Insect pests of stored products

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Chuwit Sukprakarn

Thailand is one of rice growing countries in the world, the cultivation is about 10 million hectares and annual production is 19.5 million tons. Rice is grown in all parts of the country from the Southern border with Malaysia to the Northern border with Laos and Burma with a distance of about 1,600 kilometres. Most of the rice varieties are irrigated rice and depended upon rainfall, very few are upland rice. About 20 percents are floating rice which may grow in water at several metres deep. Rainfall is the most variable climatic factor affecting rice cultivation. The average annual amounts of rainfall for the whole country is 1,500 mm (about 60 inches) per year. In the Northeastern Region, only 1,000 mm are common while in the Southern Region the usuall rainfall are about 2,000 mm and may reach up to 2,500 mm.

Besides rice production, Thailand also producing maize, sorghum, mungbean, soybean, groundnut, cassava etc.

Most of the farmers do not store the grain for a longer period except for replanting. They generally sell the grain either before harvesting or during threshing for the rent or money requirement. Nearly all of the grain and other agricultural products therefore are kept in the mills, godowns or even silos ready to be exported or distributed to the local markets. For this reason, those who operate storage facilities must know how to protect the grain from insect infestation. Government organizations as well as the farmers do not emphasise much research on stored grain insects. Very few research works on stored grain insects have been undertaken during the recent years, most of the works were confined on the insect pest attacked in the field rather than the pests of stored grain.

Losses due to insect infestation

The percentage of losses is very difficult to determine and the figures vary from 1 % to as much as 25%. The official figures however, released by the five ASEAN countries stated that the member nations lost about 25% of their paddy crop during harvesting and other post-harvest practices including storage and transportation, and the loss represents 10.5 million tons of paddy whereas FAO reported in 1977, the loss of rice within the postharvest system for Thailand ranged from 8-14%. In Thailand itself, there is no official report on losses due to insect infestation. The estimation of losses is based only upon experiments. For paddy, some investigators reported that the loss in weight for 8 months was at 1.14-3.41% when in farm and more than 5% for commercial storage while the author reported that grain loss was from 0.05-10.48% for one year storage. The report recently from the Rice Institute, 20 varieties of paddy seed when stored untreatment for 10 months, the losses varied from 2.06 to 24.30% with an average 4.54%. Other grain crops eg. maize, sorghum and pulses, these crops have already been infested by the insects from the fields and also from the poor storage condition. When the grain has no protection, the insect population will build up rapidly. Therefore, the losses and damages by insect pests are related to the storage duration. Unfortunately, there is no record on losses of these crops but it has been observed that the severe damage will occur within a few months of storage and may reach upto 50% for 6 months storage. This is one of the reasons why the farmers need not keep the grain in large quantity and longer periods.

Presently, quantity loss is not as important factor as the loss of goodwill in the international trade. The loss of goodwill between traders and farmers or between importers and exporters in the international trade could be a serious matter in future marketing. In the past, some major exporters experienced the embarrassment of some shipments being declared distressed cargoes. This was due to the presence of some quantity of insecticide on grain which may cause health hazards to human beings. Commercial losses can also occur due to the reduction of quality through adulteration or insect attacks.

The major insect pests can be grouped according to feeding behavior as follows:

Biology of Storage Insects

Insects development take place between 17°C and 35°C The optimum temperature for most storage insects is around 30°C at 75% relative humidity The insects are more critical in their response to high temperatures. Most insect pests are killed by temperatures above 40°C. Whereas most stored grain insects are able to withstand temperatures below freezing for several days but when exposed to 65°C for a few minutes all the insects are killed.

The number of important storage insects is rather small, being about 30. Most species have a short life cycle of 4-6 weeks and are universal in their taste.

A number of storage insects lay their eggs in or near produce. After hatching the larvae into the produce and develop there till after pupal stage. Other pests develop and pupate on or between the produce.

Adults of storage insects can be long or short living. Bruchids and moths are short living (up to 3 weeks). They lay most of their eggs during the first week of adulthood. Other species like Sitophilus and Tribolium can live for 6 to 12 months. Egg deposition takes place over a prolonged period.

Important parameters in the development and reproduction of storage pests, besides climate, are the kind and condition of the produce and its moisture content. So at 25°C and 70% relative humidity the development of Ephestia cautella takes about 30 days on grains, 43 days on groundnuts and 53 days on cocoa beans. Storage pests can be divided into primary and secondary pests. Primary pests like Sitophilus or Ephestia species are able to develop on undamaged produce, while secondary pests will develop only on produce which was previously damaged by other insects or mechanically. So in stored produce a certain succession of pests may occur.

In this section only the biology of some major insect pests will be briefly discussed.

