In order to prevent and control stored product pests, hygiene and chemical measures are used in the first place. The proportion and the importance of physical and biological pest control methods has been increasing, however, during the last decade. The reasons for this trend are the restrictions being placed on chemical treatment of grain in many countries as well as the ever increasing demand for "residue-free" products which comes especially from consumers in industrialized countries.
Whereas biological methods today still have a rather limited practical importance, some physical methods already have become routine in several countries. In some instances the cost of application still limits the practical use of alternative techniques. The increasing demands related to the safe use of insecticides and fumigants make these methods gradually more complicated and thus more expensive, too, so that the cost relation gently swifts in favour of environmentally sound physical and biological methods.
In developing countries technical standards still remain limiting factors for the application of methods that require special apparatus and equipment or, for example, above-average gastightness of storage structures.
These ate generally methods which aimed at separating the pests from the stored produce. While the main mechanical methods in small farm storage ate sieving, picking out, or winnowing, use is made in larger scale storage of various cleaning installations. It is important to destroy any insects found in the by-products or left-overs immediately. Larvae living inside the grain are only inadequately eliminated.
Pests can be prevented by packing the still uninfested stored produce well. This is, however, only the case if the material used is strong enough to resist any attack by the pests. It will often be difficult to obtain packaging material which meets this demand. Jute and artificial fabric sacks, plastic foil, paper or containers made of wood or cardboard are in general use. They often do not afford any mechanical protection against pests.
Packaging material can only be attacked by pests which have sufficiently strong mouthparts or teeth. This applies e.g. to the following pests:
· Rhizopertha dominica
· Sitophilus spp.
· Lasioderma serricorne (Cigarette beetle)
· Stegobium paniceum (Drugstore beetle)
· Plodia interpunctella
Processing has outstanding importance in preserving perishables. As far as staples foods are concerned the manifold traditional cassava products for long-term storage of this highly perishable commodity should be mentioned. In cereal grains, however, processing is restricted to special cases, because the living grain with its rather low moisture content constitutes a commodity which generally has extraordinary storage features.
In some cases grain is even stored traditionally as it is harvested, without any threshing or shelling. Some examples are:
· storage of unshelled rice (paddy)
· storage of maize cobs with husks
· storage of sorghum and millet in panicles
· storage of pulses and groundnuts in pods.
These practices have in common that the grains remain in their natural protective shells which cannot be penetrated by some of the stored product pests. There are some exceptions, however, like those pests that can already attack the commodity in the field or the Larger grain borer which bores holes through the husks of maize and even prefers maize on the cob to shelled one. With these storage techniques there is no risk of bruising the grain before storage during threshing which makes grain susceptible to attack by secondary pests.
The importance of proper drying should be highlighted under this section. Drying is dealt with in detail in chapter 4.3.
The only important examples of processing in cereals in order to improve storability are treatments involving heat and moisture like the preparation of bulgur from wheat in Arab countries or the widespread parboiling of rice which is particularly frequent in Asia. Both techniques change the structure, density and hardness of the grains, so that certain storage pests find the processed product less attractive than the raw grain.
Parboiling involves steeping the paddy in cold or warm water for varying periods (up to three days), steaming and drying. During this process starch cells gelatinize and heal the cracks that may be present in the grain. Advantages of parboiling include reduced insect attack, less breakage during milling, improved retention of nutrients and vitamins and an overall improvement of storability. Careless parboiling may result in unwanted colour changes and unpleasant smell in particular if husks were strongly infested by fungus as a result of insufficient drying after the harvest.
10.2.1 Airtight Storage
Airtight (or hermetic) storage prevents any pests from entering and causes the death of insects left in store due to a lack of oxygen and an excess of carbon dioxide. The most important prerequisites. for airtight storage are gastight facilities.
Airtight storage has been successfully practised in small scale using well sealed clay jars and pots or demijohns for seed storage. Underground pits are traditional hermetic storage structures known since prehistoric times. In particularly arid climates empty oil drums stored in-room have proven their suitability and have become rather popular in certain regions of West Africa. These recipients have in common that the stored grain must be very dry and protected from extremes in temperature in order to avoid condensation and mould growth.
Larger-scale applications of airtight storage are known from the time of the Second World War in Argentina and from Cyprus where in the fifties concrete lined conical bins have been constructed which were covered by concrete dome-shaped roofs. These "Cyprus bins" have been introduced to Kenya in order to store the national grain reserve.
