Losses of wheat due to inadequate storage and other post-harvest factors at the farm, village and commercial levels of up to 4 percent have been observed (McFarlane, 1989; Abdullahi and Haile, 1991), though losses in excess of 40 percent for other cereals are not uncommon (NRC, 1996). Deterioration of stored grain is influenced by physical (temperature, humidity), biological (microflora, arthropod, vertebrate) and technical (storage conditions, methods and duration) factors. Experience has shown that such losses are not easily reduced in the absence of well-integrated policies and plans to develop the total system of production, marketing, storage and distribution (Tyler and Boxall, 1984).
Food storage pests seem to have been associated with grain stores since time immemorial. Storage pests have been identified in grain stores found in the tomb of Tut'ankhamun (1345 BC) and other ancient sites (Buckland, 1981). So too, the discussion of proper grain storage techniques is not a contemporary issue. The ancient scholars Aristotle, Pliny and Vergil offered observations and recommendations of grain storage techniques, including the use of seed dressings of olive oil to kill infesting insects (Panagiotakopulu and Buckland, 1991).
Pre-harvest grain losses and damage
Modern and traditional methods of wheat harvest technology often depend upon the scale of production. Self-propelled mechanical threshers may be found in use in all countries. However, reaping by hand-pulling of plants or cycling, stooking, shocking and stacking of straw (Plate 72) followed by threshing, upon need of grain for food or sales, by stationary mechanical threshers or on oxen-trodden mud-packed threshing floors is common in many small-scale farming situations. On-farm storage of grain is common for smaller scale farmers, whereas direct sales to cooperative, government or private elevators are the norm for larger producers.
In all situations, grain must be harvested in a timely manner, before shattering, pre-harvest sprouting, bird damage or weathering, to minimize pre-harvest losses, yet must be dry enough for storage. During threshing, cracking and breaking the grain should be avoided since damaged grain invites greater damage from storage moulds and insects, and reduces marketability.
Harvesting at the proper time can minimize shattering or pre-harvest sprout damage, but untimely harvests are often beyond the control of the farmer. Grain that shatters before and during harvest not only yields no return, but may cause additional expense as a volunteer crop. Pre-harvest sprouting reduces seed viability and may result in milled flour with inferior baking properties due to an excess amount of alpha-amylase that causes excessive liquefaction of dough and results in a wet and sticky crumb (Bloksma and Bushuk, 1988).
Alternately, grain may be harvested at a moisture content higher than is safe for storage, by reaping or swathing the grain and allowing it to dry in windrows, sheaves, stooks, shocks or stacked (Plate 73). Wheat may be reaped or swathed with no loss of yield at any time after the completion of the maximum-weight phase of grainfill, which occurs when the moisture content of the kernel has declined to about 35 percent (Kiesselbach, 1925). These procedures allow wheat to dry more quickly, prevent harvest damage due to the presence of late weed growth and protect the otherwise standing grain from weathering.
Czarnecki and Evans (1986) observed that pre-harvest damage due to weathering was cultivar dependent. Five cultivars of wheat, evaluated over two years, were subjected to the effects of weathering by exposing wind-rowed grain to varying periods of rain. Moderate amounts of precipitation caused a significant reduction in test weight affecting density and packing efficiency. Cultivars differed in test weight loss and in susceptibility to bleaching.
The redbilled quelea (Quelea quelea), perhaps the most numerous bird in the world with its population having been estimated at between 1 to 100 billion (Elliot, 1983), is perhaps the greatest biological limit to African cereal production (NRC, 1996). Flocks of hundreds-of-thousands of quelea, not uncommon, can result in feeding and shattering damage of between 1 to 100 tonnes/day, depending on crop stage. A number of countries in East, Central and Southern Africa support lethal control programmes using aerial spraying of aveotoxins. Purpose-built trapping roosts have been studied in Zimbabwe to provide a means of control that is not dependent upon chemical toxins and that can provide a source of protein through processed 'poultry' (NRC, 1996).
Post-harvest grain losses and damage
Occurrence of microflora fungi and inoculum on grain is cosmopolitan, and thus prevention of some losses or avoidance is difficult to achieve. The use of fungicides that are non-toxic to the grain or to humans has not yet become common practice and drying, both prior to and during storage, remains the only practical method of control. Microflora damage grain by: (i) damp grain heating, which causes caking and fermentation or rotting; (ii) reducing food value as a result of degradation of starch and protein, mycotoxin production and production of a musty, unappetizing smell; and (iii) jeopardizing its ability to germinate through injury to the germ.
Insect and mite pests commonly associated with damage in stored wheat are listed in Table 23.4, chapter "Insects in wheat-based systems". Internally feeding insects, whether adult or larvae, feed on grain endosperm and/or the germ. Endosperm feeding results in grain weight loss, reduction in nutritive value and deterioration in end-use quality. Damage to the germ results in a reduction in seed germination, while both types of feeding will reduce seed viability and vigour. Externally feeding insects damage grain by excrement contamination, empty eggs, larval moults, empty cocoons and adult corpses. None of the pests of stored wheat are confined to wheat alone.
