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In central storage and grain-handling facilities, there are the conflicting objectives of providing for large throughputs and for long-term reserve storage.
Port installations involved in either importing or exporting grain require mechanical handling facilities that can only be justified by a high degree of utilization. The benefits of being able to handle large volumes of bulk grain quickly lie in the prompt turnapound of ships, railway freight cars and road transport, thereby avoiding costly delays.
Such facilities are not required, however, for long-term storage where grain, in exteme cases, may remain for several years. It is always advisable that emergency grain reserves of this kind be cycled into the normal distribution system on the basis of a first-in first-out (FIFO) queueing system.
Central storage facilities are usually provided with drying facilities to treat incoming grain that is above the regulation 13-percent moisture content wet basis for longer-term storage.
Various techniques are used to control insect pests in stored produce-from sunning and smoking on the traditional farm to irradiation in large-scale bulk handling. This section of the manual is concerned only with proved techniques suitable for use in small- and medium-scale storage under tropical conditions.
Special recommendations are difficult to make. A technique must be tested for a particular situation, since it may become inappropriate as a result of changes in:
(a) economics (the value of the product, relative to the cost of materials and labour);
(b) pest problems (such as occurrence and resistance); and
(c) techniques within the farming system or through the availability of new products.
Therefore, it is important to consider both:
(a) economics; and
(b) technical specifications for effectiveness against target pests and hazards to farmer and consumer.
Will the improvement resulting from the use of a control technique pay for the cost of carrying it out? This question can only be answered satisfactorily by field trials supported by an effective loss assessment.
9.2.1 Sanitation. It is crucially important to reduce the initial pest population and prevent development of any insect pests in the crop products. Before bringing a new crop into store, the following steps are necessary.
(a) Remove infested material. Do not mix new grain with old; old material that must be kept should be thoroughly fumigated.
(b) Clean the storage structure.
Figure 9.1 Cleaning storage structure
Take control measures early to prevent infestation of crops maturing in the field.
9.2.2 Natural resistance. Crop varieties differ in their susceptibility to storage pests.
Traditional varieties are usually more resistant to storage pests than new varieties. If new varieties are introduced, measures must be taken to improve storage techniques and pest control.
Some new varieties of maize and cowpea are now being selected for improved storage resistance and they are now becoming commercially available.
There are some useful general resistant characteristics in the following crop varieties:
(a) maize: good husk cover can reduce field infestation, and storing in the husk reduces the rate of pest increase;
(b) sorghum: varieties where the glumes cover the grain tend to be more resistant before threshing;
(c) rice: paddy rice is considerably more resistant to pests than milled rice;
(d) cowpeas: intact dry pods provide some protection against bruchids; if fumigation or airtight storage is impracticable, cowpeas are better stored unthreshed;
(e) cereals: grain hardness affects resistance.
9.2.3 Hermetic or sealed storage. In airtight conditions, reduced oxygen and increased carbon dioxide will eventually arrest insect and mould development.
Grain for human consumption or seed must be dry; in damp grain bacterial and enzymatic action will continue, causing tainting and loss of viability.
Bagged material must be protected; if the seal is broken (by insects, rodents or careless handling) the grain is unprotected and unventilated, and losses may be severe.
A method that has been found satisfactory in North Nigeria (a dry area) is that of storing threshed cowpeas in sealed plastic bags with cotton liners; the cotton prevents emerging insects from perforating the plastic bag.
9.2.4 Chemical control: overview. Methods of using insecticides on stored products are:
Insecticides usually show some degree of toxicity to humans, domestic animals, poultry, etc. and must be used with caution. Be sure to:
(a) read the manufacturers' instructions;
(b) choose a chemical with low toxicity to mammals and birds;
(c) stay within the recommended dosage; and
(d) protect workers with careful instruction, constant supervision and provision of protective clothing.
Insecticides are usually specific, and do not kill all insects and mites; choose a chemical approved for use in stores and/or on stored products that has either a "broad spectrum" or specific toxicity to moths and beetles. Mites may require special treatment.
Insecticides tend to lose their persistence, or effectiveness, with:
Stored chemicals must be protected from these factors to ensure their continued effectiveness. In stored products, long-persistence insecticides give long protection against pests, but increase the risk to the consumer. Insecticides vary widely in their persistence. Choose one appropriate to the job: for example, persistent chemicals for the treatment of storage structures, non-persistent ones for space-spraying.
