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close this book Programming and Training for Small Farm Grain Storage
close this folder Part II. Grain storage project training
View the document H. Determining training needs and priorities
View the document I. The role of grain and grain storage in the world food supply
View the document J. The movement of grain from harvest to consumer
View the document K. The physical properties of grain
View the document L. Moisture and its measurement
View the document M. Factors which threaten good preservation of grain
View the document N. Grain drying
View the document O. Insects and their control
View the document P. Rodents and their control
View the document Q. Recognition of storage problems in the field
View the document R. Design and field testing of improved storage technologies
View the document S. Extension of improved storage technologies

O. Insects and their control

Major Subject Areas

- Sources of insect infestation

- Basic insect biology

- Identification of major insect pests

- Selection of insect control methods

- Non-chemical and traditional insect control methods

- Control with chemical insecticides

Training Objectives

- Volunteer will be able to demonstrate and discuss insect infestation sources to farmers and extension agents as support for the deinfestation of bins and equipment

- Volunteer will be able to identify on observation the major local insect species

- Volunteer will be familiar with the life stages of the insect, and be able to identify the adult stages or major local species

- Volunteer will be familiar with and able to identify and evaluate all local insect control methods which do not rely on modern insecticides

- Volunteer will be familiar with major locally available insecticides and know which are or are not suitable for grain

- Volunteer will be able to recommend to farmers and extension agents the use of chemical insecticides, including dosages and application methods

- Volunteer will know the safety precautions for the application of all locally available insecticides For use with grains as well as first aid procedures for the treatment of insecticide poisoning

Suggested Resources

- Peace Corps/VITA Small Farm Grain Storage Manual, Section 6, Part 1; Appendix C; Sect on 7

- FAO Grain Storage and Handling in Tropical and Subtropical Areas, Sections 4, 5, 8 , and Appendix C

- Degesch Principal Storage Pests, a color insect identification chart available from: Degesch D-6000 Frankfurt (Main) 1, Postfach 2644, Federal Republic of Germany

- Regulations for local insecticide use

- List of all locally available insecticides and their suggested application and dosage

- List of government and private insecticide outlets and prices

Suggested Field Exercises

- Visit selected local farmers to practice insect identification and infestation level determination techniques:

- remove samples from all accessible parts of store, (i.e., top, bottom, sides) examining samples to determine type of infestation and desirability and type of treatment required

- observe and discuss local insect control methods with farmers to determine farmer awareness and interest

- Visit area grain markets for similar insect identification and infestation level determination

- Practice the application of all local insect control methods

- Develop posters and simple instruction booklet for insect control methods extension

- Visit grain warehouse or dock storage to examine and practice bulk insect control methods

Insects represent one of the most prevalent and destructive causes of stored grain loss. In order to most effectively control them, a basic understanding of their physical characteristics and biology is required. Such information permits the correct use of insecticides as well as ocher more simple and less potentially dangerous insect control methods.

The choice of insecticides must reflect practical and economic considerations as well as an awareness of their Potential dangers to the environment and to both the people using them and those who will eat the grain. Insecticides are poisons, some more dangerous than others. They are commonly misused, especially in developing countries, sometimes with tragic results. Such tragedies are avoidable through education. Because insecticides are often so vital to the success of a grain storage effort, it is critical that Peace Corps Volunteers know their proper use.

The Source of Infestation

Grain may become infested in a number of ways. In many graingrowing regions, infestation ,tarts in the field before the crops are harvested. This is particularly true when the rice weevil and other insects are abundant in the field at harvest or where loosely constructed 'arm storage permits insects to move from stores to the fields or from unused grain bags back and forth to fields and nearby stores. Insects are generally more or a problem in tropical climates where they wait out the non-growing season in loose grain in the fields.

In addition to field infestation, there are several other important sources of infestation of stored grain. Grain is customarily stored in the same bins, sacks, or warehouses year after year. The cracks and crevices of wooden bins, For example, fill with dust and broken grain and afford places of concealment for insects. If these containers are never cleaned, insects later emerge in enormous numbers. Thus fresh grain quickly becomes infested.

