Since the cereals and pulses are the staple food of people in India, they have to be stored by the producers in their homes and by the traders and the government agencies in godowns for one year or more. The food grains are constantly exposed to physical factors such as temperature and moisture, and biological agencies such as insects, mites, rodents, birds, fungi, etc.
Almost all the insect pests of stored grains have a remarkably high rate of multiplication and within one season, they may destroy 10-15 per cent of the grain and contaminate the rest with undesirable odours and flavours. Nearly one thousand species of insects have been associated with stored products in various parts of the world. The majority of insect pests belong to the orders Coleoptera and Lepidoptera, which account for about 60 and 8-9 per cent of the total number of species, respectively.
Under Indian conditions, there are about a dozen species of insect pests of stored grains which are most important among a large number of species recorded. Out of these, some are capable of damaging all kinds of stored food grains while others attack the broken or milled grains only. The former are known as primary insect pests and the latter as secondary pests.
Pests of Stored Food Grains:
Temperature is one of the most important factors of the environment, as the multiplication of insect pests of stored grains depends on it. The minimum temperature at which these insects are able to develop and multiply is between 15.5 and 18.3°C. Many of the insects can live for long periods at lower temperature but their activity is reduced very much. The optimum temperature for most of the species lies between 28.0 and 32.0°C.
The effect of humidity on the development of insect pests of grains and other dry products is intimately associated with that of temperature. The insects feeding on dry materials need a certain minimum of moisture in their food. The moisture requirement varies from species to species but practically all of them need more than 10 per cent moisture and the optimum is around 14 per cent.
Therefore, grain having less than 10 per cent moisture is considered safe for storage. Microorganisms also develop at high moisture content. If wheat is stored at a moisture content of 14.0-14.8 per cent at 21.1°C, it will be attacked by Aspergillus restrictus while at higher moisture content, A. ruber will predominate.
Attack of A. candidus takes place at moisture content of 16-17 per cent and of A. flavus at 18 per cent or above, with relative humidity between 85 and 90 per cent. The moist grains in storage sometimes contain toxins such as aflatoxin, ochratoxin, F-2 toxin and rubrotoxin, produced by the fungi, A. flavus, A. cohraceus, Fusarium sp. and Penicillium rubrum, respectively. The mycotoxins found in food grains are possible hazards to human and animal health.
The grain in storage also undergoes chemical changes with a change in the moisture. Increased respiration of stored grain under high temperature increases fatty acids and reduces sugar content. At high moisture levels, however, carbohydrate fermentation may occur with the production of alcohol or acetic acid resulting in a characteristic sour odour.
Storage of grain above 16 per cent moisture content at comparatively high temperatures may reduce seed viability to a great extent. Some other known effects of storage of moist grain are discoloration of the kernel, mustiness, problems of milling and separation of starch. Grain with high moisture content not only deteriorates in quality but also occupies more space as its bulk increases.
At the time of harvest, therefore, the grain should be dried until the moisture content is less than 9 per cent, which is ideal for storage; but sometimes, uncertain weather would not allow the proper drying up of grain. Again, the grain kept in damp godowns and improper receptacles may absorb moisture from the ground or from the atmosphere, particularly during the monsoon. In general, warm season and high moisture content of the grains are highly conducive to the proper development and rapid multiplication of insect pests of stored grains and pulses.
Lepidoptern Pests:
1. Angoumois Grain Moth, Sitotroga Cerealella (Olivier) (Lepidoptera: Gelechiidae):
The distribution of this pest is world-wide. In the Indian Sub-continent, the pest is more abundant in the mountainous areas or where the climate is rather mild. It is considered an important pest of stored grain such as wheat, maize, sorghum, barley, oats, etc.
Only the larvae cause damage by feeding on the grain kernels. A full-grown larva is about 5 mm long, with a white body and yellow-brown head. The adult is a buff, grey-yellow, brown or straw-coloured moth, measuring about 10-12 mm in wing expanse. The characteristic feature of this insect is the presence of the narrow pointed wings fringed with long hair, most prominent along the posterior margin.
Life-Cycle:
Breeding takes place from April to October. The insect overwinters as a hibernating larva and as the season warms up, it pupates in early spring. After emergence, moths mate within 24 hours and the females start laying eggs singly or in batches on or near the grain. The eggs are small and white, when freshly laid, turning reddish later on. A single female lays, on an average, 150 eggs, usually within a week after mating. The incubation period is about 4-8 days in summer.