In recent years various types of silos, warehouses and flexible structures have been sealed hermetically with flexible plastic liners. Experience has shown that there are materials which resist to tropical climate conditions, but there are still a number of aspects that require further improvements before this technology can become commonly available as an alternative. Some of the problems which are not yet entirely solved concern monitoring, grain quality maintenance, moisture migration and condensation.
10.2.2 inert Cases
Storage in an atmosphere of inert gases (carbon dioxide and/or nitrogen) gives insects no chance of survival. When using nitrogen (N2), a concentration of 97-99% must be maintained in order to guarantee a successful treatment. The oxygen content must be kept below 1%. In case of carbon dioxide (CO.) a concentration of around 60% provides good control. Methane (CH4), which is produced in bio-gas plants, can also be used for this purpose.
There are three essential prerequirements when using inert gases:
1. Availability of CO2 (from flasks or as a product of the
combustion of propane or butane).
2. Gas-tight stores (or bag stack seal) which allow to maintain the concentration for several weeks.
3. Low moisture content of stored produce, as otherwise condensation is likely to occur.
Detailed procedures have been developed for large permanent grain stores, sheeted bag stacks, shipping containers and small-scale packaging. The major constraints for wide use in developing countries are the high cost and availability of the gas and the lack of storage structures which retain the gas sufficiently. Carbine dioxide can be generated on the spot by using burner gas systems Depending on the gas and the application technique used, the minimum exposure time varies from 14 to 21 days
Carbon dioxide has a special potential to replace methyl bromide for quarantine purposes. When applied under normal atmospheric pressure, the exposure time must be 10 days or more in order to obtain complete insect control. With high concentrations (98%) under high pressure (up to 30 kg/cm²) however, exposure periods of 5 to 20 minutes are sufficient to produce complete mortality. The high cost of this technique (autoclaving) restricts the use to high value commodities for the moment.
The future perspectives of inert gases will certainly not only depend on costs, but also on the fate of fumigants like methyl bromide which are still widely used but probably phased out sooner or later. The use of inert gases can also provide an alternative to the use of insecticides in the future for a number of developing countries.
10.2.3 inert Dusts
Since the last ten years inert dusts (mainly amorphous silicas) have been applied commercially in increasing quantities in Australia. The following three types of use are common:
- admixture of dusts with the commodity, generally in a
proportion of 1 g/kg
- structural treatment on walls and floors with either dry dusts or aqueous slurries
- addition of dusts to the surface of grain bulks.
When used as an admixture to grain the protective effect of inert dusts lasts normally at least twelve months which is comparable to conventional chemicals. The effect of different products varies considerably and some of them cannot provide sufficient control as compared to chemical insecticides. It has also become evident, that some insects like Sitophilus granarius are not very susceptible to this kind of treatment. The admixture of dusts has the drawback of increasing the dustiness of the grain. When applied to surfaces, however, inert dusts are by no means inferior to residual insecticides.
A promising approach in large-scale storage consists in treating the surface of grain bulks with dusts in combination with another method of pest containment like cooling or fumigation. In the first case the dust supplements ventilation with cool air and kills insects in the top layer where they tend to congregate. When applied together with a fumigant inert dusts act as a gas barrier and help to provide adequate concentrations near the surface.
There is some potential for the use of inert dusts in on-farm stored product protection. This technique is comparable to the traditional use of dusts and ashes and has the advantage of a considerably reduced dosage. Whereas traditional mineral admixtures are generally effective in concentrations of 40% or more, inert dusts containing amorphous silicas are applied at rates of I to 2 weight %. Among other pests Prostephanus truncatus and bean bruchids could be controlled in laboratory trials for up to six months. In cases where the protective effect is not considered to be sufficient, a combination of inert dusts with reduced dosages of conventional insecticides may also offer a solution for the future.
In any type of storage the use of inert-dusts is only effective when the grain moisture is kept below 12% and the relative humidity of the air is rather low. In the humid tropics inert dusts would clump rather quickly and thus loose their effect, but semi-arid and arid regions provide ideal climatic conditions for this type of treatment.
10.2.4 Use of High Temperature
It is a general rule that temperatures of above 40°C at-e lethal for most stored food pests within a short time. Traditional sun-drying of the hat vest makes use of this effect. Distinction is made between heat treatments under wet and dry conditions. The considerable amount of energy and equipment requited for heat treatments at a large scale is a drawback. This process cannot be used for seeds because it endangers the germination capacity.
