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1.1 The problems of farm storage

It is estimated that between 60 and 80 percent of all grain produced in the tropics is stored at the farm level. For small farmers, the main purpose in storing grain is to ensure household food supplies. Farm storage also provides a form of saving to cover future cash needs through sale or for barter or gift-exchange. Small quantities of grain are also stored for seed. Farmers who produce a surplus may also store grain for sale later to take advantage of seasonal price rises.

Farm storage systems must provide maximum protection against deterioration of the commodity by inclement weather and pests, and also to deter theft. Traditional farm storage systems have evolved over long periods to satisfy these requirements. Most are well adapted to their environment and losses are generally low, often below 5 percent of grain weight over a storage season (Tyler and Boxall, 1984). However, for resource poor farmers living at or near subsistence even losses of this magnitude have important implications for food security. Rectifying these losses can only be achieved by subsistence farmers if changes are made to the traditional system of storage which bear no cost (other than of the farmers own labour), such as improving the design of the storage structure and using grain protectants which occur naturally in the local environment.

Higher losses may occur if the equilibrium of the traditional post-harvest system is disrupted. New, introduced crop varieties that do not possess the good storage characteristics of the traditional varieties may be more susceptible to pest attack; for example, storage losses in hybrid maize due to insect pests may exceed 25 percent by weight (Golob and Muwalo, 1984). These newer high-yielding varieties, grown by more progressive small-scale farmers, may produce grain in quantities, which exceed the traditional storage capacity. In the past, these farmers have depended upon parastatal grain marketing organizations and grain traders to take the surplus soon after harvest.

The introduction of grain market liberalisation in many countries has resulted in a rapid decline or elimination of marketing organisations and the emergence of a more important private grain trade. Farmers now have an opportunity to take advantage of seasonal price rises but the benefits can only be achieved if grain is held longer on the farm with no deterioration in quality. Thus the farmer now faces new problems associated with conserving grain safely on the farm and he/she needs advice on the most cost-effective type of storage system and methods of pest control to use.

In developing countries, the greatest losses during storage to cereals and other durable commodities such as pulses and oilseeds, are caused by insect pests. A list of the most common species is shown on page vii.

1.2 Insect control methods

Traditionally, protectants against insect infestation fall into two groups: those materials such as ashes, minerals and oil, in which physical barrier effects are responsible for the control of insects; and the use of whole plants, or parts of plants where there may be some chemical insecticidal or repellent effect.

1.2.1 Physical control

The use of chemically inert materials such ashes, sand or other minerals, powders or seeds in large quantities to fill up the interstitial space in grain bulks and provide a barrier to insect movement is quite widespread. For example; farmers living on the Lilongwe plain and in the Dedza hills of Central Region in Malawi commonly use ash from the cooking fire to protect beans stored at the home for both consumption and seed; in India sand is mixed with both cereals and pulses for storage; and in Indonesia peppers, beans and sesame seed are mixed with ashes or sand (Golob and Webley, 1980). The abrasive nature of such materials may also help control infestation by damaging the insect cuticle leading to dehydration and death. Recently, silica-based inert materials (such as silica aerogels and diatomaceous earths), have proven to be very effective in field trials under certain climatic conditions, although at present the cost and lack of availability prevent their use at the farm-level in developing countries. It has also been proposed that such materials may be used as carriers for insecticide active ingredient to increase their effectiveness under a range of climatic conditions. However, silica-based dusts have not found universal acceptance, firstly, because of the potential respiratory health hazards which may be caused by inhalation of their finely divided particles and, secondly, because of the possible damage to grain handling machinery which might result from the abrasive properties of these materials.

It is now well established that vegetable oils are very effective in controlling certain species of bruchids by their ovicidal effect. The main concerns over the use of oils however is the availability, price and potential problems with taint as off-flavours may be imparted to the grain.

Other physical methods of insect control which could be employed at the farm level are thermal disinfestation, the commodity being spread out in a thin layer on the ground or on a platform and is exposed to the sun for several days; and hermetic storage, the commodity being retained in sealed, air-tight containers such as clay pots or metal bins. Thermal disinfestation is suitable for small quantities of produce and for areas which have extended periods of hot, dry weather but protection is only afforded for a few months as the technique does not provide the grain with residual protection; the insect pests tend to be driven away from the grain by the heat but not killed. Hermetic storage is suitable for protecting small quantities of seed material but is generally too expensive for use with food grain. The use of traditional underground pits is widespread in some areas in which the physical environment is conducive to this type of storage, i.e. hot and dry with very low water tables, such as occur in Ethiopia, the Sudan and Somalia.  

