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Published research on the use of plant materials, extracts and oils for the control of stored product pests shows that, over the past 12 years, a large number of plant species from a wide range of families have been evaluated. Jacobson (1989) suggested that the most promising botanicals were to be found in the families Meliaceae, Rutaceae, Asteraceae, Annonaceae, Labiatae and Canellaceae. In this current review references to trials using plant species from over 50 families have been found. The most numerous being in the families Compositae, Fabaceae, Labiatae, Leguminosae, Solanaceae and Umbelliferae, in the remainder five or less species may be found and, admittedly in some instances only, one or two references have been located.

The plant species that have been investigated are frequently those used locally, within individual countries, as culinary spices or in traditional medicine. Some researchers infer that the material is therefore safe to use as an insecticide. When the plant species are cross-referenced in international pharmacopoeia, various citations are found which indicate their suitability for the treatment of many ailments, including colds and sores, cancer and leprosy, and for the induction of parturition and as contraceptive agents. However, these wide-ranging medicinal uses do not obviate the need for toxicological testing if materials derived from these species are to be promoted for use as food protectants.

6.1 Considerations on selection of plant materials and plant-derived oils as stored product protectants

Many researchers are attempting to validate the efficacy of traditional storage protectants, whilst others are seeking effective plant species which would be readily available in the local environment for farmer use at village level (Weaver , et al. 1991a). Other researchers are seeking plant extracts which can become the focus of local pesticide industries (Kis-Tamas, 1990). Decisions on ultimate intended usage need to be made.

6.1.1 Botanical characteristics

Desirable characteristics of botanicals for use in pest control whatever the end-use, would probably be that the plant is perennial, easy to grow and not expensive to produce, i.e. requiring little space, labour, water or fertiliser application. Plants should also show no potential to become weeds or the host for plant pathogens themselves and should, if possible, offer complementary economic uses. In addition, the insecticidal product should effectively control the range of pests encountered in local storage situations, be safe to use, pose no environmental hazard, be easy to extract, formulate and use with available skills (Kis-Tamas, 1990). Unfortunately, little indication is given in the papers examined so far, that any of these characteristics have been sufficiently considered.

6.1.2 Effect on seed viability

Only a few of the materials tested have been examined for their effects on seed germination. Even if effects on seed viability are examined, this is seldom done after a storage period of 6-12 months, which is the normal period of storage for seed grain by farmers.

It has been argued that, if a material is to be used for seed treatment, investigations of the toxicological properties of the plant or extract are not as essential as for other applications. However, the materials registered for seed treatment still need to undergo the same stringent toxicological evaluation, as the treated seed will be handled and may also be eaten in times of shortage.

6.1.3 Effect on food quality: taint

For a synthetic insecticide to be approved for use in grain storage it must fulfil ten criteria, one of which is that it must not affect the quality, flavour, smell or handling of grain (Snelson, 1987). Unfortunately, many plant materials added directly to food commodities may impart a taint to processed or unprocessed food. The traditional practices of applying pepper, turmeric and garlic to rice or beans to control storage pests appears to be acceptable. However, few investigators have carried out tasting trials to ascertain that the treated commodity is acceptable, to local people, when prepared in the customary manner. Unwanted plant material is commonly removed from the commodity, after storage, by sieving or rinsing in water.

6.2 Potential for commercial development of spices and medicinals

6.2.1 The current status of the commercial production of botanical pesticides

Only products from four plant species have found widespread use as insecticides. Rotenone is a compound produced by Derris and Lonchocarpus species and was widely used during the early part of the century in England as an insecticide. Derris elliptica and D. malaccensis occur quite commonly in East Africa and China where the roots have been used as fish poison. Rotenone has substituted for pyrethrins in mosquito coils but its production has declined because of the much cheaper synthetic analogues, which are available.

The second plant product is pyrethrum which has been produced commercially for more than 150 years. Although precise production figures are not available these have been estimated at 15 000-25 000 tonnes per annum (Kis-Tamás, 1990). More than 90 percent of the world’s production comes from five countries: Kenya, Tanzania, Equador, Rwanda and Japan. Small quantities are utilised locally, particularly for mosquito control, but most are exported, in the form of dried flower heads containing 0.9 percent active ingredient or a concentrated extract containing 20 percent, to the United States of America, Europe and Australia. Pyrethrins are not used for grain protection.