1. The rice weevil (Sitophilus oryzae L.)

Order Coleoptera, Family Curculionidae

The weevil is brown to black with two paler reddishbrown patches on each elytra, 2-3 mm long. The female lays about 200 eggs in the grain and the larva feeds and pupates in the grain. The incubation period lasts 36 days, the larval and pupal period is 20-30 and 3-7 days respectively. The life cycle is completed in 30-40 days.

2. the red flour beetle (Tribolium castaneum Hbst.)

Order Coleoptera, Family Tenebrionidae

The beetle is 2.3-4.4 mm long, reddish-brown and flat. Eyes large, emaginate; antennae clavate, 11 segmented. The egg, white, oblong or pear-shaped. A female lays 15-72 eggs in rice and the incubation period takes 3-7 days. The larva is whitish, worm-like. There are 6-7 larval instars and the larval period ranges 21-40 days. The pupal period is 3-7 days and the life cycle completed in 26-48 days.

3. The Angoumois grain moth (Sitotroga cerealella Oliv.)

Order Lepidoptera, Family Gelechiidae

A small moth with a wing span of approximately 1/2 inch which can be distinquished by its pale brown colour and the elongate sharply pointed apices of the hind wings. A female lays 30-78 white, oblong shaped eggs, singly or in small groups. After hatching the whitish larva bores into and spends its complete life cycle wishing a single grain, eventually pupating there. It takes 4-6 days for incubation period and 26-35 days for larval period. The pupal period takes 3-6 days and the moth can live for 3-7 days.

4. The almond moth (Ephestia cautella W.)

Order Lepidoptera, Family Phycitidae

The moth is greyish or pale grey with two zigzag lines on forewing with a wing span of about 12-16 mm. A female lays upto 205 eggs in cracks or crevices on grain after which the female dies soon after. The eggs hatch within 3-6 days. The whitish larvae molt 3 times and complete their life cycle in 27-83 days.

5. The rice moth (Corcyra cephalonica Staint.)

Order Lepidoptera, Family Galleriidae

A medium-sized moth with a wing span of 20-25 mm, the forewings are uniformly pale brown with the veins slightly darkened. A female lays 44-364 whitish eggs with incubation of 4-5 days. The larva is white to whitish-grey. The larval period takes 2841 days with 5-7 larval instars. The pupal period is 6-13 days and the moth can survive for 4-6 days.

6. The Cowpea beetle and Southern cowpea beetle (Callosobruchus chinensis L. & C. maculatus F.)

Order Coleoptera, Family Bruchidae

These two species are resemble in appearance and they occasionally feed on food together. The size of the Cowpea beetle is little smaller than the Southern Cowpea beetle (2.0-3.5 mm.: 3.0-4.5 mm). It is small, oval brown beetle with or without black markings on the elytrae. A female lays more than 10 yellowish eggs (average 50 eggs) on grain with the incubation period 36 days. The larva feeds and pupates inside the grain then emerges as adult. The larval and pupal period is 1820 and 3-7 days respectively. The life cycle completed in 18-33 days and the beetle can survive up to 12 days.

Prevention and control of storage insects

The principal means of prevention is to select a place and method of storing, that suit best the produce and local conditions. Many products like maize, sorghum, groundnuts etc. can either be stored as unshelled or as shelled produce.

Unshelled produce such as maize and sorghum can be stored on the cob or in the ear in rectangular or round cribs constructed from poles, bricks and chickenwire. Because of good ventilation mould problems are few, but protection against insects and rodents needs use of pesticides.

Shelled produce can be stored in bags in warehouses or in bulk in silos.

In all cases, strict hygiene is very Important. Warehouses and silos must be cleaned thoroughly of old infested produce before the new harvest is brought in. Bags should be stacked on pallets and stand free of walls and celling. Different products should be stacked separately. Food stores should be swept out every week and the sweepings must be burned immediately. The storage structures should be closed off to prevent entry by pests, airtight silos with good thermal insulation offer the best protection.

Admixture the grain or seed with inert substances such as dust or plant parts could prevent maize and sorghum from insect damage for some period, while in oil seeds and pulses the admixture the grain with edible oil like palm oil, rice bran oil or peanut oil are recommended to control the Bruchids.

Temperature Control: Since most stored product insects cannot tolerate extreme temperature, heating and cooling are logical approaches to insect control. To some extent it has been a common practice to superheat some comodities for insect control. The temperatures of 55-60°C maintained for 10 to 12 hours are effective. Actually, these temperatures kill most insects very quickly but when the grain and materials are involved, the certain temperature must be kept for several hours to ensure complete penetration.

Low temperature is probably the most important single factor in making long term storage possible and economical. The insects become inactive and eventually die at a temperature below 12°C. Freezing quickly kills many insects. Low temperature is also important in maintaining seed viability.

