CHAPTER 2: PLANTS AS STORAGE PESTICIDES
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2.1 Current status of the use of plants by small-scale farmers
The utilization of plant materials to protect field crops and stored commodities against insect attack has a long history. Many of the plant species concerned have also been used in traditional medicine by local communities and have been collected from the field or specifically cultivated for these purposes. Leaves, roots, twigs and flowers have been admixed, as protectants, with various commodities in different parts of the world, particularly India, China and Africa.
Where plants are used as storage protectants they are almost always applied to control insect pests. This is reflected in the volume of research directed to identifying insecticidal or insect repellent plants and plant extracts (see Chapters 3 and 4). Nevertheless, some work has been undertaken to determine whether plants can control storage fungi. Most workers have investigated the properties of spices as inhibitory agents of mycelial growth of Aspegillus species and of its toxin production. Syzigium aromaticum (cloves) have been found to be particularly effective, often completely inhibiting both fungal growth and toxin production (Hitokoto, et al. 1980; Mabrouk and El-Shayeb, 1980). Many commercially available spices and herbs, turmeric, basil, marjoram, anise, cumin and coriander, are able to completely inhibit toxin production but only partially inhibit fungal mycelial growth (Hotokoto, et al. 1980). Aqueous extracts of weeds and medicinal plants have also been shown to inhibit toxin production by A. flavus. These include Ricinus communis, Arnebia nobilis and Nicotiana plumbaginifolia (Bilgrami, et al. 1980). Other fungi, Fusarium solani f. Phaseoli and Verticillium albo-atrum, have been shown to be susceptible to tannins extracted from bark of various trees, including chestnut and wattle (Lewis and Papavizas, 1967). More recent studies in Ghana have confirmed these properties of spices. Both Ocimum gratissimum and Syzigium aromaticum were very effective in preventing mould growth (Awuah, R.T, personal communication). However, there is no evidence that farmers actually use spices or other plants for preventing or controlling mould growth in stored commodities.
Additional information on the mycological properties of plants is included in the following chapters. There is very little information on the rodentical properties of plants except in terms of the pharmacological and biochemical effects reported in medical studies, an area of specialism beyond the scope of this review.
Despite the cosmopolitan use of plants as grain protectants knowledge of methods and efficacy of these plants is scanty. Most of the information is derived from anecdotal sources, which provide the minimum of information. Golob and Webley (1990) list many examples of this nature and although the descriptions provide useful information regarding the types of plants used, it is neither possible to transfer technology to other environments, nor even to extend the use of the methods to other communities within the same areas.
Very few systematic studies have been conducted to determine how farmers utilise plant protectants, the methods employed and their effectiveness. Such information is difficult to acquire, requiring the investigator to have a good understanding of the farming and sociological systems operating in the target communities. The introduction of rapid rural appraisal (RRA) and participatory rural appraisal (PRA) techniques in recent years has facilitated the collection of this type of information and since 1992, some studies have been undertaken.
A survey of plants used as domestic insecticides was conducted over a seven month period in 12 out of 15 districts in forest areas of the Ashanti Region in Central Ghana (Cobbinah, et al. unpublished, in preparation). Fifteen villages or towns were visited in every district and groups of farmers interviewed in each, almost 500 farmers were interviewed individually. Twenty-six different plant species were found to be used as grain storage protectants, the most common being Chromoleana odorata (Siam weed), Azadirachta indica (neem) and Capsicum annum (chilli pepper). Insecticidal activity against insects other than those of stored grain pests was reported in more than 90 other plant species. The survey found that while approximately a quarter of the farmers used plant protectants in some form, only a small percentage rely on them entirely to protect their harvest from storage pests. Smoking maize stores was the most common method of control in most districts with the exception of the two major areas producing regular maize surpluses where conventional synthetic insecticides are preferred.
In a shorter PRA survey in northern, semi-arid regions of Ghana only 16 plants were identified as being used as grain protectants (Brice , et al. 1996). Apart from neem, none of the plants featured in the list of stored-product protectants were used in the Ashanti Region. Two of the plants, Chamaecrista nigricans and C. kirkii (both known locally as lodel), said to be the most effective, have not been recorded elsewhere, worldwide, as having this property and, consequently, do not feature in any research programmes devoted to plant protectants of stored products; these particular plants, like many of the others used in Northern Ghana, are weeds and serve no other useful purpose.
In contrast to Ghana, there is no real tradition of using plants as insecticides in Malawi. Thirty-one groups of farmers, almost a thousand people, were interviewed in a series of surveys conducted in the Northern and Central Regions. Poor quality tobacco leaves, those unfit for sale or sweepings, are used by farmers to protect stored grain. Only in one village was the dried powdered root of a shrub, Dolichos kilimandscharicus, used as a protectant of shelled maize. No other use of plants as storage protectants was recorded though a few others were used to protect growing cereal crops and vegetables and as fish poisons (Taylor, et al. 1995).