Experimentally, the eggs hatch in 8.5, 6.0, 4.1 and 4.6 days at 20, 25, 30 and 35°C, respectively. The newly emerged larva soon bores into the grain and feeds on its contents. The larval stage may last about 3 weeks. Before pupation, the larva constructs a silken cocoon in a cavity made during feeding and then turns into reddish-brown pupa. Later, the adult emerges by pushing aside the seed-coat that covers the exit.
At the constant temperatures of 20, 25, 30 and 35°C, the duration of the larval and pupal stages is 23.5, 20.2, 19.4, and 22.2 days, and 12.5, 9.1, 6.5 and 12.2 days, respectively. The female adults have a longer-life-span than the males, the duration being 10.3, 7.5, 6.6 and 4.3 days at 20, 25, 30 and 35°C, respectively. During the active season, the life-cycle is completed in about 50.6 days. Several generations are completed in a year.
Damage:
The damage is at its maximum during the monsoon. The larva bores into the grain and feeds on its contents. As it grows, it extends the hole which partly gets filled with pellets of excreta. Usually, about 30-50 per cent of the contents are consumed, but sometimes the larva finishes off the entire grain. With infestation the grains give out an unpleasant smell and present an unhealthy appearance, each grain being covered with scales shed from the moths. In a heap of grain, it is the upper layers that are most severely affected.
2. Rice Moth, Corcyra Cephalonica (Stainton) (Lepidoptera: Pyralidae):
The rice moth is distributed in Asia, Africa, North America and Europe. In the larval stage, it is an important stored- grain pest in both India and Pakistan. It also infests gram, sorghum, maize, groundnut and cotton-seed.
Life-Cycle:
The rice moth is active from March to November when all stages are noticed. It passes winter in the larval stage and the overwintered larvae pupate sometimes during February. The moths emerge in March. They are active at night and lay eggs singly or in groups of 3-5 each on the grains, bags and on other objects in the godowns. A single female may lay 62- 150 eggs during its life-span of 2-4 days. The eggs hatch in 4-7 days and the larvae feed under silken web-like shelters, preferring the partially damaged grains.
They grow in five stages and are full-fed Rice moth, Corcyra cephalonica. A. Eggs; in 21-41 days, after which they make B. Larva; C. Pupa; D. Adult silken cocoons among the infested grains for pupation. The pupal stage lasts 9-14 days and the adults live for over one week. They complete their life-cycle in 33-52 days and the pest completes approximately 6 generations in a year.
Damage:
The larvae alone damage the grains of rice and maize by feeding under silken webs. When infestation is high, the entire stock of grains may be converted into a webbed mass. Ultimately, a characteristic foul odour develops and the grains are rendered unfit for human consumption.
3. Indian Meal Moth, Plodia Interpunctella (Hubner) (Lepidoptera: Pyralidae):
The pest is worldwide. It infests grains, meals, breakfast foods, soybean, dried fruits, nuts, seeds, dried roots, herbs, dead insects, etc.
Only the larva causes damage and may be recognized in the early stages as a small whitish caterpillar, often tinged with green or pink, a light-brown head, a prothoracic shield and an oval plate. On reaching maturity, the larva is 8-13 mm in length. The adult moth is about 13-20 mm in wing expanse. It has a coppery lustre on the outer two- thirds and terminal whitish grey on the inner portions and on the end of the body. The palps form a characteristic cone-like beak in front of the head.
Life-Cycle:
Breeding continues throughout the year under favourable warm conditions; however, in cold weather or in unheated buildings, the insect over winters in the larval stage. The female moth lays 30-350 minute whitish ovate eggs, singly or in clusters, on or near the appropriate foodstuffs. The incubation period is 2 days to 2 weeks depending upon weather?
The larvae feed upon the grains or other foods and become full-grown in 30-35 days. They crawl up to the surface of the food material and pupate within a thin silken cocoon. The pupal stage lasts 4-35 days. In summer, the life-cycle is completed in 5 or 6 weeks and there are about 4-6 generations in a year.
Damage:
Extensive damage may be caused by the active, crawling caterpillars which may completely web over the surface of a heap of grains with silken threads. The adults fly about from one bin to another and spread the infestation.
