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Storage pests: an ingrained problem?

The traditional nkhokwe, used for storing maize cobs
Credit: Frank Johnson, Malawi

Market liberalization has encouraged many farmers to store a greater proportion of their harvested crops on farm. They now have an opportunity to take advantage of seasonal price rises but these benefits can only be achieved if the stored crops can be held with no deterioration in quality. Farmers face considerable storage problems as a result of infestation by moulds, insect and rodent pests and these have to be overcome if farmers are to maximize income from production of durable food crops.

Moulds

Develop on the surface and inside of stored commodities that have not been dried properly or become wet during storage. Stored products spoilt by moulds should not be consumed due to the presence of mycotoxins which are hazardous to human and livestock health (see Aflatoxins: detecting the danger).

The occurrence of moulds can be avoided by reducing the moisture content of the crop as soon as possible. Some crops may be left in the field to dry but this increases the risk of attack by storage pests. Cereal, pulse, spice and some fruit crops can be sun-dried on the ground or on platforms under which fires may be lit to increase rate of drying. Once in store, the commodity must be kept dry.

Insects

Common insect storage pests

Acanthoscelides obtectus - Bean Beetle
Callosobruchus spp. - Pulse Beetles
Prostephanus truncatus - Larger Grain Borer
Ryzopertha dominica - Lesser Grain Borer
Sitophilus spp. - Maize and Rice Weevils
Tribolium castaneum - Rust-Red Flour Beetle

The most serious pests of stored grains and pulses. They cannot easily be excluded from stores and infestation results in weight loss and deterioration in quality. Heat, moisture and waste products produced by insects may also result in further deterioration and the growth of moulds. Store hygiene is essential to prevent reinfestation of newly stored grain.

In the past, insect infestation was often a less serious problem because farmers cultivated traditional varieties which, although low yielding, were generally more resistant to attack by insects. However, the introduction of high-yielding grain and pulse varieties has resulted in increased storage losses as these varieties are usually more susceptible to insect damage.

The use of pesticides in food stores on farm level is often too costly for resource-poor farmers. And, in many cases, farmers express concern over the safety of chemicals so that they will often only use synthetic chemicals to treat grain they intend to sell and not to treat grain for home consumption. Traditional methods of insect control include smoking, sun-drying and admixing of dusts or ashes. The latter provides a physical barrier against insects but needs to be used in large quantities (min. 20% by volume). The grain then has to be sieved or washed to remove the ash/dust before use. Diatomaceous earths have also proved successful in Zimbabwe as an alternative to chemical insecticides (See Inert dusts article). Vegetable oil, such as palm or coconut oil, mixed with grain offers some degree of protection and is particularly effective when used with pulses. However, the large volume of oil required makes treatment expensive.

Insecticides of botanical origin have also been long used as traditional storage protectants. They are low cost, locally and readily available, and do not require any packaging. Plant extracts, whether used as a dried, powdered dust or in solution often contain a variety of toxic chemical constituents that help to reduce insect pest problems by repelling pests or by actually killing them (see Plant protectants against storage pests).

Drying cowpeas in the sun
Credti: B.M.D Tran, NRI

Use of entomopathogenic fungi (e.g. Beauveria spp.) as mycopesticides may also provide an environmentally safe and cheap alternative for controlling storage insects. On-farm trials1 in Kenya are currently assessing the potential of selected Beauveria spp. isolates in protecting maize grains from attack from insects such as the Larger Grain Borer.

Rodents

The impact of rodents on food availability and health, particularly in poor communities, is severe. Existing knowledge and technology on rodent control is inappropriate, in most cases, for the control of rats in rural villages. Rodenticides are not an option as such toxicants are usually too expensive, not readily available, and likely to be dangerous if used incorrectly. The most effective means of controlling rodents is to practise good store management and to have adequate rodent proofing although this may prove difficult if crops are stored within the household. (see Rising above the rat trap)

Traditional farm storage systems have evolved to provide maximum protection against mould deterioration, provide a barrier to insect and rodent pest attack and deter theft. Most are well adapted to their environment and losses are generally low (<5%). However, although some systems may be able to cope with farmers' changing storage and marketing needs, in some cases, modifications or entirely new storage practices may be necessary and farmers will need advice to meet these requirements. For this to be effective, farmers should be involved in the analysis of their problems and in identifying appropriate systems even though this approach is more time consuming and complex than the common promotion of a single basic message by Agricultural Extension Services. Participatory approaches are proving to be successful and are resulting in greater adoption of new or modified strategies that are appropriate to farmers' needs. Training and advice for extension officers in appropriate participatory methods is available through research institutions, such as NRI, who are working on these issues.

1 Insect Pathology Dept at CAB International's Africa Regional Centre, Nairobi, Kenya

Further information:
Pest Management in Farm Granaries
, 1997 NRI publication ISBN 0 85954 490 7 £7.50
Available from CAB International Email: orders@cabi.org or http://www.cabi.org/bookshop
Traditional procedures and methods of storage protection published and available from GTZ. Also available at http://www.fao.org/inpho

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