Insects can develop physiological and behavioural resistance to insecticides. Excessive or inappropriate use of chemicals will lead to the insects becoming resistant; therefore, use the right dose and use insecticides only when strictly necessary.
9.2.5 Fumigation. Chemicals used to attack insects through their respiratory system are known as fumigants.
Fumigants may be formulated as:
carbon bisulphide
carbon tetrachloride
ethylene dichloride
ethylene dibromide
hydrogen cyanide
methyl bromide
phosphine
The concentration of a fumigant is measured in mg/l of space occupied.
The CT product is the concentration of the fumigant multiplied by the time in hours that will give a 99-percent kill of the pest concerned. Table 6 gives details of some CT products for commonly used fumigants; it serves as a guide only to their effectiveness because other factors may be involved.
TABLE 6. CT product for specified insect pests (in mg/l/hr)
| Chemical | Insect species |
|||
Rhizopertha |
Sitophilus |
Tribolium |
Trogoderma |
|
| Carbon bisulphide | 294 |
325 |
560 |
700 |
| Carbon tetrachioride | - |
4 500 |
2000 |
- |
| Ethylene dichloride | 636 |
1 200 |
365 |
2 080 |
| Methyl bromide | 0.60 |
1.0 |
0.50 |
331 |
Some characteristics of the more common fumigants follow.
Carbon bisulphide appears to be a reasonably good fumigant, but it is very inflammable; a spark caused by a nail hitting a stone will cause the fumigant to explode, and for this reason it is now rarely used.
Carbon tetrachloride seems to be a poor fumigant, but in fact it penetrates grain very well and is often mixed with other fumigants which do not penetrate in order to carry them through the produce. It is not inflammable, but over a period of time it may injure the livers of those who use it.
Ethylene dichloride is inflammable and does not penetrate well. Its use is not recommended.
Ethylene dibromide appears to be effective, but it is absorbed very rapidly; its penetration through the grain is therefore very poor.
Methyl bromide is an excellent fumigant, but it has no smell and is very poisonous. It can therefore only be used by trained teams. It is not generally available.
Phosphine is an excellent fumigant and fairly easy to use. It is used as a mixture of aluminium phosphide and ammonium carbamate. These are stable if kept in sealed containers, but when exposed to the air they take up water and release phosphine, ammonia and carbon dioxide. Phosphine normally contains impurities which make it spontaneously inflammable, but in the presence of ammonia and carbon dioxide it is safe. The chemicals are formulated so that there are about 30 minutes available to distribute the mixture before the gas is released. The gas has a strong and unpleasant smell and is therefore easy to detect. Phosphine is the only fumigant that will not interfere with germination if the grain is to be used for seed. The others may affect germination if exposure to the fumigant is excessive or repeated.
Fumigation in practice involves consideration of the scale of the operation, and safety.
In small-scale fumigation, grain and other products may be fumigated with carbon tetrachloride in a dustbin or 150-l drum. About 150 ml are poured over the surface of the grain, the lid is sealed with paper glued round the junction between lid and base, and the grain is left for 14 days. If it is to be used for seed it should be aired after fumigation; otherwise germination may be affected.
In larger-scale fumigation, phosphine fumigation may be carried out using formulations which consist of tablets, pellets or powder in envelopes. The makers issue instructions for the quantities to be used. Sacks of grain may be fumigated under plastic sheeting: the grain is stacked on a sheet of plastic, covered with another sheet, and after the fumigant is inserted the top and bottom sheets are rolled together. Sandbags are placed on the roll to give an airtight seal.
If it is not possible to put a covering sheet over the grain the building should be sealed thoroughly before fumigation. Alternatively, if feasible, a sheet should be put over the whole structure before fumigation.
Safety must be a primary consideration. All fumigants can kill people as well as insects, and some may cause serious disorders in humans who are exposed to low concentrations over a long time. Consequently, stocks of fumigant should not be kept in offices or stores where people are working.
Accidents can happen, so two people should always be present when fumigating. Fumigation should only be carried out by trained staff operating under proper supervision.