4. Almond Moth, Ephestia Cautella (Walker) (Lepidoptera: Pyralidae):
The pest is also known as the dried currant moth and the fig moth. It is worldwide and is a serious pest of dried fruits such as currants, raisons, dried apples, dates, berries, figs, almonds, walnuts, tamarind seeds, etc. It has also been recorded on lac, malted milk, dried mango, pulp, garlic bulbs, various cereal grains and grain products. Although it is primarily a pest of stored products, the almond moth is known to maintain itself on figs and date-palm in some countries. The adult moth has greyish wings with transverse stripes on the outer region and the wing expanse is about 12 mm.
Life-Cycle:
The female lays whitish eggs indiscriminately in cracks and crevices of the receptacles or on the food stuff. While feeding, the larvae spin tubes in the food material and are full-grown in 40-50 days. The full-grown larva is white with pinkish tinge and measures 1.5 cm. The larvae pupate inside the cocoons and pupal stage lasts about 12 days. The life-cycle is completed in about two months and there are 5-6 generations in a year.
Damage:
The caterpillars make tunnels in the food materials. The number of silken tubes is sometimes extremely high and these clog the mill machinery where the infested grains have been sent for milling.
Beetles and Weevils:
1. Khapra Beetle, Trogoderma Granarium Everts (Coleoptera: Dermestidae):
Although a native of India, this pest has spread to many countries and has been reported from England, Germany, Israel and the USA. In the Indian Sub-continent, it is a very destructive pest of wheat and other grains, particularly in the north-western dry regions of Pakistan, Rajasthan, Haryana and Punjab. Apart from wheat, the insect has also been recorded on sorghum, rice, barley, gram, maize, poppy (Papaver samniferum), pulses, pistachio, walnut and other dried fruits.
Only the larvae cause damage. A newly emerged yellowish-white larva is about 1.5 mm long and has a brownish head. When full-grown, it is about 4 mm in length and is brownish, with yellow- brown transverse bands across the body which has long hairy bristles. The integument between the segments and the ventral surface of the body is pale yellow.
The adult is a small dark- brown beetle, 2-3 mm long, with a retractile head and clubbed antennae. The entire body is clothed in fine hairs. The males are distinguished from females by being smaller (usually half the size of the females) and darker, with more elongated terminal points of the antennae.
Life-Cycle:
The insect breeds from April to October and hibernates in the larval stage from November to March in cracks and crevices of walls and floors or in other sheltered places. Copulation takes place 2-3 days after emergence, a male being capable of fertilizing more than one female. One to three days after copulation, the female begins to lay white translucent eggs on the grains, singly or sometimes in clusters of 2-5.
The eggs are rather cylindrical, rounded at one end and narrow at the other. A female may lay 13-35 eggs in 1-7 days at the rate of 1- 26 eggs per day, the largest number being laid on the first day. The incubation period varies from 3-5 days in June to 6-10 days in October.
The viability of the eggs varies from 86 per cent in September to 58 per cent in October. The male larva is full-fed in 20-30 days and the female larva in 24-40 days. Pupation takes place in the last larval skin among the grains. This stage lasts 4-6 days. The adults are incapable of flying. There are 4-5 generations in a year.
Damage:
The greatest damage is done in summer from July to October. The grubs eat the grain near the embryo or at any other weak point and from there proceed inwards. They usually confine themselves to the upper 50 cm layer of grains in a heap or to the periphery in a sack of grains. If the infestation is severe, the devastation is complete, reducing the grain to a mere frass. Since the larvae are positively thigmotactic, they can be collected by merely placing gunny bags on a heap of grain.
2. Rice Weevil, Sitophilus Oryzae (Linnaeus) (Coleoptera: Curculionidae):
The pest is world-wide and is found practically throughout India. It is the commonest and, perhaps, the most destructive pest of stored grain throughout the world. It is distinct from another allied species, the grain weevil, Sitophilus granarius (Linnaeus). Both the species are similar in size and appearance and are found together feeding upon rice, wheat, maize and other grains. The rice weevil may, however, be found in the paddy fields as well.
Both the adults and the grubs cause damage. The full-grown larva is 5 mm in length and is plump, fleshy legless creature, having a white body and a yellow-brown head. The adult is a small reddish-brown beetle, about 3 mm in length, with a cylindrical body and a long, slender, curved rostrum. Its elytra bear four light reddish or yellowish spots and thorax is fitted with round depressions. Unlike S granarius, the metathoracic wings of this insect are well developed.