10.2.5 Use of Low Temperatures
The effect of low temperatures ranges from reducing feeding activity and mobility to complete stop of development and to death. Extensive technology is required in order to achieve low temperatures in the store and the costs of the energy are very high. It may be necessary in individual cases to store valuable seeds in cold stores. As grain has a low temperature conductivity it is difficult to cool large stacks or bulks of grain. In addition, there is the danger of condensation during cooling procedures.
10.2.6 Treatment using Short-wave Radiation
Stored product insects can be killed by exposing them to short-wave (g-) radiation. The radiosensitivity of pest species varies. Cereals can be disinfected with a dose of 0.5 Kilogray (kGy), pulses at below 0.2 kGy. Eggs and larvae are the most sensitive insect stages. At the prescribed dose no alteration of physical, chemical and organoleptic properties of the treated product is reported. There are some commercial applications of this method, especially in potatoes and vegetables which have remained limited up to now. Around 40 countries have introduced registrations for this kind of treatment in certain products.
The advantages of irradiation include:
- no residues
- uniform penetration into grains
- no development of resistance to be expected
- instantaneous treatment.
Inconvenience of irradiating stored products are:
- higher cost than chemical treatments due to high initial
- irradiation means an additional handling step
- need for centralised facility
- limited capacity of irradiators
- slow acceptance by end user.
As there are [TO residues, the treatment can be applied to the final package of food. There is no residual effect, so that irradiated food must be protected from new infestation by means of suitable packaging or other methods. because of the technical facilities required, the cost involved and the lack of acceptance by the consumers it does not seem probable that y-irradiation will soon gain much importance for the treatment of grains.
Every living organism has natural enemies or diseases. They ensure the equilibrium of the population. Biological control makes use of such natural antagonists of pest species. The main advantage of biological control methods lies in the fact that they are in most cases toxicological safe. Before they are used, however, any ecological side-effects must be precisely investigated and taken into account. Practical application of biological control methods against stored product insects is still very limited because of some special features of the storage environment:
- in industrialized countries there is generally a zero
tolerance concerning any kind of "filth" in food including beneficial
- antagonists of stored product pests are naturally very susceptible to the commonly used broad-spectrum insecticides
- antagonists do not find very attractive living conditions in larger storage facilities like silos (e.g. low humidity, lack of nutrients for adult parasitoids, etc.)
Pests can be kept at a low level using biological methods but cannot be eradicated, As storage pests are tolerated up to a certain level in small farm storage, there are excellent possibilities for the use of such control methods in this kind of stores.
Furthermore, increasing restrictions concerning the use of fumigants and synthetic insecticides have made the application of biological agents in stored product protection more attractive. It should also be kept in mind that the tolerance for contamination with any kind of "filth" may vary. In traditional granaries generally small numbers of insects are tolerated. The same applies to feed grain. It is also evident that there are stages in the production cycle where standards need not to be as high as for finished goods or grain for export.
The following antagonists are promising biological control agents due to breakthroughs in research and practical work during the last years.
With the release of the histerid beetle Teretriosoma nigrescens against the Larger grain borer (Prostephanus truncatus) in Togo and Kenya a milestone has been set in biological control of stored product pests affecting granaries at the small-scale farmer level. The Larger grain borer had been accidentally introduced to Africa at the end of the seventies, spread rapidly and caused losses which had never been observed before (up to 30% after six months of storage). All previous efforts of containment of the new pest had rather poor results or were not easily accepted by the farmers, so that GTZ and the Natural Resources institute (NRI) set up projects in order to investigate the possibility of biological control.
Out of several investigated agents the antagonist T. nigrescens (like the Larger grain borer a native of Central America) turned out to have the highest potential for this purpose. After a thorough research of the predator's impact and safety aspects, T. nigrescens was introduced to Togo and released in early 1991
The monitoring of the release showed a substantial loss reduction due to the antagonist in the field. These findings encouraged the set-up of national control programmes in other countries where releases and follow-up work are still continuing. The procedures for breeding of the predator, introduction, release and monitoring have been well documented and published, so that interested governments can easily adopt this technique.