Uninfected grain should not be placed for storage or shipment in sacks previously used for grain storage unless they have been insect-sterilized by heat or fumigation. Sacks may be heattreated by boiling; sunning is also partially effective. On a thatch, sod or clay roof, it may be relatively ineffective. On a hot metal roof, it may be 100% effective.

In any discussion of controlling insects in grain, we need to recognize that the insects The are dealing with exist all over the world in other food and foodstuffs, in food refuse and in nature unrelated to people's grains and foods. These existed in nature long before there were stored grains. Because insects exist outside of stored grain, sanitation on and the use of tight storage bins are extremely important factors in their control in grain stores. Unless each storage site is cleaned, the insects will be there waiting. Unless it is closed tightly, the insects will move in, and the grain will require repeated inspections and probably treatments.

Insect Biology

There are many control procedures available. Before selecting the best possible technique, an understanding of insects and the relation of controls to biological factors is essential. Insect control consists of a combination of many interrelated factors.

In the tropics beetles and moths are generally the most common insect pests causing losses and deterioration to stored food grains; in some areas termites and ants may also be a significant problem. Among the insects which live in stored food rains, a few begin their attack several weeks before harvest. There are other species which are unable to attack until the crop is almost dry or during the postharvest field drying period. As drying advances, certain of the insect pests are eliminated because there is no longer enough moisture available to support their needs. Insects do not breed successfully in an environment where the relative humidity is maintained at less than 40% or with temperatures below 10°C. As the temperature and humidity conditions diverge from the optimum, the time taken to develop from egg to adult increases, and the number of eggs laid become fewer. Some species tolerate the high humidity conditions with which fungi are associated, principally because they are mold feeders or require that the produce be decomposed by mold development in order to be suitable for them to eat. Most species do not tolerate prolonged temperatures above 42°C.

Insects have six legs. They have a hard outer skeleton or skin called the cuticle. The body is divided into three distinct regions: the head, the thorax, and the abdomen. In adult insects the head has mouth parts for biting and sucking, large compound eyes, and two antennae or feelers. The thorax carries the three pairs of legs and usually two pairs of wings. The abdomen contains part of the food canal and the reproductive organs. The adult insect lays eggs loosely in food, cements them onto food grains, or bores a small hole with its mouth parts in which an egg is securely laid. The egg develops and hatches a small worm-like larva, which is unlike the adult. The larva feeds, cut in order to grow, it must cast or shed its skin in a process called molting. This process of feeding and molting continues until the larva has reached its maximum seize. At this stage it stops feeding and may form an other shell or in the case of moths spin a shelter or cocoon in which it changes shape and becomes a pupa. The pupa, which may look like a folded-together adult insect, remains immobile and does not feed; it develops into tine fully formed adult insect which will push or bite its way out of the covering.

Since it is very difficult to see the eggs or the very young larvae of insect pests, and since they do not lave large holes in the grain, the farmer may assume that grains and kernels are uninfested. However, the presence or a few adult insects walking on or flying over stacks or bulks of produce usually indicates that there are many more insects inside one bulk of grain.

Under good conditions insects breed very quickly, the life cycle from egg to adult being completed in a few weeks and each female insect laying a large number of eggs. Under ideal temperatures and humidity (28°C and 65-80% relative humidity) and with adequate food, a pair of flour beetles is theoretically capable of increasing to millions in six months.

Identification of Major Insect Pests

With some careful practice, Volunteers can learn to identify the most common insect pests. Local entomologists or plant storage protection officers are likely sources of information and instruction. The Peace Corps/VITA Small Farm Grain Storage Manual has black and white pictures of common grain insect pests; and both the Degesch color insect chart and the FAO Handling and Storage of Food Grains in Tropical and Subtropical Areas may be helpful for Volunteers who wish more detailed information. For identification purposes it is best to place dark-colored insects on a light surface such as a piece of white paper.

Selection of Insect Control Methods

The selection of a control method is influenced by many factors:

- Temperature

- Moisture

- Type of storage

- Type of insect

- Length of storage

- Price of grain and insect control.