Life-Cycle:
The rice weevil breeds from April to October and hibernates in winter as an adult inside cracks and crevices or under wheat bags in the godowns. During the active season, the females lay eggs on the grain by making a depression with the help of their mandibles. After an egg has been laid, the hole is sealed with a gelatinous secretion. A single female may lay as many as 400 eggs.
The eggs hatch in 6-7 days and the young larvae bore directly into grain, where they feed and grow to maturity. Then, they pupate inside the grain. The pupa, at first, is dirty white, but later on becomes dark brown. The pupal stage lasts 6-14 days. On emergence, the adult weevil cuts its way out of the grain and lives for about 4-5 months. At least 3-4 generations are completed in a year.
Damage:
Heavy damage may be caused by this pest to wheat, rice, maize and sorghum grains, particularly in the monsoon. It has also been reported feeding on oats, barley, cotton-seed, linseed and cocoa. The weevils destroy more than what they eat.
3. Red Flour Beetle, Tribolium Castaneum (Herbst) (Coleoptera: Tenebrionidae):
This insect is world-wide and is the most common pest of wheat-Hour. It also feeds upon dry fruits, pulses and prepared cereal foods, such as cornflakes. Both the larvae and adults cause damage. The young larva is yellowish white and measures 1 mm in length. As it matures, it turns reddish yellow, becomes hairy and measures over 6 mm in length.
Its head, appendages and the last abdominal segment are darker. The adult is a small reddish-brown beetle, measuring about 3.5 mm in length and 1.2 mm in width. Its antennae are bent and bear a distinct club formed by the three enlarged terminal joints. The last antennal segment is transversely rounded. Another allied species, T. confusum Duval, is also often present in the wheat flour.
Life-Cycle:
The insect breeds from April to October and passes the winter mostly in the adult stage. In the active season, the adults copulate one or two days after emergence. The females lay white, transparent, cylindrical eggs in the flour or in the frassy material among the grains and other foodstuff. The surface of freshly laid eggs is sticky and, therefore, flour or dust particles easily adhere to them.
A single female may lay as many as 327-956 eggs. The incubation period lasts 4-10 days. The worm like larvae undergoes 6-7 moultings and they are full-grown in 22-25 days at 30°C. Pupation takes place in the flour. The pupa is yellowish and hairy. The pupal stage lasts 5-9 days. The development period from egg to the adult is 26-30 days in summer, but is longer under unfavourable conditions of temperature and food.
Damage:
The greatest damage is during the hot and humid monsoon season. Although this insect is incapable of feeding on sound grain, it does considerable damage to flour and flour products and also to grain damaged by other pests. The larvae are negatively phototactic and are always found hidden in the food. The adults, however, are active creatures, capable of short flights but are mostly found concealed in flour. In severe infestation, the flour turns greyish and mouldy, and has a pungent, disagreeable odour making it unfit for human consumption.
4. Lesser Grain Borer, Rhyzopertha Dominica (Fabricius) (Coleoptera: Bostrichidae):
Originally, an inhabitant of India, this insect has now spread to the rest of world. It has been reported as a pest from Algeria, Greece, United States, New South Wales (Australia), Japan and China. It is known as a destructive pest of wheat and has also been recorded on other grains and foods such as rice, maize, sorghum, barley, lentils, army biscuits, ship biscuits, stored grains, dried potatoes, corn flour, beans, pumpkin seeds, tamarind seeds and millets.
Both the adults and larvae cause damage. The larva is about 3 mm long, dirty white, with a light-brown head and a constricted elongated body. The adult is a small cylindrical beetle measuring about 3 mm in length and less than 1 mm in width.
It is shining dark brown or black with a deflexed head, covered over by a crenulated hood-shaped pronotum. The antennae are 10-jointed and terminate in a prominent tripartite club, the first two segments of which are triangular and the apical segment oval. No morphological difference separates the two sexes.
Life-Cycle:
The pest breeds from March to November and in December; it enters hibernation as an adult or as a larva. In March, it resumes activity and by the first week of April, the females begin to lay eggs. Polyandry and polygamy are common in this insect. During opposition, there are spurts of vigorous feeding, alternating with heavy egg-laying.
A single female can lay 300-400 eggs in 23-60 days at the rate of 4-23 eggs per day. The eggs are laid singly among the frass or are glued to the grain in batches. They measure about two-thirds of a millimetre in length and about one-fifth of a millimetre in diameter, and are almost cylindrical and rounded at one end and somewhat pointed at the other.
