Alternative and supplementary control measures
Contents - Previous - Next
The currently available options for grain storage and the pest control methods usually associated with each storage technique are reviewed in Table 8.11. In Table 8.12 (see Table 8.12. Pest control techniques: current options.) the pest control methods themselves, including some of the biological techniques referred to only briefly in the previous Table, are reviewed more comprehensively with regard to their principal advantages and disadvantages.
These Tables, together with those presented at the outset of this chapter (Tables 8.1 and 8.2) should make it fairly clear that most insect control techniques are inherently related to certain forms of storage and, moreover, that none of them is perfectly complete and without disadvantages. Chemical control techniques, although discussed separately in this chapter, should also be seen to depend upon good basic storage practice and to be supplementary to the control achieved by other techniques.
The effects of various physical factors upon insect development and control have been discussed already. The particular measures that are important as supplements to other insect control procedures are cleaning and drying. Those which may provide alternatives to other forms of control are cooled grain storage, hermetic storage, thermal disinfestation and, in some circumstances, mechanical disturbance.
The cleaning and drying of grain for storage are essential measures and the techniques are described elsewhere in this bulletin. Practical difficulties in achieving the desired freedom from excess moisture and foreign matter are frequently encountered. There can be no doubt that failures to overcome such difficulties do occur and that these lead to increased insect infestation. The rate of insect development may be somewhat accelerated and, more importantly, the spectrum of infestation will be greatly increased (Table 8.4). Practical recommendations take best account of this when they acknowledge the difficulties that may occur but emphasise the need for cleaning and drying to be done as thoroughly as possible, especially when grain is to be stored for a long period. The longer the expected storage period, the greater the need for efficient cleaning and drying.
Techniques for the storage of damp grain, in hermetic conditions, under controlled atmospheres or with mould-suppressant treatments, have been developed but these are regarded as unsuited to the storage of grain for use as human food (Christensen, 1982). However, the practical value of ventilated cribs for the storage of maize on the cob and other grains on the head or in the pod, when insufficiently dry for sealed storage, should not be overlooked. Advice on optimum design for maize cribs, with particular reference to the humid tropics where the restriction of crib width to facilitate drying is important, is given in FAO Agricultural Services Bulletin No.40 (Anon., 1984).
The development of other temporary storage procedures, especially for underdried rough rice, has received much attention in countries where the introduction of new cultivars has led to massive production increases and, sometimes, to the regular harvesting of grain in wet weather. Limited applications of mould-suppressant chemicals, such as propionic acid, have been found effective and may be acceptable for short holding periods (5-7 days) prior to proper drying (Kamari and Yon, 1980). The use of admixed desiccants, such as common salt (sodium chloride) or wood ash, may also be of limited usefulness.
Aeration and cooling, by natural aeration in small, ventilated stores (e.g. maize cribs), or by forced aeration in larger stores, can significantly retard the development of insect infestation. Where it is possible to reduce the temperature of a grain mass to 17°C or less the infestation will be effectively suppressed although not eliminated. Suppression could be achieved, by selective aeration, in many parts of the tropics where early morning temperatures are of this order. More attention should perhaps be given to this (Gough and McFarlane, 1984; Calderon et al., 1989). The particular importance of maintaining relatively cool storage conditions for seed grain stored in tropical climates is well known (Christensen, 1982). The trade-off between design costs, for improved thermally-insulated storage structures, and the cost of drying the grain to very low moisture-content, to counteract the effect of high temperature, has been analysed by O'Dowd et al. (1988).
The principles of hermetic storage are outlined elsewhere in this chapter. Small-scale applications in the tropics are not uncommonly reported and attempts have been made to encourage the use of this technique in many parts of the tropics. However, it can only be cost-effective, in practice, where the storage management objectives will accomodate the principle and where suitable containers are available at a reasonable price. It is best regarded as a technique for selective application to particular commodities or to particular stocks clearly identified as reserves for protracted storage. Large-scale applications are likely to be handicapped by the cost of maintaining airtightness in large structures and by the common commercial requirement that grain stocks should be renewed at regular intervals (Hyde et al., 1973). However, considerable interest in the technique remains (Calderon et al., 1989).
Thermal disinfestation techniques include simple exposure to the heat of the sun, a traditional procedure that can achieve disinfestation in thin layers of exposed grain but which may often, in practice, do no more than drive off any adult insects or free-moving larvae. At the other extreme is the sophisticated technique, based on fluid-bed grain drying systems, described by Dermott and Evans (1978). Between these extremes lie opportunities for using solar drying equipment for grain disinfestation (McFarlane, 1989) and the occasional use of conventional hot-air grain dryers for this purpose in the reconditioning of infested grain. All of these techniques need careful management to ensure an effective kill of all stages of the insects in the grain without causing physical (thermal-stress cracking) or physiological (germinability loss) damage to the grain. This can be achieved in the simplest and most sophisticated systems, it is least likely to be achieved by the use of conventional hot-air dryers. Thermal disinfestation (like fumigation) provides no ongoing protection against reinfestation and, moreover, if heated grain is put into storage without sufficient cooling any subsequent infestation may develop very rapidly.