1.2.2 Chemical control

The use of the fumigant gas aluminium phosphide (phosphine) at the farm level is becoming increasingly more common where it is available to farmers, particularly in the Indian sub-continent and West Africa. This trend is a matter of some concern as farmers are rarely sufficiently trained in the safe handling and use of such products and its misuse is likely to lead to accidental poisoning and the development of insect resistance. Its use is to be discouraged where possible although if its use continues to spread it may be necessary to ensure that extension workers are sufficiently knowledgeable and trained in the application of the fumigant to be able to train farmers in its safe use and handling.

Conventional insecticides, as dilute dust formulations which are admixed with the commodity, are safe and effective, providing that only those chemicals that have a label recommendation for use on stored grains are used (such as pirimiphos-methyl [Actellic: Zeneca Agrochemicals], methacrifos [Damfin: Ciba Geigy] and fenitrothion [e.g. Sumithion: Sumitomo]), and proper attention is paid toward user safety aspects. In many developing countries availability of suitable products is poor, and often inappropriate, dangerous chemicals, such as fenthion, lindane and DDT, may be used instead. The direct application of a pesticide to a food commodity, known as ‘admixture’, will always produce a residue which will be more or less persistent depending upon the chemical nature of the pesticide used (and climatic conditions and, to a lesser extent, the nature of the commodity to be protected). This may however create a potential hazard or, at least, a source of possible anxiety to the user of the commodity.

Application of chemicals to surfaces of store walls and floor by spraying with wettable powder or emulsifiable concentrate formulations to produce residual surface treatments are not recommended due to the very short effective persistence, often less than one day under tropical climatic conditions (Gudrups, 1996). Such residues will often contribute to the development of insect resistance, which occurs when insects are exposed to sub-lethal concentrations.

Insecticides used in places where foodstuffs are stored or handled, especially when used for admixture with commodities, should always be chosen from those that have international and local approval for the particular purpose, and established maximum residue limits that allow effective application rates. Most developing countries have very limited legislation controlling the use of chemicals and instead rely upon recommendations made by United Nations Organizations to identify acceptable compounds which are safe to handle and consume. International regulation pertaining to the use of insecticides on food stuffs is governed by the United Nations Codex Alimentarius Commission (CODEX). Codex specifies chemicals and residue limits, which are permissible in traded grains and other food commodities. Its specifications are based upon recommendations provided by the FAO and WHO Joint Meeting on Pesticide Residues (JMPR). This body accumulates scientific evidence concerning insecticide efficacy and toxicity and recommends maximum permissible residues for different food commodities. These maximum residue limits (MRL), expressed as quantities of active ingredient (ai) of the chemical, take into account the acceptable daily intake for human beings of each compound. Only a small number of the insecticides marketed for the protection of agricultural produce have received clearance by Codex and have MRLs recommended for durable food crops. Some of those that have approval include the pyrethroids: permethrin (2 mg/kg on raw cereals), deltamethrin (1 mg/kg), bioresmethrin (5 mg/kg), phenothrin (2 mg/kg) and fenvalerate (2 mg/kg); and organophosphorus compounds: malathion (8 mg/kg), pirimiphos-methyl (10 mg/kg), fenitrothion (10 mg/kg), bromophos (10 mg/kg) and chlorpyrifos-methyl (10 mg/kg).

Organophosphorus compounds (organophosphates) are the most widely used group for storage protection. In the absence of insect resistance, which may affect the choice of an insecticide, the choice between various organophosphates relates mainly to marginal differences in toxicity and specificity. In general, pyrethrum and the synthetic pyrethroids are more expensive than the organophosphates, but their relative efficacy against the bostrichid beetles R. dominica and P. truncatus makes them cost-effective where these pests are a problem; pyrethrum, a plant extract, is however photolabile and quickly becomes ineffective in daylight.

The control of insect infestation in maize stored on the cob also presents special problems. Unless the sheathing leaves are removed, any insecticidal spray or dust may fail to reach the insects on the grains themselves. Some control of infestation is possible even with the sheath intact with insecticides like pirimiphos-methyl (Golob and Muwalo, 1984), but the control achieved will depend upon many factors including grain moisture content, ambient humidity, the condition of the cob sheath, the type of storage structure used and the pattern of infestation. In general, the best solution is to dry the cobs as quickly as possible and then shell the grain so that an admixture treatment can be applied more effectively. This is certainly the best treatment where P. truncatus is a serious pest (Golob, et al. 1985), provided that natural drying in the crib does not take more than one to two months and that initial infestation levels are not exceptionally high.