The Larger grain borer problem has not been entirely solved in Africa by the release of T. nigrescens but the pest can now be contained rather efficiently by means of suitable integrated pest management measures. Unfortunately T. nigrescens has no or only a very low impact on other storage pests e.g. Sitophilus spp. or Tribolium spp.
Some other predators like the anthocorid bug Xylocoris flavipes are frequently encountered antagonists in traditional granaries which show a good potential to reduce pest populations provided that they are not suppressed with broadspectrum insecticides. Even if they are not purposely used as control agents they can naturally contribute to loss reduction within the framework of integrated stored product control in a pesticide-free environment and thus deserve special protection.
Recent investigations have opened perspectives for the use of tiny parasitoid wasps in cereal stores. These species are generally very specific to certain stored product pest species as hosts. Special mention should be made of Trichogramma species which parasitize the eggs of moths. Some strains have been identified that perform well in storage conditions. The use of Trichogramma requires repeated (inundative) releases in certain intervals of time in order to insure a long-term effect.
As far as bruchids in grain legumes are concerned the specialized egg parasitoid Uscana lariophaga offers some perspectives due to its strong impact on Callosobruchus maculatus in stored cow-pea in West Africa.
A number of larval parasitoids like Anisopteromalus calandrae, Choetospila elegans and others are frequently found in traditional granaries which are not treated with chemical insecticides. Their impact can be considerable and they should be taken into account in integrated pest management concepts for small scale storage.
10.3.3 Pathogenic Agents
Pathogenic agents (bacteria, viruses, protozoa), which are specific to certain species, have proven their potential to provide satisfactory control of pest populations in the field. Bacillus thuringiensis is the most widely used of all biological control agents. In storage conditions it has the following advantages:
· it has a highly toxic effect on storage moths
· it remains effective for several months
· a surface treatment is sufficient.
The pyralid moths Plodia interpunctella and species of Ephestia are particularly sensitive against this bacterium. Unfortunately, pronounced resistance has already occurred in several instances, so that the future value of B. thuringiensis as an alternative to synthetic insecticides cannot be easily estimated. There is a variety called B. thuringiensis tenebrionis with a certain potential in controlling stored product beetles, especially Rhyzopertha dominica, which requires still further research.
Other pathogens like fungi, viruses and protozoans have been under investigation but none of them has gained importance in cereal stores up to now due to their limited lethal effect or their toxic side effects (mycotoxins) warm-blooded beings.
These methods involve more than other control methods the pests to be controlled actively in their own destruction. Use is made of the natural reactions of storage pests to stimuli from the environment.
The use of baits has been in practice for centuries. Food is mixed with poison and offered to the target animals. Baiting is the best and environmentally safest method if all necessary precautions are taken.
Occasionally this technique is used to attract and control insects. The main use of baiting is, however, still in dealing with rodents (See section 11.7).
Pheromones are natural stimulants emitted by insects to establish a kind of communication system. Sexual attractants (mostly issued by the female) as well as aggregation pheromones (which have an equal effect on both sexes) have been synthesized from storage food pests. Pheromones are in most cases not really used as control agents, but rather serve in the following tasks:
· Studies of the species composition
· Recognizing infestation (monitoring)
· Estimating the population density
· Defining the date for the application of control measures
· Checking the success of control measures
Pheromones have been isolated and identified from more than 30 species of stored product insects. The most common applications are pheromone-baited traps for survey, detection and monitoring of pyralid moths, Cigarette beetles and dermestids related to processed foods. Mass trapping of male moths has proven not be cost efficient. Among others pheromones are commercially available for the following important stored product pests:
Lasioderma serricorne (cigarette
Prostephanus truncatus (Larger grain borer)
Rhyzopertha dominica (Lesser grain borer)
Stegobium paniceum (Drugstore beetle)
Tribolium castaneum (Rust-red flour beetle)
Tribolium confusum (Confused flour beetle)
Trogoderma granarium (Khapra beetle)
Sitotroga cerealella (Angoumois grain
Ephestia cautella (Tropical warehouse moth)
Ephestia kuehniella (Mediterranean flour moth)
Plodia interpunctella (Indian-meal moth)
Males of the pyralid moths E. cautella, E. kuehniella and P. interpunctella can even be captured with the same compound which renders monitoring comparatively economic in this case.