When freshly laid, the eggs are glistening white, but later on a pink opaque line appears on them. The incubation period is about 5-9 days but under controlled conditions, the eggs hatch in 9.7, 6.4, 4.7 and 5.7 days at 25, 30, 35 and 37°C, respectively.
Just before emerging, the larva cuts a circular hole in the pedicel end of the eggs and comes out of it. It then crawls about and feeds upon the flour produced by the boring beetles or it burrow into slightly damaged grains and passes its entire life inside, feeding upon the starchy contents. The larva moults 4- 5 times and is full-fed in 52.7, 31.3, 23.7 and 25.6 days at 25, 30, 35 and 37°C, respectively.
The pupal stage, like the larval stage, is passed within the grain or in the grain dust. The adult, on emergence, remains inside the grain for a few days until it is mature. It then cuts its way through the side of the grain and comes out. The period from the egg to adult stage is completed in 30 days in the summer.
Experimentally, the duration of the pupal and adult stages at 25, 30, 35 and 37°C was observed to be 6.7, 4.6, 3.8 and 3.9 days, and 87.0, 79.0, 54.9 and 39.9 days, respectively. The males have a longer life-span than the females. There are 5-6 generations in a year.
Damage:
The adults and grubs cause serious damage to the grains by feeding inside them and reducing them to mere shells with many irregular holes. The adults are powerful fliers and migrate from one godown to another, causing fresh infestation. When the infestation is severe, the adults produce a considerable amount of frass, spoiling more than what they eat. The flour, so produced, serves as nourishment for the young grubs until they are ready to bore into the grain.
Bruchids:
1. Gram Dhora, Callosobruchus Chinensis (Linnaeus) (Coleoptera: Bruchidae):
The gram dhora or pulse beetle has been reported from the USA, Mauritius, Formosa, Africa, China, the Philippines, Japan, Indonesia, Sri Lanka, Myanmar and India. It is a notorious pest of gram, mung (Phaseolus aureus), moth (Phaseolus aconitifolius), peas, cowpeas, lentil and arhar (Cajanus cajan). It has also been reported on cotton seed, sorghum and maize.
The larva does the damage by feeding inside the grain. In the early stages, it is whitish with a light-brown head and later on it acquires a creamy hue. The mature larva is 6-7 mm long. The adult beetle measuring 3-4 mm in length is oval, chocolate or reddish brown and has long serrated antennae. There is a pair of white elongate prominences in the middle of the hind margin of the thorax, a spine on each of the inner and outer edges of the end of the hind femur, and truncate elytra, not covering the pygidium.
Life-Cycle:
The pest breeds actively from March to the end of November. It hibernates in winter in the larval stage. At the end of March, the adults appear and copulate immediately after emergence. A day later, the female starts laying small, oval, scale-like eggs which are glued to the grain. In this species, more than one egg may be laid on a grain. Thus, two or three (up to 8 have been reported on a single grain) larvae may develop in separate chambers.
In the allied species, C. analis (Fabricius) (predominantly on mung), only one egg is laid on one grain. A single female of C. chinensis may lay 34-113 eggs at the rate of 1-37 per day. The highest egg production is in May and October and the least in April, June, July and December. The eggs hatch in 7- 14 days in April, 4-6 days in September and 8-16 days in November. The viability of the eggs varies from 3.6 per cent in May to 76.9 per cent in August and September.
The young larva bores into the grain and completes its development inside. The larval stage is completed in 10-12 days in August and September, and 26-38 days in November. The hibernating larvae take 117-168 days to complete their development. The full-grown larva migrates towards the periphery and comes to lie next to the seed coat where it turns into an oval white pupa.
The pupal stage lasts 4-28 days, depending upon the season. The adult escapes by cutting a circular hole in the seed-coat and such grains can be spotted easily. The average life-span of an adult is 5-20 days. A preponderance of males occurs throughout the active season. The insect passes through 7-8 overlapping generations in a year.
Damage:
The damage is at its peak from April to September and is considerably reduced in October-November. Damage to the pulses infested with this insect is very high and quite often each and every grain is infested so that the pulses become unfit for human consumption. Infested grain is often converted by the traders into flour which has a characteristic off-flavour and should not be marketed.
2. Mung Dhora, Callosobruchus Analis (Fabricius) (Coleoptera: Bruchidae):
The distribution of mung dhora or pulse beetle is comparatively limited. So far, it has been recorded from Germany, Rhodesia, Myanmar and India. It is a pest of mung (Phaseolus aureus), mash (Phaseolus mungo), moth (Phaseolus aconitifolius), peas, cowpeas and other pulses. Both the larvae and adults cause damage. The larva is recognized by its creamy-white, oval, flabby body.