Mechanical disinfestation techniques also show a range of refinement from the simple turning of grain through bulk-handling systems (Joffe, 1963) to the use of sophisticated percussion machines (entoleters) in flour mills. As with thermal disinfestation, the treatment provides no ongoing protection and may cause physical damage to the grain which, if it is returned to storage, may therefore be made susceptible to infestation by a greater range of insect species.
Traditional grain protectants
The occasional use of abrasive mineral dusts, natural desiccants like wood ash and various plant materials with repellent or insecticidal properties is well known and documented (Golob and Webley, 1980). Recent interest in such materials, intensified by a common concern to reduce, if possible, the general dependence upon synthetic pesticides by promoting the use of alternative materials, has produced a flood of information on experiments that have tested many plant materials. Regrettably, much of the published information is of limited value because practical aspects, including availability and acceptability for use as food grain protectants, are generally overlooked. However, a new bibliographic database on this research has been produced (Rees, Dales and Golob, 1992) which sorts more than 1000 references to work, mostly published since 1980, according to the materials used and the insect species against which they have been tested. The authors point out that the majority of papers in the database describe laboratory experiments or small-scale trials at research stations and that the conclusions drawn by the authors therefore have little significance for practical application. They indicate the need for further work that focusses attention on practicalities. This should, incidentally, reduce the currently over-extended list of candidate materials to more realistic proportions.
It is fair and useful to note here that there have been a few exceptional papers on work in this area. Some recent work in Colombia (Baler and Webster, 1992), for example, included practical on-farm trials which assessed a vegetable oil, kitchen ash and black pepper as protectants for stored beans and included realistic evaluations of economic effectiveness and acceptability. The latter aspect included effects on germination, palatability and cooking time, which were found to be insignificant. All three treatments gave effective protection against A. obtectus for several months, taking 4% grain damage as the economic damage level.
Other workers have identified various commonly available cooking oils, notably palm oil but also groundnut oil and coconut oil, as being particularly effective (and used in some countries) for the protection of pulses against bruchid beetles. The oil obtainable from the seeds of the widely-grown neem tree (Azadirachta inidca) has also been found effective but comprehensive evaluations of its economic acceptability are less easily identifiable. Makanjuola (1989) gives a good account of laboratory investigations and field trials in Nigeria that tested other materials from the neem tree, including water-based leaf extracts, for the protection of cowpeas and maize. The results showed good protection of cowpeas (against C. maculates) for five months but only moderate protection of maize (against 5. zeamais) and found that seed extracts were more effective than leaf extracts.
Modern biological methods
Irradiation techniques and controlled atmosphere storage are included here, although they may also be regarded respectively as physical and chemical techniques, because their use depends upon radical interference with biological systems or processes.
Watters (1972) and Banks (1976) give useful reviews of the possible applications of various irradiation techniques. There has been much subsequent research, especially to determine suitable dosage rates and operational procedures, with regard to safety as well as efficacy, but the use of irradiation as a direct control measure remains limited by basic problems of capital cost, running costs and other aspects of practical feasibility. The method shares with fumigation and thermal disinfestation the obvious disadvantage that it confers no protection against reinfestation. Insect resistant packaging of grain or grain products, immediately prior to irradiation, would seem the most logical adjunct in countries where socio-economic circumstances favour the adoption of this sophisticated and relatively expensive control technique. The indirect applications of irradiation, to achieve the suppression of pest populations through the release of sterilised males of the pest species, appear unlikely to prove economically attractive for the widespread control of grain storage insects.
Controlled atmosphere (CA) storage has become an important addition to the available options for stored-grain pest control. Extensive information on CA storage is now available and recent symposia on this research area have presented several comprehensive compilations, the most recent by Champ, Highley and Banks (Ed.) 1989. The present position and future prospects are usefully reviewed by Banks, Annis and Rigby (1990).
Conventional biological control techniques for possible application in stored-grain pest control, including control by the use of predators, parasites, insect diseases and sterile males, the use of pheromones for pest monitoring, mating disruption or enhanced mass trapping, and the use of resistant crop varieties, are summarised in McFarlane (1989), based on papers by Dobie (1984), Haines (1984) and Hodges (1984). There are published reports of the successful practical application of a number of these techniques, notably in the USA (McGaughey, 1978; Arbogast and Mullen, 1990; Brower and Mullen, 1990; Brower and Press, 1990), but the area of most interest for application in tropical countries is the use of crop varieties with resistance to storage insects as well as preharvest pests. The conceptual impact of some of these biological control techniques is indicated in Figure 8.4. It should be noted that control by the use of a resistant variety will generally retard the increase of infestation and grain damage, thereby prolonging the period in which damage remains relatively low, while control by predators or parasites can be expected to suppress the pest population and the consequent grain damage but is unlikely to restrict insect numbers or grain damage to a low level.
