Pests of Stored Products and Damages Caused

On–farm storage is usually practiced by farmers. Staple foods and grains for family consumption are stored on–farm for many months. Most cash crops are also stored on – farm for a while and later moved into warehouses or silos in town and cities.

Damage to stored grains, seeds and foodstuff is of particular importance because it occurs post-harvest, and cannot be compensated for. Post-harvest crop losses are often very high, averaging about 20 – 30%.

Cowpea, sorghum and other small grain (rice and wheat, especially) often suffer very badly in storage. The more susceptible varieties show deterioration after only 3 – 4 weeks in storage and in some instances, 100% losses occur after only 50 days.

Pests of stored products

The more important pests of stored products are a few moths in the family Pyralidae, a few mites (Aerina), beetles belonging to a diversity of families, rats and mice.

The produce is usually grain of many types, dried pulses (usually shelled), nuts of other seeds, some dried fruits and berries, dried leaves and dried roots and tubers.

Categories of storage pests

The pests may be categorized on the basis of their feeding behaviour as follows:

Primary pests: These insects are able to penetrate the outer coats of grains and seeds, and include;

Ephestia spp. Trogoderma, Rhizopertha, Cryptolestes and Sitophilus spp.,as well as rat and mice.

Secondary pests: These are only able to feed on grains already damaged by primary pests or physically damaged during harvest e.g. Oryzaephilus spp.

Fungus feeders: A number of insects (mostly beetles) that are regularly found infesting stored products are usually feeding on the fungi growing on the moist product. Some species, however, may be both fungus feeders and secondary pests, e.g. some Psocopetera.

Scavengers: These are polyphagus, often omnivorous, casual or visiting pests. These include cockroaches, crickets, ants, some beetles, rats and mice.

Some pests are clearly more specific in their dietary requirements than others. For example, the Bruchidae only attack pulse grains; some caterpillars and beetles attack dried fruits, some beetles attack only grains, some only in flours and processed products. Many are confined to animal products and dried proteinaceous materials.

Damages caused by storage pests

The relative importance of the different species of storage pests depends on the nature of damage done.

1. Direct damage

This is the most obvious typical form of damage. It is often measured as a direct loss of weight or reduction of volume.

However, neither is accurate since although produce is eaten, there is an accumulation of frats, faeccal mater, dead bodies, etc. all the insects, mites and rodents are responsible for such damage.

2. Selective eating

Some insects prefer the germ region of seeds and grains. Thus a fairly low level of damage will severely impair germination of stored seeds.

In stored food grains, there will be serious reduction in quality resulting from the loss of the protein, minerals and vitamins that occur in the germ region. This preference is shown particularly by Ephestia larvae and Cryptolestes.

3. Heating of bulk grain

When grain or any other similar produce is stored in bulk, stagnant air trapped within the produce becomes heated by the insect metabolism and “hot spot” develop.

The moisture from the insects’ bodies and the stored grains condense on the cooler grain at the edge of the “hot spot”. The condensed waster causes caking, leading to fungal development and may even cause some rains to germinate.

4. Webbing by mothlarvae

The pyralid larvae in stored products all produce silk webbing which if present in large quantities may dog machinery and otherwise a nuisance.

5. Contamination

For export crops and produce to be sold, the presence of insects and dead bodies, exudate, frass, faeces, urine, hairs etc causes a general loss of quality and value.

Export crops are mostly destined for Europe and America where infestation control legislation is particularly stringent.

Many consignments have been rejected at the point of entry owing to presence of rodent hairs, urine or faecal matter, certain insect pests or residue of insecticides. In some parts of the world, many consignments of sorghum and maize grain have been rejected by breweries for similar reasons.

Important Pests of Stored Products

1. Cowpea bruchid allosobruchus maculates

It is wide spread and the most important pest of stored cowpea across the world. Alternative hosts include soybean, pigeon pea, green grains, chickpea and other pulses.

The larvae bore into and feed on the beans. Infestation usually start from the farm and are laid on maturing pod in the field. The infested pods are harvested and taken into the farm stores where further development takes place.

Adults are 2.5 – 3.5mm long brownish beetles. The females have obvious dark patches in the centre of each side of the elytra and at the anterior and posterior tips. These markings are less distinct in the males. There is a tooth on the apical end of the hind femur, larvae are usually found only within the bean.