The adult is an oval beetle, somewhat smaller than C. chinensis. The female is of chocolate colour with a black trapezoidal area surrounded by white streaks on three sides. This area is located in the centre of each elytra towards the outer margin. The exposed pygidium is black with a central longitudinal white streak. The male is uniformly chocolate with a tinge of straw. The other species of pulse beetle common in India are C. maculatus (Fabricius) and C. phaseoli Gyll.
Life-Cycle:
The insect breeds from March to December and then over-winters in the larval stage within the infested grain. In the first week of March, the adults escape by cutting out a circular hole in the seed coat. Soon after emergence, copulation takes place which lasts 3-15 minutes. The females lay oval, white eggs singly or in batches within 72 hours of mating.
The eggs are glued to the grain and the number laid in 2-22 days by a female varies from 11 to 150. The highest number of eggs is laid in August (average 95 eggs) and the lowest in July or December (average being 63). Viability among eggs varies from 14.5 per cent in August to 72.4 per cent in May, which probably depends upon the prevailing humidity.
The incubation period is 3-6 days in May-August, 4-8 days in April to October, 8-13 days in November and 18 days in December. Experimentally, the eggs have been hatched in 4.9, 3.8, 4.1 and 5.3 days at 25, 30, 35 and 37°C, respectively. There was no hatching at 40°C. The young larva, on emergence, bores into the grain and feeds there. The development is fast during the summer but during the winter larvae take as long as 95-145 days to complete their development.
When full-grown, the larvae pupate inside the grain. Experimentally, the duration of the larval and pupal stages at 25, 30, 35 and 37°C was 15.8, 10.2, 19.9 and 26.1 days, and 10.4, 5.6, 6.1 and 7.9 days, respectively. The life of adults varies from 5 to 10 days, the male having longer life. The insect completes 9-19 overlapping generations and the sex ratio varies at different times of the year.
Damage:
This pest causes the maximum damage from April to August, when all stages of development are present. The larvae feed and breed inside the grain, consuming the entire contents. When the infested grain happens to be in a poorly aerated receptacle, a foul smelling fungus also develops.
Other Pests of Stored Grains:
The other insect species recorded as minor pests on stored grains and products in India are the saw-toothed beetle, Oryzciephilus surinamensis (Linnaeus) (Coleoptera: Silvanidae); the flat grain beetle, Cryptolestes pusillus (Schonherr) (Coleoptera: Cucujidae); the cadelle, Tenebroides mauritanicus (Linnaeus) (Coleoptera: Trogossitidae); the long-headed flour beetle, Latheticus oryzae Waterhouse (Coleoptera: Tenebrionidae); the tobacco beetle, Lasioderma serricorne (Fabricius) (Coleptera: Anobiidae) and the black fungus beetle, Alphitobius laevigatus (Fabricius) (Coleoptera: Tenebrionidae).
Control of Storage Pests:
The effective management of storage pests may be ensured by drying the grains properly before storage, storing new grains in the clean godowns or receptacles and plugging all cracks, crevices and holes in the godowns thoroughly. If infestation of grain has already taken place or is imminent on account of a peculiar situation in which the grain is stored, then application of chemicals becomes necessary. The chemical controls to be practised under different situations are variable.
1. Surface Treatment:
The grain kept in jute bags can be protected by spraying insecticides on the surface of the stack. The godowns are disinfested before storing the grain and new infestation can be prevented by surface treatment. Disinfect old gunny bags by dipping them in 0.0125 per cent fenvalerate 20EC or cypermethrin 25EC for 10 minutes and drying them in shade before filling with grains or use new gunny bags. Disinfect empty godowns or receptacles by spraying 0.05 per cent malathion emulsion on the floor, walls and ceiling.
2. Seed Treatment:
The grain meant for seed can be protected by mixing dusts with it. At present, mixing of malathion 5 per cent at the rate of 250 g per quintal of seed is recommended. The grains may also be treated with 25 ml of malathion 50 EC or 2 ml of fenvalerate 20EC or 1.5 ml of cypermethrin 25EC or 14 ml of deltamethrin 2.8EC per quintal of seed by diluting in 500 ml of water.