Pheromones can be excellently used in combination with traps. There are a number of different trap designs according to pest species and purpose. The most convenient and frequently used moth trap is the delta trap which consists of waxed cardboard folded twice to form a three-sided prism open at both ends. The three inner surfaces are covered by a sticking material. The bottom surface is provided with a capsule containing the pheromone. The flying insects are attracted by the pheromone and become stuck to one of the adhesive surfaces. There are also more complicated and thus less economic designs like funnel or wing traps. Small glue traps are used to locate moths in difficulty accessible places.
The range of flight traps is rather limited. Inside warehouses insects respond in a distance of up to 10 m. In order to provide an effective grid of coverage, traps must be placed approximately 10 m from each other. Outdoors tests with the
Larger grain borer have proven a maximum attraction of nearly 500 m which is dependant on wind conditions.
For flying beetle species (e.g. P. truncatus) similar trap designs exist. Crawling beetles can be captured with grain probes which are inserted vertically into bulk produce and work even without pheromones catching insects that pass by. It goes without saying that luring beetles with pheromones enhances specific catches. There are also corrugated cardboard traps containing a pheromone capsule and treated with an insecticide (e.g. for T. granarium). These traps, as well as window traps make use of the stored product beetles' tendency to enter hiding places. For T. granarium a vertical wall mount trap has been devised which uses the wall-climbing behaviour of this species. There are still other designs available for certain species.
Pheromone traps for crawling insects operate over even shorter distances than flight traps. The maximum distance for most designs is about 1.5 m, so that a complete coverage can hardly be achieved. It is recommended to concentrate such traps on vulnerable points at the entry of storage facilities or at places where insect congregations are likely to occur.
Food attractants, which act on the sense of smell, draw storage pests over a greater or shorter distance. They can be used in practice like pheromones. In some cases, as for T. granarium pheromones may even be combined with food lures in order to improve the attraction.
Some plant extracts have the effect of repelling stored product pests. This applies e.g. for Neem, which has been mentioned in section 220.127.116.11.2. Tests results have shown that the application of these substances is limited under practical conditions.
10.4.5 Growth Regulators
Attempts have been made to use substances which interfere with the insects' complicated mechanism of development and moulting. By using these substances, it is possible to disturb development to such an extent that no progeny capable of reproduction are born. In this context substances with a structure resembling juvenile hormones (juvenile hormone analoga) should be mentioned. Their application leads to the development of intermediate forms in the larval or pupal stages which cannot survive.
Common growth regulators and juvenile hormone analoga include methoprene, fenoxycarb and diflubenzuron. They are sufficiently persistent in stored grain, but have a rather poor effect on Sitophilus species. Growth regulators still cannot be used effectively enough in most circumstances to make them a viable alternative to insecticides. A potential exists, however, for the application of methoprene against Cigarette beetles or organophosphorus resistant strains of Rhyzopertha dominica and Oryzaephilus surinamensis. Tests have also proven a potential use of methoprene in combination with an organophosphorus compound.
10.4.6 Crop Varieties Tolerant to Storage Pests
A larger number of high-yield varieties coming on the market in context with the "green revolution" have proved to be more susceptible to infestation by storage pests than the local varieties. The following criteria may be responsible:
· reduced hardness of the seed coat
· change in compounds like higher protein content
· more attractive smell due to the change in composition of the grain
· maize husks which do not completely cover and thus protect the cob
Making use of the differences between varieties can be seen as an excellent prophylactic means of control, providing the tolerant varieties meet the necessary quality standard. Varieties with tolerance against stored product pests should therefore have priority in breeding programmes.
With few exceptions like the use of inert dusts for structural treatments, none of the methods listed in this chapter can at present be regarded as being a viable alternative to the use of insecticides. They represent, however, parts of integrated control strategies against stored product pests and can contribute to a considerable reduction of the application of chemicals in future.
Fumigation and Controlled atmosphere Storage of Grain, ACIAR Proceedings No. 25, Canberra
La conservation du nib (haricot) avec l'huile de neem. Fiche Technique de la Protection des Vgtaux, Lom-Cacaveli, 26 pp.
Post Harvest Problems, GTZ, Eschborn, 258 pp. + 32 pp. appendix.
HIGHLEY, E., E.J. WRIGHT, H. J. BANKS & B.R. CHAMP. ed.
Stored Product Protection. Proceedings of the 6th International Working Conference on Stored-product Protection, CAB international, Canberra, 2 volumes, together 1274 pp.