1.3 Plant materials

The wide-scale commercial use of plant extracts as insecticides began in the 1850s with the introduction of nicotine from Nicotiana tabacum, rotenone from Lonchocarpus sp, derris dust from Derris elliptica and pyrethrum from the flower heads of Chrysanthemum cinerariaefolium. Both D. eliptica and Lonchocarpus belong to the Fabaceae family and have been found to contain 3-20 percent rotenone and also rotenoids, the chemical components which show insecticidal activity. C. cinerariaefolium belongs to the family Asteraceae and the flower heads contain six identified active compounds, pyrethrins I and II, cinerins I and II and also jasmolins I and II.

Several other traditionally used plant preparations have found local commercial markets, for example Ryania speciosa (Ryania) (Flacourtiaceae) which contains an insecticidal alkaloid, and Haplophyton spp. (Apocynaceae) which have been used in the West Indies and Mexico for crop protection. Nicotine, an alkaloid from some Nicotiana species (Solanaceae) and the related compound anabasine, from Anabasis species continue to be used against various insect pests in spite of being highly toxic to mammals. Constituents of many aromatic plants used for flavouring or medicinal purposes have been found to possess insecticidal properties. Recent surveys of desert and semi-desert plants have revealed a range of sesquiterpenes, benzopyrans, chromenes and prenylated quinones that are repellent or cytotoxic (Bell, et al. 1990). Some plant families may accumulate a restricted number of anti-insect chemicals, so-called secondary metabolites, whilst others possess a wide variety of different structural compounds. As can be seen in Chapter 5, the most numerically dominant insect repellents are the alcohols, alkaloids and terpenes.

There is renewed interest, for various reasons, in both plants and other alternative materials, such as silica-based inert dusts and insect growth regulators, for use as stored-product protectants. These include the development of insect resistance to synthetic insecticides, fears over misuse and overuse (during application) and the potential problem of insecticide residues affecting consumers, wildlife and the environment. Also, the increased cost of insecticide development has led to a reduction in the number of new pesticides being evaluated by chemical companies for use in storage applications. In addition, in developing countries, the high cost of using imported synthetic insecticides may result in a drain on limited reserves of foreign exchange. These problems are compounded by the difficulties and expense of packaging and distributing chemicals in a form which is suitable for on farm use, requires no sophisticated equipment for application and is cheap.

The use of locally available plants avoids the need to establish complex mechanisms for pesticide distribution; the community can collect or grow the plants itself. The use of plants materials for storage protection is sustainable; they can be continuously propagated year after year; biodegradable; and do not have any negative impact on the environment as long as care is taken to avoid the propagation of plants from foreign ecosystems which might, therefore, become established as weeds.

In considering the natural materials required for effective pest control it is clear that large quantities are often required for satisfactory control. In some instances this may result in some harm to the local environment; for example if large quantities of ash are required, deforestation may result.

Many non-cultivated plants have been used traditionally in foods and will therefore present no risk to health. Although commonly regarded as safe, many plants contain noxious compounds, which may render them unsafe for both animals and humans to consume. Therefore, the toxicity of plant materials, either directly admixed into foodstuffs or of extracts which are applied onto the produce, may be unacceptable and even long-standing use does not guarantee a level of safety which can be recommended unconditionally.

In 1980, Golob and Webley produced a bibliography summarising traditional methods used by farmers throughout the world to protect stored products. They also collated published scientific research on the use of extracts of plant materials. In 1993, Rees, et al. produced a bibliographic database of 1 100 references citing the use of alternative methods to conventional synthetic insecticides, for the control of stored-product insect pests. These included the use of plant materials, extracts and oils. This database was used to form the basis of a review by Dales (1996), which represents a continuation of the original bibliography by Golob and Webley (1980), providing a review of articles between 1980 and 1996. The three publications have been used as the framework for the present review.

This review is structured in six chapters. The different methodologies used to determine the effectiveness of plant materials and the merits of each regarding the potential use of plant materials are outlined in Chapter 2. Chapter 3 lists, alphabetically, plant species used for medicinal and culinary purposes with insecticidal properties against tropical insect pests of stored cereals and pulses. Plant species where only scant anecdotal information regarding their insecticidal properties was available were omitted. Properties of oils extracted from plants are listed in Chapter 4. The toxicology of selected plant species is considered in Chapter 5 and the development and feasibility of using spices and medicinal plants as plant protectants is discussed in Chapter 6. 

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