulse beetles grow inside whole grains; flour beetles follow the damage.
Learn the signs: exit holes, windows, white eggs and dust.
Insects multiply fastest at twenty-five to thirty-five degrees, so keep grain dry and cool.
Inspect every two to four weeks, sieving from several depths.
Clean the store, sort the grain, never mix old and new, and store dry grain airtight. Leave fumigation to licensed operators in sealed stores.
Our next two videos cover drying grain and the moulds that make toxins.
For the full stored-grain guide, visit laisha.ai.
And before you go: Laisha Lens number one is out, our free guide to November, for every climate zone on Earth.
North of the equator, the cool-season crops go in:
winter wheat and garlic in the temperate north,
wheat, barley and lentil on the autumn rains of the Mediterranean,
and on the plains of South Asia the whole rabi season,
wheat, mustard, chickpea and potato.
South of the equator it is spring, and the tropical south sows soybean, maize, cotton and rice on the first rains.
Lens also covers crop residue without fire, the pests worth walking the field for, and a study plan for agri students.
Open My farm's November, choose your zone and up to three crops, and it builds your field jobs, week by week, with the pests to scout for. It is free, and needs no account. Our free November Lens, with your farm's month plan, is on laisha.ai slash lens.
From all of us at Laisha: may your fields do well this season. Thank you for watching.