Moisture Control: Most of the stored grain insects are unable to survive and reproduce in grain whose moisture content is below 9 per cent. Most favorable grain moistures for insect development ranges from 12 to 15 per cent. If, by various means, it is possible to reduce and maintain the moisture below than favorable for reproduction and development, then we have in effect, controlled the insects.

All agricultural products should be well dried before storage especially for storing in silos. A high moisture content tends to increase insect and mould development; to bacterial deterioration, and chemical changes in the produce. When the crop is ripe it still has a high moisture content. Under dry weather conditions the crop is usually left in the field to dry, but in the humid tropics artificial drying is often necessary.

Produce should not be stored at moisture contents higher than indicated below.

Chemical Control: For the protection of stored produce against the insects the following groups of pesticides are used:

a) insecticides
b) fumigants

For the protection of store produce, pesticides are often applied shortly before use or mixed with the produce, which limits the choice of pesticides which can be used and rates of application. It will be obvious that, for the protection of stored produce, only pesticides can be used with a rather low mammalian toxicity whose residues easily degrade to innoxious compounds which can be excreted. Insecticides which are accumulated in the human body e.g. DDT are of course completely unsuitable for use on stored produce.

Insecticides may be used for spraying wall, floors and ceilings of warehouses or storerooms in order to kill a residual infestation. The insecticides can also be sprayed directly on bagged produce. This may prevent or delay reinfestation of insect-free produce. Insecticides may be mixed with the produce. This can give complete protection for a long period and may also kill pests which have already infested the produce. The best way however to disinfect produce, warehouses or storerooms, is by means of fumigation. The fumigants used penetrate into the grain or compressed products like tobacco and kill all insects. Some fumigants kill also micro-organisms. After fumigation, reinfestation must be prevented by insecticides or by storing the produce in an insect proof silo or container.

Resistance to pesticides is developing fast in storage pests. If this occurs the best way of protection if probably a combination of fumigation with storage in insect proof containers or silos.

Insecticides

For the protection of stored products only a few insecticides are in common use.

a) Malathion: This is a safe insecticide which can be admixed to or sprayed on shelled (threshed) or unshelled (unthreshed) grains. On stored produce only premium grade malathion must be used. (LD50 = 1400 mg; tolerance 8 ppm for raw cereals (FAO/WHO) The general recommendation is to mix 100150 9 2% with 100 kg produce.

Malathion dust has some limitations.

1) The product must be dry, (moisture content not higher than 13.5%) otherwise the malathion breaks down very fast.

2) The formulated malathion dust mostly has a rather short shelf life (not more than 6 months).

Malathion can also be used for spraying walls, floors or the outside of stacks (1000 mg a.i./m²).

b) Ryrethrins: This is a very safe botanical insecticide but costs are high (LD50 = 1500 mg; tolerance 3 ppm for raw cereals) (FAO/WHO).

Pyrethrins are mostly admixed with a synergist to increase their effectiveness and stability and to reduce costs. The shelf life of dust formulations is rather short. Rates of application are:

100 g 0.2% pyrethrins + piperonyl butoxide (1:5) per 90 kg cereals and 100 9 0.1% pyrethrins + piperonyl butoxide (1:5) per 90 kg beans.

c) Other insecticides

During the past years a number of other insecticides have become available, the use of which is permitted in several countries.

The most important are the following:

bioresmethrin
bromephos
chlorpyrifos-methyl
fenitrothion
pirimiphos-methyl
tetrachlorvinphos

Rates of application are indicated in the table below.

Fumigants

A fumigant is a chemical which at the required temperature and pressure can exist in the gaseous state in sufficient concentration to be lethal to a given pest organism.

Many fumigants are available and several are commonly used throughout the world. Any confined space which can be made airtight, may be used for fumigations, e.g. silos, railway, trucks, shipholds, plastic bags, etc. Bagged produce is mostly fumigated under gasproof sheets.

After fumigation a small amount of unchanged fumigant may remain as a residue.

The following fumigants are commonly in use:

a) Methylbromide. It penetrates easily in large stacks of bagged produce but, without a special circulation system its use in large silos is limited because of unsatisfactory distribution of the gas in the grain bulk.

In flat storage, for instance in barges, this does not play a role. Under atmospheric conditions a fumigation takes 24 hours. Under vacuum conditions only several hours are needed.

Methylbromide is highly toxic and rather sophisticated equipment such as gas cylinders piping systems, gas masks and gas detectors are necessary. The fumigation has to be carried out by trained personnel.

Rates of application under atmospheric pressure are 16-32 g/m³ for 24 hrs depending on temperature, commodity and the insect or mite species to be controlled.

b) Phosphine. This fumigant is available in the form of tablets (pellets or sachets). Moisture absorption liberates phosphine which is very toxic to insects. The tablets must be evenly distributed through the grain by adding them to the grain flow when a bin is filled.