If phosphine is in use, one person should have a respirator with the correct canister. Phosphine fumigation is extremely easy to carry out, and increasing numbers are using it without a proper knowledge of the dangers. Some fumigants are supplied in containers that cannot be resealed, which can be dangerous if some of the material is left in an office or even a home. When the cover is lifted after a period of fumigation there will be a high concentration of the gas for a short time and this can be dangerous. Ideally, the material should only be handled by crop protection staff; but as these are so few and the results of fumigation can be so good, there is commercial pressure to make the products widely available. Since it seems inevitable that untrained people will use phosphine, the media (press, radio and television) should be prompted to give as much guidance as possible on its proper use.
9.2.6 Biological pest control. These methods have been effective in some situations. Bacillus thuringiensis has been used for controlling some species of insect pest in stored cereals. Cats provide effective control of small numbers of rodents in and around farm steads, but they should not be used in warehouses.
9.3.1 Insecticidal dusts. The method of dispersing dusts usually involves using an admixture of dilute dust at 2.5- 15 ppm active ingredient, depending on the insecticide, at the time of loading/bagging.
Figure 9.3 Mixing insecticidal powder with grain
The appropriate quantity of dust is measured into a perforated tin or punctured plastic bag and sprinkled onto the produce layer by layer. For bulk grain the dust is mixed more effectively by shaking in a tin with the produce, shovelling on a groundsheet or mixing in a revolving drum. For larger-scale operations commercial devices are available.
There are several problems with this type of insecticide.
Applications of dusts include use in cribs and bulk stores but they are more effective in the latter. They are also only suitable for dry conditions.
(a) It is difficult to obtain accurate dosage and a thorough admixture.
(b) Application can only be carried out at loading, when it makes an extra demand on labour; so it is often not properly done.
(c) Breakdown of the active ingredient can be particularly serious with local formulations in which the carrier is not sufficiently "inert"; no scope exists for reapplication.
(d) Insect resistance depends on the insecticide used and the insect species, rather than on the formulation.
Among suitable chemicals are the established (malathion, lindane/gamma BHC) and the potentially better (pirimiphos-methyl, synthetic pyrethroids).
9.3.2 Insecticidal sprays. The method of using sprays is to allow 10-15 ppm active ingredient (ai), contained in the minimum of water necessary to give an even coverage (about 0.3-2 l/tonne, depending on the applicator). Such a small quantity of water will not cause moulding. The insecticide can be applied with a small domestic applicator (Shell/ox-type), but a knapsack sprayer reduces labour requirements.
Application of sprays differs with the type of storage facility used. In warehouses, the following procedures are used.
(a) Bagged produce. Each layer of bags is sprayed as the stack is built; this should give protection for several months, but in case of reinfestation, the stack should be resprayed and fumigated for effective penetration.
(b) Space spraying. A non-persistent insecticide is sprayed to kill the adults of flying insects, especially warehouse moths; used in conjunction with fumigation under sheets.
(c) Fogging. For the same purpose, an electric applicator delivers very fine droplets, which hang in the air, maximizing effectiveness.
(d) Surface treatment. A persistent insecticide is sprayed on walls, roof and floor of the storage structure.
In cribs, the insecticide is sprayed directly onto the produce. If there is significant field infestation it is advantageous to spray each basketful during loading; otherwise, insecticides should be applied to the outside of the crib after loading, and reapplied at intervals as necessary (monthly is suggested).
Problems with sprays include the following:
(a) breakdown of chemicals in the highly ventilated crib environment (al though this implies a minimum residual toxicity for consumers);
(b) poor penetration with some types of structures; and
(c) lack of availability of sprayers and suitable chemicals.
Among suitable chemicals are the synthetic pyrethroids, usually used for space-spraying and/or control of the larger grain borer; dichlorvos for automatic fogging (Note: this is highly toxic to mammals); and malathion or pirimiphosmethyl for general use (since it costs less and has lower toxicity).
9.3.3 Fumigation. Methods include fumigating produce in containers, or fumigating surfaces. The produce is placed in drums, plastic bags under tarpaulins or plastic sheets. After addition of the chemical, the produce must be kept in airtight conditions for at least three days for Phostoxin or about one day for EDB, depending on the doses applied. For fumigation of stacks in warehouses, it is necessary to spray the roof and walls simultaneously to prevent reinfestation. Grains must be protected from subsequent reinfestation.
Applications of fumigants differ with the type of crop they are used on. They are indispensable for export crops such as groundnuts, coffee and cocoa. At the small-farm level, fumigation might be justifiable for seed material of high-value crops such as grain legumes.
