Integrated pest management (IPM) in the control of storage insects
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The characteristic problems of stored-grains pest control in the tropics, with particular reference to developing countries, are concisely described by Taylor, Golob and Hodges (1992). They indicate the crucial importance of effective pest control for a broad range of storage entrepreneurs, including resource-poor farmers, commercial operators, and national marketing organisations; and a wide variety of storage situations in which simple, traditional on-farm systems and large scale bag-storage systems predominate. Bulk storage occurs quite commonly for small bulks, at farm level; but is less common than bag-storage in large scale operations except at large grain mills and at some central marketing depots.
Pest control measures, in general, have to be integrated into an operational system, be it large or small in scale, if they are to be effectively applied. This is a basic principle, not a novel concept, but it connects well with the modern idea of Integrated Pest Management (IPM). The use, in that term, of the word 'management' is appropriate, especially with regard to the need for the integration of pest control measures into management systems, but it should be remembered that the two words, 'management' and 'control', are almost synonymous. The fundamentally important emphasis should be placed upon the word 'integrated' .
Integrated pest control can be defined as the acceptable use of practicable measures to minimise, cost-effectively. the losses caused by pests in a particular management system. For the measures to be cost-effective they must be appropriate to and acceptable into that system. They may be simple or complex but they must suit the system objectives and its technical capabilities. Furthermore, in this context, cost-effectiveness requires that all costs and benefits, including sociological and environmental effects, should have been taken into account.
The term integrated pest management is used to imply that a flexible and technically informed approach is also required. In defining this term it may be considered necessary (McFarlane, 1989) to specify the inclusion of scientific and cost-effective pest monitoring procedures which permit judicious adjustments to the timing, choice and intensity of control actions. It may also be advisable to point out that specific pest control measures, as distinct from general crop or commodity husbandry practices, should generally be omitted unless the circumstances warrant and permit their cost-effective inclusion.
Insect pest management for stored grains, like preharvest pest management, can thus be seen, historically, as a traditional approach in which good husbandry is the primary requirement. Unfortunately, it must also be acknowledged that the advent of readily available, relatively inexpensive synthetic insecticides has led to considerable over-dependence upon these hazardous tools with, in some cases, a consequent neglect of basic good husbandry. The current emphasis upon integrated pest management is, in effect, a reassertion of the need to put traditional good husbandry in place as the fundamental basis of pest control. In grain storage, as with other durable agricultural products, it is good commodity management and good store management which are the major prerequisites (Tables 8.1 and 8.2).
The various options for more intensive insect pest control, which are also listed in Tables 8. l and 8.2, include several which are themselves based upon traditional concepts of pest management. Thermal disinfestation, cooling and hermetic storage are examples. These latter two methods are also examples of the opportunities, provided by the process of storage, to manage the generally enclosed storage environment in such a way that insect pests are prevented from multiplying or, as in efficient hermetic storage, effectively eliminated. Preharvest problems of insect pest control are rarely, if ever, so easily managed!
Control of the storage environment is thus an essential element in grain storage pest management. It involves, primarily, the controls on in-store climate and infestation-pressure which can be achieved by technically sound store design and construction. Equally important, however, is the climatic control attainable by scientific management of the commodity to ensure that the stored grain is itself both dry and cool when loaded or, in ventilated stores and bins with aeration equipment, that the storage procedure achieves drying and cooling sufficiently rapidly. In a fully loaded store it is the stored grain itself which largely determines and stabilises the temperature and humidity conditions in the store.
Commodity management can also control, to a considerable extent, the initial insect infestation level in the stored grain. However, in tropical countries, where preharvest infestation by storage insects is hardly ever completely preventable, the ideal of loading insect-free grain into the store is not often attainable. Special facilities to completely disinfest the grain before loading may not prove cost-effective. The common alternatives, if early disinfestation is required, are to treat the grain, at intake, with a suitable admixed insecticide or to disinfest the loaded grain by in-store fumigation.
Control of grain quality before storage, to minimise the intake of heavily infested and badly damaged or uncleaned grain, is feasible and is commonly practiced to a considerable extent. Even at the small farm level it is possible to segregate the crop at harvest, especially with maize on the cob and unthreshed sorghum and millet, selecting relatively undamaged material with good storage potential and setting aside the more evidently infested or otherwise damaged material which, if there is no other option, can at least be used first. By such means, the rate of deterioration due to insect infestation can be considerably retarded in the main stock of stored grain. There is little doubt that some subsistence farmers use this form of commodity management fairly effectively. Certainly, one can sometimes observe onfarm grain stocks, that have received no special insecticidal treatment, with relatively little insect damage after several months storage at an ambient temperature that would permit the rapid increase of any well-established initial insect population.
Scientific approaches to grain storage pest management, having regard to grain storage as a part of the food production and distribution management system, have sometimes referred to the biological ecosystem concept as a means of comprehending grain storage processes and problems. In a recent contribution Dunkel (1992) has applied ecosystem principles in a broad analysis directed towards an improved understanding of physical and biological interactions including socio-economic factors. The purpose was to generate improved understanding of the stored grain ecosystem and to identify objectives for future postharvest research. This treatment of the subject should serve to enhance the growing awareness of storage as a system within a system and to stimulate systematic and objective analysis of grain storage problems. Whether or not one prefers to use the term 'ecosystem' is less important and this somewhat academic term should not be allowed to obscure the main issues.