Before treatment, the grains should be spread in thin layer on pucca floor or polythene sheet. After treatment, the grains should be mixed thoroughly and then put into the containers. Against pulse beetle (dhora), cover the pulses stored in bulk with 7 cm layer of sand or sawdust or dung ash.
3. Fumigation:
Fumigation is a process of creating lethal concentration of a toxic gas for the time necessary to destroy insect infestation in a closed space. If there is any sign of infestation in the grain, whether stored in receptacles or in godowns, it is best to fumigate it with one of the poisonous gases available for the purpose and then to sieve and dry it before returning to the receptacles.
To ensure a complete mortality of the infesting pests, it is advisable to repeat the fumigation after one month. Since the fumigants are highly poisonous to human-beings and animals, fumigation should be carried out only if the godowns are away from the residential buildings and only if it is possible to make them air-tight.
If the godowns are in an awkward position and these conditions are not met with, the grain, whether in bags or in bulk, should be taken out in the open and fumigated under a gas-proof tarpaulin or a tent. Metallic drums or wooden boxes can also be used for fumigating small quantities of grain.
The chemicals used are available as liquids, gases or solids. In India, ethylene dichloride and carbon tetrachloride mixture has been recommended for fumigation of food grains in storage at farm level, and hydrogen phosphide in the form of aluminium phosphide or methyl bromide for protection in warehouses, godowns and silos.
Suitable fumigant which can be used under ordinary conditions of storage is the mixture of ethylene dichloride and carbon tetrachloride at the rate of 1 litre for 20 quintals of grain or 35 litres per 100 m3 of space with exposure period of 4 days. This fumigant-mixture is comparatively safe. Methyl bromide is also becoming popular because of its high toxicity to insects and low cost.
It is used at the rate of 3.5 kg per 100 m3 of space with 10-12 hours exposure. The fumigant, hydrogen phosphide (aluminium phosphide), is currently used for large scale fumigation of food grains in those storage structures which are away from living premises. It is available in tablet form and can be used at the rate of one tablet (3 g) per metric ton or 25 tablets per 100 m3 of space with an exposure period of 7 days.
Procedure for Fumigation:
It is desirable to do mud plastering of storage structures in advance, leaving only exit door or openings in the wall. For small quantities of grain, if an EDB ampoule has to be used, it is inserted a little below the surface of the grain. The aluminium phosphide tablets when used are inserted into the grain kept in bulk or distributed uniformly among bagged grains. The opening is then immediately sealed with mud plaster and polythene sheet.
The storage structures thus sealed should be left undisturbed for a period of seven days or as long as the grain is not wanted. After the required period of exposure to the fumigant, grain should be aerated well for a few hours until no smell of the fumigant is left. The treated grains should be cleaned and sieved properly before they are consumed.
Precautions:
The following precautions should be strictly followed during fumigation:
i. The fumigants should be used only in airtight stores or under tarpaulins in the open by specially trained persons, because these compounds are deadly poisonous.
ii. Inhalation of fumigant vapours should be avoided by wearing gas masks and protective clothing.
iii. The fumigants should be applied at the recommended dosages. EDB ampoules should not be used for the fumigation of milled products, oilseeds and moist grains, to avoid excessive absorption.
iv. Splashing of the liquid fumigants over hands and face should be avoided.
v. Aluminium phosphide must not be used in living quarters. Its use in a godown next to the living rooms may also be hazardous.
4. Use of Improved Storage Receptacles:
The grains can be best protected by using improved insect-proof receptacles of following types:
A. Indoor Bins:
(i) Domestic Metal Bins:
Several types of domestic designs of metal bins have been developed, the capacities of which range from 3 to 27.5 quintals. These are indoor bins and may be kept in a room or verandah under a roof. The bins are made from galvanized iron sheets and are moisture, rodent and insect-proof. These are found suitable for storage of wheat, paddy, maize, pulses and seed grains.
(ii) Gharelu Theka:
This type of storage structure consists of rat-proof metal base, rubberized cloth containers and bamboo posts, having different storage capacities ranging from 2 to 3 metric tonnes.
(iii) Pucca Kothi:
The improved structure called ‘Pucca kothi’ is constructed using burnt bricks plastered with cement mortar. The structure is constructed in two compartments with a capacity of 1 metric ton.
(iv) Welded Wire-Mesh Bin:
The bin is fabricated using wire-mesh with hessian cloth lining inside so that the air may circulate freely through it. The structure is mounted on prefabricated steel elevated base to prevent the entry of rodents. The storage capacity of bin is 2.8 metric tonnes. The food grains like maize and paddy can be stored in this bin even at slightly higher moisture level without deterioration in quality.