The damage done by a single insect, in terms of actual food consumption, is generally quite little. For grain weevils, for example, it amounts to only 10 to 20 mg per insect during the larval feeding stage. It is the capacity for very rapid population growth, however, that makes insect infestation a major cause of food loss in storage (McFarlane, 1989).
The principle factors that lead to the proliferation of insects in stored grain are temperature and humidity. In general, the condition suitable for most insect pests is a temperature around 30°C, with a relative humidity of 40 to 80 percent. Above 40°C, all species will eventually be killed, while reproduction ceases at temperatures below 20°C, with dormancy induced at temperatures below 10°C. At less than 40 percent humidity, reproduction is inhibited.
Occurrence of mites in stored grain is related to high grain moisture content. Thus, under normal storage moisture conditions, harm due to mite damage is minimized. Losses caused by mites have been little studied, but it has been found that the nutritive value of monogastric animal feed heavily infested by Acarus siro is seriously reduced (McFarlane, 1989). Mites may also cause allergic responses in animals eating infested feedstuffs and in workers handling them.
Grain deterioration in storage can be minimized or prevented by keeping the grain dry (less than 12.5 percent grain moisture), cool (less than 10°C) and free from insects. Concerted efforts should be made to eliminate grain storage insects from remnant grain left in the storage bin. A small number of resident insects in a bin, or introduced with the grain, can lead to a serious infestation if the grain is warm, or if the grain remains in storage a long time. Alternately, brief, high-temperature treatment of grain has been found to disinfest all stages of Sitophilus granarius in wheat (20 minutes at 70°C; Zewar, 1993) and other storage insects (two minutes at 55°C; Lapp et al., 1986).
Sitophilus weevils may lay their eggs on or in the grain as it ripens, or during the period between harvesting and storage. Most insect pests of stored grain, however, attack the crop only after harvest, and the infestation often originates from residual populations resident in storage structures and from other stored commodities. Infestation may occur during transport through contact with contaminated grain or from aerial contamination. The primary source of infestation, however, is often due to unhygienic storage conditions, poor maintenance of storage facilities since the previous crop was removed, or by contact with previously contaminated grain.
Coordination between the three principle approaches to protection of stored grain - physical, biological and chemical - is collectively known as integrated pest management (IPM). Cook and Veseth (1991) refine this concept further recognizing: (i) start by thinking of ways to take maximum advantage of nature's own controls; (ii) then decide on inputs that are occasionally needed to enhance or supplement nature's controls; and finally (iii) decide on inputs to be made before and during harvest and during grain cleaning, transport and storage to control economically storage losses.
Customary pest control techniques are:
controlled climate, including cooling and aeration to reduce insect reproductive rate and humidity;
controlled and airtight storage atmospheres;
heat treatment (more than 60°C) to disinfest grain;
physical barriers to protect uninfested grain;
fumigation with penetrating gases;
contact insecticides to control flying insects;
admixing an insecticide or grain protectant;
trapping to control rodents;
resistant crop cultivars;
statutory regulations affecting the quarantine or control of storage insects.
These techniques are based on the elementary concepts and objectives of pest management. The basic concept involves the storage of clean grain in clean, cool, well-protected conditions for the shortest time possible. The concepts are:
The objectives are:
Monitoring insect infestations
Frequent monitoring of grain storage conditions to identify possible problems associated with storage moisture and temperature and pest infestation is important. Due to their small size, visual assessment of insect infestation in grain stores is often difficult or ineffective, unless large numbers of insects are present. Traps baited with synthetic aggregation phe-romones have been developed for detection and monitoring of stored grain insect pests. Low cost, combined with species specificity, make pheromone-baited traps ideal monitoring tools in developing countries (Campion et al., 1987).
Unfortunately, many resource-poor farmers use grain storage structures that are inadequate to protect grain from rain or dampness and rodent or insect infestation (Table 27.1). Recently, appropriate storage structure technology demonstrated by Chinese instructors, through support from the Food and Agriculture Organization of the United Nations (FAO), has been introduced to Africa. These airtight grain stores are made from clay and straw, are simple in construction, are low cost and have the potential to reduce significantly post-harvest grain losses (NRC, 1996).
Small-scale and commercial grain storage structures
Associated pest control methods
Bagged or small container storage
Sealed pots, gourds, etc.
Self-disinfesting, if sufficiently airtight
Admixing a grain protectant
Wrapped bags in sealed pits or bunkers
Self-disinfesting, if sufficiently airtight
Small-scale bulk storage
'Smoking' over an open fire
Self-disinfesting, if sufficiently airtight
Large-scale bulk storage
Bunkers and bins
Admixing a grain protectant
Source: McFarlane, 1989.