Table 8.1. Prerequisites and options for on-farm storage pest management
|Basic IPM||Additional measures|
|Site and store management (protection from birds, rodents and weather plus basic hygiene)||Maintenance of
conditions favourable to natural control:
- by cooling (where feasible) by insect parasites, pathogens, etc. and/or
Thermal disinfestation by solar heat and/or Treatment with traditional additives (if sufficiently available and effective)
|Commodity management (cleaning, drying, etc.)||or
Treatment with synthetic insecticides (if suitable formulations sufficiently available and effective)
Hermetic storage (pits or metal drums, etc.)
Table 8.2. Prerequisites and options for storage pest management at main depots
|Basic IPM||Disinfestation||Prevention of reinfestation|
|Site and store management (protection from birds, rodents and weather plus basic hygiene)||Insecticide
by the treatment Residual insecticide sprays?!
Physical protection! (Sheeted stacks or packaging) or
management (cleaning, drying, etc.)
- with bulk storage if appropriate
Provided by the system
Provided by the system
Provided by the system
Notes: * May entail double handling for in-bag storage. Efficacy doubtful. Extra management skills and/or other inputs required.
The integration of various control techniques, within the framework of integrated pest management, has become a focus for research in stored products work (Evans, 1987a). The importance of a multidisciplinary approach to stored grain research has also been stressed (White, 1992). This is very valid but it is useful to recall that a great deal of the research done in the past has been of this nature. It was pointed out (McFarlane, 1981), at a stored products pest management symposium held in 1978, that the need for an interdisciplinary approach is generally well-known. Entomology, mycology, chemistry, engineering and food science are commonly involved, but effective integration of technical solutions is often lacking; possibly because some of the more pragmatic disciplines, notably economics, sociology and business management, are not always sufficiently involved. It is the interface between the research team and the storage managers, whether these be individual farmers or a large storage organisation, which may sometimes be the most crucial barrier to progress.
A clear perception of the need for solutions which can be integrated into the management system, because they meet the business objectives and can be accommodated within existing management capabilities, is probably the most important requirement (Hindmarsh and McFarlane, 1983). In this context it is of some interest, although not very surprising, that a correlation has been found amongst farmers in India between 'grain hoarding capacity' (which relates to the farmers' existing business objectives and capabilities) and the adoption of improved storage practices (Thakre and Bansode, 1990).
Modern theories of pest management have also generated the concept of economic control thresholds (ECTs). An ECT is most simply defined as the level of pest damage which justifies, in cost/benefit terms, the expenditure of resources upon control actions (Hebblethwaite, 1985). It is always a variable threshold because the costs and benefits of any action will depend upon the situation and its circumstances. An ECT is situationspecific. This is especially true, and not only for postharvest pest control, when one considers the great differences in opportunities and constraints between the small farm level in developing countries and more sophisticated levels of operation. Nevertheless, it is possible to generalise to some extent (Figure 8.1. Conceptual control thresholds for insects on stored grains). For insect control in grain storage the ECT is likely to be at or very close to zero:
In both cases the assumptions are made that at least one potentially cost-effective package of control measures is available and that the package provides for sustained control: including efficient prevention or control of reinfestation. In most sophisticated situations, where consumer quality standards are likely to be high, these will be valid assumptions. Otherwise the ECT could not be zero and the required quality standards would have to be met by reconditioning the product. The losses, including reconditioning costs, would then have to be borne by the system or the consumer. A real economic loss would have occurred, masked by marketing tactics. Such events do, undoubtedly, take place.
Where grain is being stored for domestic use, or for eventual sale in an uncritical market, the ECT may be well above zero. This can be true even when all parameters of economic damage are considered because, in fact, the true economic significance of a low percentage of insect damaged grain may be virtually nil. The actual loss in real food value is likely to be negligible and the market value of the associated weight loss, if saved, may not offset the full costs of pest control actions.
The need to minimise the cost of insect pest control is a major factor militating against the extensive use of some of the more 'environmentally friendly' measures which might otherwise be preferred to the continued use of suitable fumigants and contact insecticides. However, it must also be acknowledged that chemical pest control measures, in grain storage, are often not only the cheapest but also the most reliably efficacious of the possible options. Where this remains true, and for as long as such treatments are accepted as generally safe, one important requirement for IPM in grain storage will be to ensure that chemical measures are recommended only where they can be safely and efficiently used and only when they are economically justifiable.
The economic justification of control measures, including where necessary the use of chemical pesticides, should take account of all costs and benefits. Many of these may be assessable only in subjective terms that are greatly dependent upon local attitudes and sensibilities. However, measurable losses of quantity and certain quality parameters can be objectively determined. A manual of methods for the evaluation of postharvest losses (Harris and Lindblad, 1978) is available and provides useful information, subject to a need for modification of some methods in particular circumstances. A critical review of the methodology (Boxall, 1986) gives further guidance based upon experience gained, since 1978, from field work in many developing countries. Choices among methods, several of which have become subjects of considerable controversy, should always be made with due regard to the actual circumstances and the prevailing objective. There is no single 'best method' for all circumstances and, for practical purposes, operational facility and repeatability are generally more important that fine precision. From the storekeeper's viewpoint a demonstrable loss reduction from, say, 5% to 4%, however statistically significant, may be of little or no practical importance; whereas a reduction from 10% to 5% (or from 2% to 1% in a sophisticated storage system) may be of considerable interest: provided, always, that the demonstrated reduction can be achieved in routine practice.
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