Current possibilities for integrated pest management
(i) Farm level improvements
As suggested by Dobie (1984) and many other authors the development and use of improved grain cultivars, with resistance to storage insects as well as to preharvest pests, could provide the key element in IPM for stored grains. This would be of particular importance for loss reduction at farm level because, if the improved cultivars were both agronomically suitable and acceptable in all other respects to farm-level users, the adoption of this lPM strategy by farmers should be quite straightforward and would require no change in their traditional approach to grain storage. It would permit the renewed realisation of traditional concepts of safe storage, for a substantial period, by good husbandry alone (Figure 8.4. Pest population growth (solid line) and increase of grain damage (solid/broken line) as affected by different pest management regimes., diagram A). It must be understood that this would not, in most circumstances, reduce on-farm storage losses to less than the customary level generally accepted by farmers storing their own preferred varieties. However, it would reverse the trend towards increased losses which has been observed in those areas where farmers have been encouraged to plant high-yielding varieties which, typically, are more susceptible to damage by storage insects. Moreover, there should be a net gain provided that improved resistance to storage insects can be coupled successfully with high yield characteristics.
Tactical opportunities for supplementary improvements in grain storage by small-holder farmers are indicated and discussed by Golob (1984) with particular reference to maize grain but considerable relevance to most other grains. They include realistic modifications to traditional storage structures to enhance their performance or to adapt performance to seasonal climatic change. The relative efficacies of various grain protectants, including some of the common traditional materials, are also considered. It is clear that several of these do have some value as a means of further extending the safe storage period but it remains true that reliable formulations of suitable contact insecticides, where these are available to the farmer at a reasonable price, are likely to prove more cost-effective so long as they are properly applied and judiciously recommended. Recommendations for widespread use, without regard to the particular storage objectives of individual farmers, are unlikely to be generally adopted.
A need for improved grain stores, modelled on larger-scale bulk storage bins suited to more sophisticated management, is a popular idea that should be treated with considerable caution. There are examples of such developments that have proved successful but a great many more have failed because the real needs and management capabilities of small-scale farmers have not been perceived.
(ii) Improvements in large-scale storage
The main technical options for insect control in large-scale storage, which generally occurs in developing countries at the main depot level or in large grain mills, are summarised in Table 8.2 and have been discussed elsewhere in this chapter. Table 8.2 indicates those techniques which require additional measures for sustained control and those which provide, in the technique itself, this essential element. Measures intended to prevent re-infestation that are of doubtful effectiveness, for reasons already discussed, are pointed out as are those techniques which are likely to require substantial management inputs to ensure success. Bulk storage, which can reduce pest problems or facilitate pest control, should also be considered but should not be regarded as a panacea. The advantages and disadvantages of bulk storage, with particular reference to its use in the humid tropics, are discussed in Champ and Highley (Eds.) 1988.
The choice amongst the technical options to develop cost-effective packages of measures for well-integrated pest control cannot be made without reference to particular situations. As has been previously stressed, it is the storage management objectives, together with the technical and financial constraints, that must be identified and analysed in each case. However, it is of interest that recent decades have seen a marked swing towards the use of physical barriers against re-infestation in combination with improved conventional fumigation or the introduction of controlled atmosphere storage techniques. Notable developments in this direction have been for milled rice storage in China (and in S.E.Asia; Annis et al., 1984), but this approach has considerable technical merit and is potentially of more general application.
The attainment of fully integrated pest management in large-scale storage will depend largely upon the development and adoption of improved pest-monitoring procedures, with increased capability for measuring pest population levels as a parameter of grain damage and quality loss, so as to ensure as far as possible the most cost-effective timing of pest control actions. Here again, in developing countries, recent advances in this direction have been particularly concerned with milled rice storage (Haines et al., 1990).
Increased attention to the monitoring of re-infestation pressure is noted by Desmarchelier (1977) as a requirement for the more cost-effective use of admixed insecticidal protectants. It is recommended here also that judicious use of grain cooling techniques to achieve a net enhancement of insecticidal efficacy in such treatments. Even for those insecticides which show a positive correlation between temperature and toxicity to insects the increase in chemical persistence, at lower temperature, outweighed the reduction in toxicity.
These several lines of research exemplify the possible refinements of established insect control procedures that are required for improved storage pest management. Such approaches are likely to prove more beneficial than attempts to devise complex packages of control measures, including as many as possible of the various available options, with the mistaken idea that IPM necessarily calls for such complexity. As was stated at the outset, the essence of IPM is the integration of cost-effective measures with management objectives and capabilities. With regard to this purpose, current efforts to develop computer packages ("Expert Systems") to guide management decisions are of interest. Systems with potential relevance to grain storage in the tropics include one, announced by the Australian Centre for
International Agricultural Research (ACIAR), which will provide advice to optimise the use of grain protectants, and another, from NRI, which addresses more generally the application of pest control in grain storage.
Contents - Previous - Next