Females lay up to 90 eggs singly, attached to pods or seeds. The eggs are round, small and grey – white. They hatch in about 6 days and the larvae bore into and feed within the seeds.

Each larva develops within a singly seed only. Pupation occurs in the seed about 26 days after the eggs were laid. Adults emerge after 7 days and do not feed. The whole life cycle takes about 4 – 5 weeks and there may be 6 –m 7 generations per year.

Non–chemical control: Dried bean stored in pods are more resistant to attack. Crops should be grown at least 1km from the nearest cowpea store. Prompt harvesting in areas at risk will also reduce attack levels.

Treatment of cowpea seeds with fresh or refined palm oil and vegetables oil can reduce infestation of C. maculates by reducing e.g. laying. Mixing cowpea with dried powder of hot pepper, ash, sand, neem, kernel powder also gives protection.

Small quantities of cowpea stored in the house for consumption can be protected against C. maculates by heating to 64⁰C for 10 minutes and then cooling and storing in insect – proof jugs. Similar control can be achieved by storing cowpea in smoky conditions such as in the kitchen.

2. Rice weevil (Sitophilus oryzae) and maize weevil (Sitophilus zaemais).

They are some of the most destructive pests of stored grains. They can damage grains beyond use. They are most active under warm – humid conditions. S. zeamais primarily a pest of maize, but will attack rice, sorghum and other stored grains.

Infestation can begin in ripening crop in the field and continue in store. S. oryzae is primarily a pest of rice, but will also attack maize, various cereals and their products. E.g. biscuits and pulses. It is less likely than s. zeamais to infest ripening crops in the field. Both have been found in dried cassava.

Adults and larvae fed on grains. Attack may start in the field and continue in the store. Larvae tunnel and fed within the grain.

After pupation, adult cuts 1.5mm diameter circular hole in the grain through which they emerge. Attack leaves the product susceptible to moulds. The grains are also contaminated with insect excreta.

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S. zea mais and S. oryzae are similar in appearance, both are dark red – brown to brown – black inclour with four pale red – brown oral spots on the elytra, but these are often indistinct. Adult S.oryzaeare 2 – 3.3mm long and adult S. zeamaisare 3 – 3.5mm long.

Females lay up to 150 eggs and drop a single egg in the hole they have dug in the grains. The hole is then sealed with a gelatinous fluid. Eggs hatch after about 8 days and larvae develop over 6 – 8 weeks within the grain.

They mould from pupation occurs in the grains and last 5 – 16 days. The adult may live up to 6 months and there may be up to 7 generations per year.

Non–chemical control: Sitophilus cannot breed when grain moisture is 9% or less. Therefore, grains stored in clean dry conditions are less likely to suffer attack. Seed treatment with wood ash or refined palm oil also suffers less attack.

Early harvesting will reduce infestation. Infestation can also be reduced by storing maize as unhusked cobs. Storage space should be free of weevils before new grains are stored there.

Rodents: The most important of these are the rats and mice.

General precautionary measures for control of pests in stored products. For the control of pest in stored products, the following should be done:

Harvesting: The grain should be harvested as soon as they are mature to avoid infestation in field.

Drying: The grains should be dried properly, below 15% moisture level before storage.

Dry grains are less easily attacked. Suggested levels are groundnut – 9%, maize, sorghum and millet

12%, rice and wheat – 14%, beans, cowpea etc. – 15%. Moist grains are readily attacked by Aspergilus and penicillin.

Good building: All holes or cracks in the wall, floor, roof, window and doors should be repaired and prevent access to rats, mice and insects. The building must be sealable if fumigation is contemplated.

Use of sealable container: On a large scale, this would include silos that are long – term grain storage as part of the national strategic reserves. Some of these silos are her metrically sealed. Some of the non – farm stores may be built of bricks or may be small plastic sack or jerry cans or large earthen ware jars. If reasonably airtight, they may be fumigated after being packed.

Wooden sac, cloth or thin gauge polythene should not be used as they fail to cult off oxygen and may allow penetration of moisture into the grain.

In summary, post-harvest crop losses due to storage pests are often very high and cannot be compensated for. To increase the shelf life and increase market value of stored products, proper control measures should be put in place.

Storage pests are categorized into primary pests, secondary pests, fungus feeders and scavenges based on their feeding habits.

Some effects of storage pests include direct and selective eating of grains, bulk heating of grains, webbing of produce by moth larvae and general contamination.

Description of some storage pests damages done and control measures were also given in this article.

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