Demonstrations in Ethiopia have proven that ferrocement (a form of reinforced concrete) silos can be built relatively inexpensively, using unskilled labour, with losses of less than 1 percent per year. The Volcani Institute in Israel has pioneered development of simple, inexpensive, easily movable grain stores especially suitable for handling emergency food supplies and for storing excess grain from bumper harvests in drier environments.
For damage to stored grain due to rodent infestation, prevention is better than a cure. Contrary to damage caused by insects or fungi, rodents can infest stored grain regardless of temperature or humidity, the only requirement being access to the grain store. Storage structures should be built with rodent-proof air vents, roofs and door jams. In addition, surrounding areas should not be considered general storage areas and should be kept clean from debris. Proper use of fumigants and rodenticides is outlined by Appert (1987). He comments, "it is vital that the negligence, carelessness and fatalism that are all too prevalent should be replaced by constant, impeccable vigilance."
Control of insects by synthetic insecticides
Chemical fumigation and treatment of grain against rodent and insect damage should be conducted when cost-effective and when sustainable techniques are available that are environmentally and user safe (see Appert, 1987). Discussion of appropriate or specific insecticide use, dosage and precautions will not be discussed here due to the specificity of licensing agreements and recommendations in a rapidly evolving environment.
A common means of insect control in stored grain in the Asian subcontinent is through the use of insecticide (e.g. malathion, chlorpyrifos-methyl or deltamethrin) impregnation of jute bags or prophylactic tarps, with effective control of up to nine months observed (Rai et al., 1987; Yadav and Singh, 1994).
Synthetic sex pheromones have been successfully used to control storage insects by disrupting mating opportunities. This approach, however, is most suited in concert with other good storage management practices, where relatively low insect population densities have been achieved through conventional techniques and when store ventilation rates are low (Campion et al., 1987).
Inert insect repellents
Inclusion of silica-containing dusts and ash with stored grain has been found to control effectively storage insects. The action of silica dusts and the ashes of materials such as rice husk, which contain at least 90 percent silica, is abrasive. The grain moisture content and the ambient relative humidity consequently influence their effectiveness for insect control. The drier the conditions, the more likely it is that a high level of control will be achieved. Rice husk ash has been tested as an insecticide, mixed with stored grains (Krishnamurthi and Rao, 1950), and has been used by farmers in Japan and India. Powdered rice husk has been recommended for the protection of wheat against S. oryzae (El Halfawy, 1976).
In Nigeria, Emebiri and Nwufo (1990) report on effective control of S. zeamais and Tribolium castaneum in maize through use of trona, a crystalline carbonate/bicarbonate that occurs naturally in several parts of Africa. Interestingly, termite mound soil has been observed to cause a high degree of adult mortality in S. zeamais (Firdissa and Abraham, 1999). Desmarchelier and Ahern (1988) observed that insecticide (fenitrothion) mixed with bentonite or halloysite clays increased the protection period and decreased the insecticide dosage rate required, by up to 80 percent, to protect wheat against S. oryaze.
Diatomaceous earth mixed with wheat at the rate of 0.35 percent was found to be effective for at least one year (La Hue, 1970). Desmarchelier and Dines (1987) observed that the efficacy of dryacide (diatomaceous earth coated with silica aerogels) at 0.05 to 0.20 percent on wheat decreased in the following order: S. granarius > S. oryzae > T. castaneum = Rhyzopertha dominica. When wheat treated with dryacide was milled without prior cleaning, less than 3 percent of the dryacide carried over into the flour, and treatment did not affect flour quality (Desmarchelier and Dines, 1987). The efficacy of diatomaceous earth against insects depends on several physical properties including the ability of the particles to adhere to the grain surface, particle size distribution, diatom shape and pH (Korunic, 1997), and the temperature and relative humidity of the grain, with cool, dry conditions favoured (Aldryhim, 1990).
Vegetable oils, such as palm oil and peanut oil, are particularly effective as protectants against bruchid beetles (Shaaya et al., 1997). Adult bruchid beetles were little affected by the presence of oil admixed at 5 ml/kg, but egg mortality increased (Schoonhoven, 1978). Embelin, isolated from the fruit of Embelia ribes as a grain protectant for wheat in storage, was found by Chander and Ahmed (1987) to be effective in order of decreasing effectiveness against Corcyra cephalonica > Ephestia cautella > R. dominica > Trogoderma granarium > S. oryzae. In China, 0.2 percent cassia oil applied as a seed dressing kept stored wheat free from insect damage for up to eight months (Xu et al., 1993).