Fumigants can be very dangerous if incorrectly used; they should not be used in domestic living quarters. Another problem is that they have no residual action.
There are two types of formulation of chemicals used in fumigation.
(a) Phosphine gas (e.g. Phostoxin) is supplied as tablets of aluminium phosphide, which release phosphine on contact with moisture in the air. It is convenient to use, but requires airtight conditions for three to four days for total kill, and longer in cool conditions.
(b) With ethylene dibromide, methyl bromide, and carbon tetrachloride there are various combinations and formulations available (e.g. Trogocide). All are volatile liquid fumigants. Capsules and sachets are available for small scale applications and pressure cylinders for large-scale ones. They are difficult to use and there is some residual toxicity and a possible consumer hazard: a shorter time is required for fumigation-normally less than one day-depending on the formulation used. Not recommended for farm- or village-level use, and only to be carried out by trained personnel.
All insecticides are also toxic to mammals to some extent.
The toxicity is usually expressed as an LD50. Technically, this is the dose required in mg of active ingredient (ai) per kg of the body weight of the consumer, under specified conditions (method of application and time span), to kill 50 percent of the test population, which are usually rats.
The LD50 figure of a chemical is a reasonable indication of its toxicity to humans, and thus of the hazards involved in its use. It should be noted, however, that some compounds are highly active against particular sorts of animals; for example, some organophosphorus insecticides, such as fenthion, are very toxic to birds, including chickens.
For toxicity of recommended insecticides, see Section 9.6.
Commercial insecticides consist of a quantity, usually small, of the toxic compound (the active ingredient) with other substances such as:
(a) inert "spreaders" such as talc, etc. in dusts and water in sprayers;
(b) surfactants, enabling the compound to be mixed with water and adhere to the pests and stored produce; and
(c) synergists, which may be included to increase the effectiveness of the active ingredient (e.g. piperonyl butoxide with pyrethroids).
The mixture is described as a formulation which comes in forms with particular characteristics:
(a) dusts, for dry application;
(b) wettable powder (wp) for mixing with water for spraying;
and
(c) emulsifiable concentrate (ec) for spraying.
The concentration of the active ingredient in the formulation will always be stated, either directly, as in "malathion 5-percent dust" (i.e. 25-percent active ingredient); or indirectly, as in "Actellic 25 ec" (i.e. 25-percent active ingredient in solution).
Dosages may be expressed in three ways:
(a) the quantity of crude product (cp) (i.e. the solution from the bottle to be used), as in "40 ml in 51, to be applied on 1 tonne";
(b) a concentration for spraying, usually referring to the active ingredient concentration, as in "use a 10-percent solution"; or
(c) a concentration as a proportion of the quantity of the produce, referring to the active ingredient concentration, as in "apply dust at a rate of 10 ppm ai".
It is important to be able to convert from one basis to another. For example, the instruction "Actellic should be applied at 15 ppm se" means that 15 g of active ingredient should be applied to every million g (or 1 tonne) of produce.
(a) Starting with a 5-percent dust, this means that 100 g of crude product (cp) contain only 5 g of active ingredient. 15 g are contained in 300 g of cp, so 300 g of dust per tonne (or 30 g to every 100-kg sack) are applied.
(b) Starting with 25 ec, this contains 25 ml ai per 10 ml cp. Taking 1 ml as weighing approximately 1 g, to obtain 15 g of ai, [15 x 100] / 25 (i.e. 60 ml) of solution are required.
The required amount of insecticide for spraying can be mixed in any convenient quantity of water; for example, in using Actellic 25 ec to spray a crib, either a Shelltox-type hand pump (in which case a 60-ml solution in 250 ml of water is required) or a cp3-knapsack sprayer (for which 60 ml might be mixed with 51 of water) may be used. In both cases, the whole spray solution would be applied to 1 tonne of produce.
If a solution concentration is specified, calculations proceed similarly:
(a) to apply a 7.5-percent (ai) solution, starting with 25 ec formulation, 7.5 ml are required in every 100 ml of spray, equivalent to 75 ml in every 1;
(b) there are 25 ml in each 100 ml of formulation;
(c) to obtain 75 ml se, 300 ml of formulation are therefore needed;
(d) water is then added to make the total up to 1 l.
Note: 300 ml cp + 700 ml water provide 1 1 of spray.
Insecticides for use with stored products include the following.