The tablets can also be inserted in or between bags which then must be covered by air-tight sheets. Since the development of phosphine starts some hours after application, the use of phosphine is easy, but gas masks are necessary when aerating large stacks.

Rates of application are 1-1 1/2 tablet per m³ for bagged produce under plastic sheets or 2-5 tablets per ton for grain in silos. A fumigation with phosphine takes 5-7 days, besides the ease in application phosphine has some other advantages. There is less chance of affecting the germination capacity than with other fumigants like methylbromide. However, a preliminary test is always useful. The penetration of phosphine is probably better than that of methylbromide and costs of fumigation are usually less. Till now phosphine has not caused residue problems.

c) Liquid fumigants like carbon tetrachloride or mixtures of carbon disulphide, ethylene dibromide or ethylene dichloride and carbon tetrachloride are easier to handle as they are less toxic to man. The liquid is poured on the produce or left in trays to evaporate. Such a fumigation takes several days depending on the temperature and quantity of fumigant used.

Airtight storages

When grains are stored in an airtight container, the oxygen content in this container will decrease slowly due to the metabolism of the grains, insects and microorganisms until there will not be enough oxygen for any insect development. Airtight storage is an attractive way to protect produce against insects without pesticides, but often the costs of constructing suitable silos prevent their general use. For airtight storage on a small scale, oildrums or plastic bags may be used.

The lowest O2 concentration below which insects cannot survive is about 2%. On the other hand a high CO2 concentration (36%) together with a high O2 concentration (15-21%) is lethal to storage pests too. The relative humidity has a strong influence on the effect of the gas concentrations. In general, a low relative humidity increases the mortality cause by low O2 or high CO2 concentrations. A new development is the use of CO2 for long-term preservation and fumigation of cereal grains which has been found more effective and easier to apply than to decrease O2 concentrations.

Table

Sieves

Hand-held sieves

Hand-held sieves are commonly used for assessing the foreign matter content of small samples of grain. Round-framed sieves with a diameter of 300 to 310 mm are preferred, although square-framed sieves with sides 300 to 310 mm long may be used. Each set of sieves should be provided with a bottom pan (receiver), for the collection of material passing through the screens, and a lid to prevent spillage during the sieving.

Comparability of the results of using hand-held sieves depends primarily on uniformity in their manufacture and it is essential to use sieves made by a factory whose products are approved by a standards organisation.

Hand-held sieves should be used in a uniform manner if comparability of results is to be maintained. Firstly, the sieve should be held level in both hands directly in front of the body, with the elbows tucked in to the waist. Secondly, using a steady motion, the sieve should be moved approximately 25 cm to the left and back through the centre position, smoothly 25 cm to the right and returned to the centre position. This sieving operation should be repeated exactly 30 times, taking about 30 seconds to do so. No forwards and backwards or up and down movements are permitted, although a final gentle tap of the sieve will help to clear it of any material hanging from the perforations before the bottom pan is removed. When sieves with slotted screens are being used, it is important to ensure that the long sides of the slots are parallel with the movement of the sieve.

Sack sieves

The foreign matter content of grain is more accurately assessed when the contents of whole sacks are screened, although this is obviously more time consuming than the hand-sieving of small samples.

A sack sieve should possess two essential features a hopper for feeding grain gradually on to the screen and a screen and a screen that moves during the sieving operation. The slope of a moving screen should ensure that the grain is kept in motion towards the discharge end. Lateral movement of the screen, as in a rotary type of sieve, is more efficient in separating out foreign matter than the end-to-end movement of other kinds of mechanical sieve.

Some standard stove sizes for cereal grains

Locally produced varieties of grain may require sieves to have screens with specifications significantly different from those indicated in Table 20. Samples of grain should be checked against standard sieves of different specifications before quantities of sieves are purchased for grain quality assessment.

Care and maintenance

Sieves of standard quality are precision instruments that should be used and handled with care, and always kept clean and dry. A proper sieve brush should be used for removing dust and other material from a sieve after it has been used. Pieces of material stuck in the perforations should never be pushed through from the top surface of the screen. This can distort the holes and affect the accuracy of the sieve. Instead, the screen should be tweed upside down and tapped sharply or the material should be pushed out of blacked perforations with the finger tip.

Newly manufactured sieves are often coated with a thin film of oil or wax. This must be removed with warm water and detergent before the sieves are used.

If a sieve is not going to be used for some time, it should be thoroughly cleaned and coated with oil before storage to prevent possible deterioration. it must be cleaned before reuse.

Sieves are subject to wear and tear despite due care and attention and they should be checked periodically for accuracy, by comparing them with standard sieves. This is normally the responsibility of a national standards organisation or central laboratory in control of all grain quality matters.

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