(v) Reinforced Cement Ring Bins:
This design consists of pre-fabricated RCC rings placed one over the other with gripping joints at the edges.
(vi) Paddy Straw-Mud Structure:
The improved paddy straw-mud structure of 400 kg capacity is made from paddy straw rope plastered on both sides with specially prepared mud. Externally, the structure is further plastered with water proof mud to prevent the entry of moisture. The structure is mounted on a raised brick masonry platform to prevent the entry of rats. It is commonly used for paddy storage.
B. Outdoor Bins:
(i) Flat and Hopper Bottom-Metal Bins:
These bins are fabricated using either steel or aluminium metal sheets of different capacities ranging from 2.0 to 10.5 metric tonnes. The bin can be either erected on brick masonary base, brick masonary columns or prefabricated steel elevated base. These bins are suitable for storage of wheat, paddy and maize under different climatic conditions.
(ii) Composite Bins:
These are the outdoor flat bottom storage bins fabricated with steel and timber in combination with varying storage capacities ranging from 3 to 14.5 metric tonnes.
(iii) Partly underground and Above Ground Structures:
A prototype structure of 10 m3 or 7.5 metric tonnes capacity which is partly underground and partly above ground has been constructed. The underground part of the structure is of RCC or brick, while the above ground part of the structure is of galvanized iron sheet. This bin is suitable for construction in areas where water table is low.
(iv) Seed Storage Bins:
Quite popular in rural areas, these bins are either circular or square in shape with storage capacities ranging from 1 to 15 quintals.
(v) Ferro-Cement Bins:
The bins are constructed by using rich cement mortar and closely spaced chicken wire-mesh. The bin is cylindrical in shape and has flat bottom and a dome-shaped roof. It is treated on the outside with suitable moisture proofing paint like bituminous aluminium paint. The capacity of the ferrobins ranges from 0.6 to 3.0 metric tonnes. They are lighter in weight than the cement concrete structure.
(vi) Pusa Bin:
This design consists of two brick walls of 10 cm thick each using sun dried bricks with polythene sheet sandwiched in-between. The structure is constructed on a brick masonry platform plastered with cement mortar. A mud slab is provided as the top on a wooden frame structure. The polythene sheet is also provided at the top and the base to make the structure completely moisture-proof and airtight.
(vii) Improved Godowns:
For larger bag storage of food grains, it is advisable to construct improved godowns, specifications for which are available from the Bureau of Indian Standards, New Delhi. The bag storage structures are conventional type godowns either with angle iron trusses, tubular trusses, raiser type trusses, precast trusses, wooden trusses or godowns with RCC flat roof or shell type roof.
(viii) Bulk Storage Installations:
In addition to these godowns, large scale storage installations with adequate storage and mechanical grain handling facility are used by grain handling authorities in the country like Food Corporation of India, Marketing Cooperative Federations, Central and State Warehousing Corporations, etc.
For bulk food storage, silo elevators have been installed in India at Hapur, Khurja, Jagraon, Gobindgarh, Moga, seaports and many other places. The storage bins or silos have a capacity of 10,000 tonnes each and provide for bulk storage of grain. The elevators are provided with mechanical operations for receiving and issuing out grain with the help of a mechanical system on scientific lines.
The super structure consists of twenty cylindrical silos with a capacity of 500 tonnes each. The silo is about 6.7 metres in diameter and 23 metres high. At the head of the silos stands the head house which is 9.1 metres in diameter and 47.3 metres high. The auxiliary shipping tanks, grain cleaning and disinfesting equipment and automatic weighing machines are housed in this building.
The grain can be stored up to five years without deterioration or loss. There is practically no access to insects or rodents. They are also provided with aeration and fumigation system whereby the grain can be periodically aerated and fumigated to maintain good quality.
(ix) Vacuum Process Storage:
The vacuum process storage (VPS) is a modern concept in storage system. Under the VPS system, the PVC containers, manufactured for the purpose, are filled with food grains and air is sucked out of them instantly. Thus, vaccumised containers could be operated like stone boxes and stacked like gunny bags in the open field. The food grains in such containers never get infected or catch fungus in the absence of air. Besides, the food grains thus stored never get dis-coloured and suffer damage in quality even though the stocks are kept for 3-4 years.