Gamma BHC/lindane
Malathion
lodafenphos (e.g. Nuvanol, Elocril)
Synthetic pyrethroids (e.g. Permethrin)
Pirimiphos-methyl (e.g. Actellic)
Dichlorvos (e.g. Nuvan)
Propaxur (e.g. Baygon)
Traditional systems rely on natural ambient air for drying. Some drying takes place in the field before the crop is harvested. Producers of small quantities of commodities dry un-dehusked maize, for example, by exposing the cobs to the atmosphere. Larger producers use a variety of naturally ventilated structures to reduce the moisture content of the cobs to about 20 percent, when they are considered reasonably safe against insect attacks.
Round, slatted-wall structures are extensively used for drying in the humid tropics. They range from 1 to 3 m in diameter and are up to 2.5 m high. Platforms are used to store un-dehusked maize cobs, often arranged in a specific pattern, and a fire may be lit beneath the platform. A thatched roof provides protection from rain.
The main limitation in using traditional ventilated structures is the long period of field-drying required before the crop can be loaded into the crib. This leads to much higher levels of infestation at harvest time which may increase during storage.
improved systems of maize harvesting, drying and storage should aim at the following:
(a) much earlier harvesting (when moisture content may be as high as 35-percent wb, reducing levels of field infestation);
(b) drying in improved traditional structures, with which farmers are familiar; and
(c) providing protection from infestation during storage.
A series of trials to measure the effect of the size and shape of cribs on the drying rates of dehusked maize cobs and on the levels of infestation and damage were carried out under the direction of the African Rural Storage Centre (ARSC) in Ibadan and Benin, Nigeria; these trials gave the following results:
(a) un-dehusked and dehusked maize cobs in cribs dried at the same rate;
(b) un-dehusked cobs with more than 26-percent mcwb became mouldy;
(c) orientation of rectangular cribs was relatively unimportant, except in areas with a prevailing wind;
(d) dehusked maize cobs required protection from insect damage, if they remained undisturbed in the cribs after drying;
(e) the drying rate of maize cobs in cribs, particularly narrow ones, was similar to the drying rate of cobs in the field;
(f) when initial moisture content exceeded 28-percent mcwb the 600-mm-wide crib gave the best results;
(g) roof overhang was important in reducing rain damage to the top surface layer of the stored cobs. Surface wetting of the sides or ends of the cobs was not important in most seasons;
(h) any wall material offering a minimum of 10-percent fairly evenly distributed open area was satisfactory;
(i) any material that sheds rain was suitable for roof covering;
(j) most effective insect control during storage was achieved by monthly spraying of the outside of the crib with a pirimiphos-methyl preparation.
The recommended design is shown in Figures 5.4 and 10.1 and Section 5.6.2. The base of the storage compartment is 1 m above ground level and is carried on 1.5-m vertical supports, separately from the longer vertical supports carrying the roof and walls of the crib. Rat guards are fitted to the vertical support below the crib floor level.
The width of the crib is 600 mm for very humid areas and up to 1 500 mm where drying conditions are better.
The capacity of the crib is given in the following table for each m in length.
Crib width |
Weight of cobs at harvesting |
Weight of grain |
(mm) |
(kg) |
(at 14% mcwb) |
600 |
500 |
300 |
1 000 |
850 |
500 |
1500 |
1 275 |
750 |
The crib may be made as long as required. Any materials used for its construction must support the weight of the cobs and provide a minimum of 10percent open area.
A crib made of sawn timber uprights, wire netting walls, corrugated-iron roof and metal rat guards will be three times more expensive than a similarly sized crib using teak pole supports, bamboo slats for walling, a corrugated-iron roof and metal rat guards. A crib made entirely of home-grown materials from the farm or forest will be approximately half the cost of a crib made with teak poles, and onesixth of that made with sawn timber.
Figure 10.1 Traditional ventilated structures
These costs refer to capital cost per tonne stored per year, and take into account the expected life of the cribs.
Cribs constructed from home-grown materials require more maintenance than the more expensive constructions and even then may only remain serviceable for four seasons. Teak pole cribs may serve for eight to ten seasons if protected against termite attack.
Pamphlets describing the construction and use of an improved maize crib were produced by the Rural Structures Unit in the Ministry of Agriculture in Kenya and by the FAO/DANIDA African Rural Storage Centre, Ibadan, Nigeria.