Neem is the only other plant, which has been the subject of significant commercialisation. In the Indian sub-content, neem extracts have been sold for very long periods of time by small-scale producers who serving relatively small markets. In recent years, as neem has increased in popularity the number of commercial suppliers of neem-based products in India has mushroomed (for example the Jawan Crop Protector, a water-based formulation containing azadirachtin and nimbidin, is marketed by McDa Agro Pvt., Ltd of Bombay; Wellgro and RD-Repellin produced by ITC in Andhra Pradesh; Neemark by West Coast Herbochem of Bombay; Nimbecidine by T Staines & Co. of Coimbatore). However, as a pesticide neem has restricted use because of its limited shelf life.

Several western companies have begun to market neem products. One of the first to market neem in the United States, W.R. Grace, has obtained a patent for its production method because it claims to have introduced a stable formulation which is significantly different to those found in India. However, much controversy has arisen regarding this attempt to patent as opponents claim it infringes the rights of indigenous communities in the countries of origin who possess native knowledge of the tree. W.R Grace has since sold its neem products and trade names.

The only other plant to be exploited commercially is Acorus calamus. A preparation containing 70 percent b -asarone, is marketed by Alrich of Germany.

This controversy over neem could be precursor to future problems likely to occur in the commercialisation of other plant-based, naturally -occurring compounds. It is therefore probable that other products will be produced locally in the country of origin, for sale within that country. Local production is unlikely to lead to the development of sophisticated products which need to conform to stringent international regulations regarding toxicological and efficacy testing, so that these products will not be approved by CODEX for use on traded grains and so they will not find markets in the developed world. Instead they will be restricted to local use within a single or perhaps neighbouring countries. Local production should mean the farmer obtains a cheap product when he/she requires it.

Before any of the candidate plants listed in Chapters 3 and 4 can be commercialised even for local production and consumption, it must be shown to be safe to use.

6.2.2 Further research

From the evidence available to date the most promising candidate plant materials for consideration as future grain protectants are Azadirachta, Acorus, Chenopodium, Eucalyptus, Mentha, Ocimum, Piper and Tetradenia together with vegetable oils from various sources. However, in order for plant materials to be seriously considered further information is required.

Standardised laboratory tests need to be undertaken, examining the residual effects of these materials over six to twelve months duration against key insect species. These include A. obtectus and C. maculatus on pulses, Sitophilus spp. and P. truncatus on maize, and Sitophilus spp., R. dominica and Lepidopteran species on rice and wheat. The residual effects of the materials on the commodities could then be critically evaluated and compared for both repellent/antifeedant action and also contact adult mortality and reduction in F1 production.

The plant materials indicated in the provisional list are not exclusive, other plant materials may be considered more effective for country-specific situations. Feasibility studies on the local availability of appropriate extraction and application techniques will also be necessary.

6.2.3 Toxicological testing and registration

Toxicity studies on the effects of the plant materials and their extracts on non-target organisms will need to be undertaken. The use of any plant materials with insecticidal activity is likely to involve some unwanted exposure of human and domestic animals to toxic substances. The use of whole-plant material is likely to be less controversial since by the nature of any extraction procedure concentration of the active ingredients will inevitably result in greater exposure of toxins to non-target organisms. Any form of extraction procedure would also require complete toxicological testing and registration and no organisation should consider recommendation of any such materials that do not have formal recommendation. Since the cost of testing and registration is prohibitive and no commercial company is likely to feel that the market is large enough to justify such an outlay, it would appear that the best chance of use of plant materials in developing countries is of the whole, unchanged material, the use of which would not require registration.

6.2.4 Introduction of plant protectants at the farm and village level

The backing of the village elders or respected members is necessary for successful introduction of any new control practice. A suitable infrastructure for the introduction of plant materials to be used as protectants at the village level must also be available. Despite the fact the some of the materials may be known to farmers, information regarding recommended application rates and handling practices will need to be imparted. This is most likely to be effected through the extension services and in areas in which these are already established would require only training of extension personnel in the understanding and use of such materials. Where no suitable extension services are available the introduction of such materials may be suitably handled by non-governmental organisations, although if they are not already locally known acceptance and trust will not be instantly forthcoming.

6.2.5 Environmental and safety considerations

The plant materials may be commonly found growing wild in the local environment or may require special cultivation. In the case of the latter example especially farmers will need to have a strong enough belief in the likelihood of the plants success as an insect control agent in order to be prepared to invest time and money in the production of the materials. If the plant is not indigenous to the area feasibility studies would be required to determine if its production under certain climatic conditions is feasible and that no unforeseen problems are likely to arise, for example with invasion by the plant of agricultural land.

Special consideration would be required by the recommending body to ensure that simple safety precautions, if required, are adequately explained and understood by all members of the community, which may be exposed to the plant material.  

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