It is clear that there is a dearth of information concerning actual use of plants by farmers. There are, without doubt, very many plants used as grain protectants by rural communities, which have yet to be identified. If local production of plant protectants is to be encouraged then it is essential that farm practices are recorded and more information acquired. However, it may prove to be impractical to record treatment application rates and methods because these may vary considerably. Very little information has been acquired which describes how farmers apply plant protectants; this applies to the three studies mentioned above. This is because farmers are either unable to describe the procedures with sufficient accuracy or their accounts vary considerably from one farmer to another. Thus research programmes which pursue optimal methods of using plant protectants on grain must strive to develop the most cost-effective procedures for application as well as identifying active components.
2.2 Current research
Research workers have generally investigated the efficacy of locally available plants for controlling insect pests. Research is being undertaken in many countries including India, Bangladesh, Pakistan, the Philippines, Japan, Rwanda, Nigeria, Ghana, Kenya, Egypt, Israel, United Kingdom and the United States of America. However, most of the published work provides insufficient information to enable most plant materials to be used for practical pest control purposes, and data on mammalian toxicity, is also lacking.
Both fresh and dried plant material have been examined and although various parts of the plant have been assessed in these trials (rhizomes, roots, stems, bark, seeds, and fruits) most have used leaves. Fresh or dry powdered material is added to the commodity, usually at 1 to 5 percent weight for weight (w/w) although higher concentrations, e.g. 18 g 100 g-1 for citrus peel (Don-Pedro, 1985) have been used.
Various methodologies have been used to determine the effectiveness of plant materials and their extracts. Almost all trials were laboratory-based and of short duration and therefore do not necessarily reflect responses which would be observed under real farm conditions; laboratory experiments cannot account for variables such as fluctuating ambient climatic conditions, the effects of store design and structure and the continuos disturbance of grain by livestock, a multitude of insect pest species and the family itself. Although laboratory investigations can provide useful indicators regarding efficacy of plant products it is essential that trials are also carried out in the field, on farms. Unfortunately, there a very few examples of on-farm or simulated field trials so this is an area of research which must be facilitated and resourced as a priority.
A few trials have been undertaken at research stations in developing countries and these partially simulate on-farm conditions. These trials have mostly utilised plants which have shown considerable promise in the laboratory. Some examples are as follows: Application of Azadirachta indica (neem) seed extract at 8 percent by weight to wheat in jute bags in the Sind, Pakistan was considered to be as effective as 5 mg kg-1 pirimiphos-methyl, reducing insect populations after six months storage by 80 percent; calculations demonstrated positive benefits over costs, with ratios of 4.7-7.4 (Jilani and Amir, 1987). In Nepal, dried powdered rhizome of Acorus calamus (sweetflag) has been found to be effective in almost eliminating insect damage when applied to maize cobs in traditional storage barns (Duwadi, V.R., personal communication).
Attempts to control Prostephanus truncatus, the Larger Grain Borer, using extracts of neem leaves were found to be relatively unsuccessful in simulated storage experiments in both Tanzania (Golob and Hanks, 1990) and Ghana (Compton, J personal communication). Assessment of on-farm use by farmers themselves, using participatory research techniques is lacking.
The methods of examination of investigations undertaken in the laboratory to date can be classified into two groups: direct admixture of plant material or utilisation of a solvent extract or essential oil.
2.3 Admixture of plant materials
Trials of plant material admixed with a commodity, usually directly assess contact toxicity to adult insects, number of eggs laid, number of eggs which hatch or the percentage of first filial (F1) adult emergence. Alternatively, indirect measurements are made by recording the loss in weight due to insect feeding damage, or the number of damaged grains. Assessment of adult mortality may be observed within 3 to 24 hours of treatment, or may be delayed to 7 or 15 days after treatment. Trials, which assess repellency, usually incorporate modifications of the Loschiavo food preference apparatus, described by Laudani and Swank (1954), which was developed to assess the repellency of pyrethrum applied to maize. The apparatus consists of a rimmed platform, which has circular evenly, spaced holes around the outer edge. These accommodate paper cups, which can be filled with treated and untreated commodity. The platform is enclosed by a lid, which is fitted with a central hatch/access portal, through which insects can be lowered to the test platform. The insects are then allowed free movement, across the surface of the base, for a specified period of time. The numbers of insects present in each cup are then recorded. Adult mortality and subsequent F1 emergence may also be assessed.