For parts of the world where pesticides are expensive or in short supply, Tembo and Murfitt (1995) observed a synergistic effect between vegetable oils (groundnut, rape-seed and sunflower) and pirimiphos-methyl insecticide against S. granarius in wheat grain. Treatments with the vegetable oils combined with the insecticide at half the recommended dose were as effective as the insecticide at the recommended rate, both achieving complete control within 24 hours of exposure. The same treatments remained effective after 90 days of storage.
Botanical insect repellents
Plant products are seen to be an effective means of 'appropriate technology' suitable to small holder farmers for protecting stored grain from insect damage. Tissue from various botanical species have been used to improve rural storage of grain, for example, the dried fruits of Capsicum spp., the powdered roots of Derris elliptica and the leaves and seeds of the neem tree, Azadirachta indica. Effective treatment has been observed with leaves from Eucalyptus globulus, Schinese molle, Datura stramonium, Phytolacca dodecandra, and Lycopersicum esculentum causing high adult weevil mortality for S. zeamais (Firdissa and Abraham, 1999). Ignatowicz and Wesolowska (1996) and Paneru et al. (1997) observed the toxic and repellancy efficacy of powders prepared from sweet flag (Acorus calamus) rhizomes in Poland and Nepal.
Aromatic plants, common as culinary herbs and spices, have often been found effective in the control of grain storage insects. Acetone extracts of mace and nutmeg (Myristica fragrans) were found to be moderately contact toxic and strongly repellent to S. oryzae, with significant reductions in insect F1 generation noted up to 23 weeks after a 200 ppm treatment (Su, 1989a), though Callosobruchus maculatus, Lasioderma serricorne and T. confusum were less affected. Similar effects were noted by Su (1986, 1989b) with seed and oil extracts of coriander (Coriandrum sativum) and dill (Anethum graveolens).
The insecticidal activity of oil extract from bay leaf (Laurus nobilis) and ground dust of rosemary (Rosmarinus officinalis) against S. granarius in wheat under store-house conditions was noted by Kalinovic et al. (1997). Essential oils from star anise (Illicium verum) and a citrus (Clausena dunniana) used to treat jute bags at 0.45 mg/cm2 protected wheat for up to four months (Xu et al., 1993). Labiatae oil ZP51 was found to be a highly effective seed fumigant, with 94 to 100 percent mortality obtained after seven days for T. castaneum, S. oryzae, R. dominica and Oryzaephilus surinamensis (Shaaya et al., 1997).
Ho et al. (1997) determined that for seed treated with steam-distilled oil from fresh garlic (Allium sativum) the eggs of T. castaneum and S. zeamais failed to produce F1 progeny at application concentrations of more than 2 000 ppm in rice and more than 5 000 ppm in wheat, respectively.
Ghanaian plant species were screened by Niber et al. (1992) for their toxicity to three storage pest species including S. oryzae on wheat. Extracts of Cissampelos pareira (Menispermaceae) and Erythrophleum suaveolens (Leguminosae) were very effective and more promising than neem (A. indica) extracts. One percent slurries or powders of Ricinus communis (Euphorbiaceae) and Solanum nigra (Solanaceae) were the crude materials most toxic to the three pest species (Niber, 1994). Chimbe and Galley (1996) assessed the insecticidal activity of powders and petroleum extracts from 12 medicinal plants from Malawi against S. oryzae in wheat; Dicoma sessiliflora (Asteraceae) and Neorautanenia mitis (Fabaceae) extracts were found to be most effective.
It is often assumed that natural plant extracts are safer than synthetic chemicals, but this is not always true. Derris dust contains the toxin rotenone, while azadirachtin in neem has antifeedant properties.
CROP CULTIVARS RESISTANT TO STORAGE INSECT DAMAGE
To reduce the rate of insect population increase, a resistant cultivar should reduce insect fecundity (effective egg production), and/or extend the development period, and/or cause high insect mortality (McFarlane, 1989).
Variation to susceptibility of grain to insect damage by S. zeamais has been observed in maize. In southern Africa, a maize breeding project involving screening of local national programme germplasm and development of improved maize populations is currently being conducted in Zimbabwe. Pixley (1997) reports that high weevil mortality rates and significant differences of the mean development period among genotypes suggest antibiosis as a mechanism of resistance. Freechoice tests have also demonstrated that maize weevils prefer some hybrids for feeding, which indicates the importance of non-preference/preference mechanisms of resistance.
Sinha et al. (1988) assessed the vulnerability of common wheat cultivars to nine major stored-product beetles. In general, whole seeds were less susceptible to insects than crushed seeds, with kernel hardness accounting for the variability observed between cultivars for whole seed susceptibility. Differences in fecundity were not observed. Chunni and Singh (1996) observed considerable varietal variability among 64 cultivars in their resistance to S. oryzae. Susceptibility was found to be correlated positively with grain size and negatively with hardness, crude fibre and protein contents of the grains, while oil content did not reveal any correlation.
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