2.4 Extracts of plant materials
Extracts of plant material rely on the solubility of the active components. Various solvents are utilised, the most commonly used being chloroform, petroleum ether, hexane, methanol, ethanol or acetone. Unfortunately, few workers in this field have reported on attempts to isolate and identify active compounds within the extract. Moreover, the weight or yield of the active component, from a particular extraction technique, is rarely given. Therefore, its concentration on final application to the commodity is not known, although serial dilutions of extract are frequently subjected to insect bioassay. Essential and vegetable oils may be extracted from appropriate plant species in the laboratory or alternatively commercial preparations may be examined. Vegetable oils are applied at 5-10 ml kg-1 commodity although higher values, in excess of 50 ml kg-1, have been recorded (Don-Pedro, 1989b). Extracts have been applied to filter paper in various forms of insect bioassay: repellency trials, antifeedant tests and vapour/fumigation trials.
2.5 Repellency trials
In repellency trials the standard methodology described by Laudani, et al. (1955) is used. Extracts containing active components at various concentrations up to a maximum of 800 ug cm-2 are applied to filter paper or aluminium foil laminated paper. A strip of the treated paper is attached, edge to edge, to an untreated strip and covered by a glass arena, so that the joined edge bisects the ring. Insects are then introduced into the arena, the species generally used being T. castaneum. Usually ten adult insects of unknown sex and age (in a few experiments 120 adults, aged 7-14 days have been used) are released in the arena and the numbers present on the treated and untreated halves are recorded twice daily at 0900 hrs and 1600 hrs for five consecutive days. The average counts for each five day period are then converted to percent repellency. Results are assigned a repellency class by using the following scale: Class 0, <0.1 percent; class I, 0.1-20 percent; class II, 20.1-40 percent; class III, 40.1-60 percent; class IV, 60.1-80 percent; class V, 80.1-100 percent. Repellency may be assessed immediately after application and reassessed at one to four weeks or from one to four months after application.
2.5.1 Antifeedant tests
In antifeedant tests, extract is applied to wafer discs, filter paper or to paper packing material. The number of holes produced by boring insects are then counted per unit time, usually for a seven day exposure period.
2.5.2 Vapour or fumigant toxicity assessment
For assessments of vapour or fumigant toxicity of essential oils the extract is usually applied to filter paper, left to dry and then suspended in a glass fumigation chamber. Insects are introduced into the chamber and adult mortality, at a given concentration, is then recorded within a defined period of time. Generally these trials extend over 24 hours.
2.6 Problems associated with current research
Most of the papers reviewed are questionable with regard to the methodologies employed for evaluating plant materials, particularly with respect to the amount of commodity initially treated and the number of insects used in the trial. Frequently only one, five or ten pairs of insects were used in each replicate and these were placed on commodities treated at various rates. In addition, observations are generally made on the effectiveness of the plant material over one generation of insects following application, and rarely are the materials re-evaluated over subsequent time intervals to assess the residual effects of the materials. Occasionally trials have been described which indicate the level of protection provided by a material over several months, and which has only been investigated with respect to effectiveness against adults, which have died before producing eggs. In such instances, comparison with the untreated control does not offer a real indication of the residual activity of a compound. In other trials, which have been carried out under normal conditions in farm stores, treated and untreated small sacks of commodity have been left to acquire natural infestations. This method of experimentation does not really quantify the persistence of a plant material although, if the control and treatment are subjected to the same level of infestation pressure, the period for which the treated sacks remain relatively uninfested does give some measure of practical efficacy, including effective residual protection.
In many of the papers reviewed, although the effectiveness of the plant material or extract against particular insect species has been described, their mode of action is not outlined. For example, although the failure to produce an F1 generation is frequently reported, an explanation is not given as to whether this was due to direct adult mortality before egg-laying, egg mortality, direct larval mortality or indirectly due to larval repellency from the treated commodity and hence death by starvation. A knowledge of these factors is crucial in attempting to understand the effectiveness of a particular treatment.
2.6.2 Identification of active ingredient
Various solvents have been used in extraction techniques. However, when the plant extracts are tested, few workers have attempted to isolate the insecticidal or non-insecticidal constituents present. Extensive cross-referencing has been necessary to provide an indication of the components or constituents, which may be present in these extracts. Many of the plants which have been tested are found across a broad geographical range, however, only in a few instances has a particular plant species been examined in more than one country. It is well established in plant chemistry that differing climatic, soil and seasonal conditions can affect the type and quantity of the components isolated from extracts. For example, Acorus calamus is widely distributed in ponds and lakes in Asia, North America and Europe. The rhizomes of the plant contains ß-asarone, a compound which shows toxic and sterilising effects against insects. Three caryotypes of A. calamus have been found: the tetraploid caryotype (4n=48) found in India, East Asia and Japan contains 70-96 percent ß-asarone whilst the diploid caryotype (2n=24) found in North America and the triploid caryotype (3n=36) present in Central Europe contain less than 15 percent of the active ingredient (Schmidt and Streloke, 1994). Researchers in the future will need to take full account of these strain variations when evaluating the potential of particular plant species to avoid utilising plant material from sources with low insecticidal activity.
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