As already stated, insect pests of stored grain have certain temperature and moisture requirements which directly affect their proliferation as well as their ability to damage grain and resist chemical control. As a group, grain-damaging insects are mostly subtropical origin and do not hibernate. They have developed little resistance to low temperatures so that in cool areas they are rarely abundant enough to cause serious damage to stored grain. Temperatures there are not immediately lethal indirectly do cause the death of many insects by rendering them inactive and preventing them from feeding.

While each species has its own low temperature dormancy level, most of the grain insects slow down appreciably below 15°C. This also means that the uptake of fumigants or other protectants is slowed, and higher dosages are required. Moreover, the insects will move back into cracks and crevices and became less active.

By 10°C, while the insects are not technically dormant, activity is further depressed and mating and egg-laying usually stop. At 5°C true dormancy occurs. However, mites continue to be active down to 5°C if the moisture level is favorable.

Subject to certain upper limits, the rate of development and reproduction of all grain-infesting insects increases with rising temperatures. A grain temperature of 21°C is considered to be favorable for insects. At 21°C or higher, severe damage to stored grain from insects may be expected whereas below 16°C serious damage is not likely to occur. Temperatures above 35°C shorten the adult life span and are unfavorable for the reproduction of most graininfesting insects. Temperatures above 38°C cause the death of some insect pests, and temperatures of 60°C kill them all.


Grain moisture is an Important factor in the life of insect pests because they depend on their food supply for the moisture needed to carry on their life processes. Up to a certain point, increasing the grain moisture favors a rapid increase in the number of insects. Beyond that point, microorganisms take over and destroy them, except the fungus feeders. At the point that microorganisms the over, the affected grain is totally destroyed as well. If the moisture content of the grain is low, the water required for carrying on vital life e processes must be obtained by breaking down the food reserves in the fatty tissues of the insect's body.

Moisture requirements differ with different species of insects. Weevils are unable to reproduce in grain with a moisture content below 9%, and the adults soon die. Adult rice weevils normally survive for only one week in 3% moisture, wheat at 29°C; at 9% moisture, about 70% die by- three weeks and few live for more than seven.

Type of Storage

Storage types can be broken down to closed and open storage systems. Sealed gourds, metal bins, barrels, underground its, and some mud bins are examples of closed systems. Open systems are generally more varied including thatch baskets, open piles, bunches hanging from trees, and piles above cooking fires. Note that closed systems are not necessarily hermetic.

Closed systems keep insects already infesting the grain from escaping and keep insects outside the storage container from entering. Open systems permit the movement of insects in and out of the stored grain. Open systems are most common in areas of high humidity where grain does not dry naturally to levels low enough to permit closed system storage without danger of mold growth and rot.

Closed systems permit the use of both fumigants and contact insecticides while open systems can only be treated with contact insecticides unless they are enclosed with some type of gas-tight envelope such as a plastic tarp, which permits fumigation. Fumigation of open storage systems without the use of some form of gas-tight envelope is not only ineffective in killing insects in the grain but also can be lethal to people and animals.

Insect Type

The type of insect pest infesting a bulk of grain will sometimes determine the type of control method to be used. In general, fumigation and airtight storage is effective against all types of grain-infesting insects. The Angoumois grain moth, because it can only penetrate some 10 cm into the grain mass, can be controlled by a surface spraying or dusting with a contact insecticide. The insecticide information sheets in the Peace Corps,/VITA Small Farm Grain Storage Manual gives details of the insects controlled by some of the most common insecticides.

Length or Storage

Grain which farmers intend to store for only a few weeks or even up to three or four months may not require any form of insect control other than the standard measures of cleaning the grain containers and good sanitation measures around the storage sites. Local farmers often know how long grain can e stored without insecticides before major infestation build-up. This is often the time when farmers decide to sell their grain, passing the impending infestation problems on to someone else. Ideally, to reduce insect-caused grain losses throughout the entire pipeline, insect control measures should be applied to ail stored grain when it is first put into storage. However, this is not always in the economic interest of the farmer. If grain is to be sotored by the farmer for three months or more, insect control measures such as the use of insecticides or airtight storage are generally required from the time the grain goes into storage. Grain is treated far more effectively when it is first put into storage than after the infestation level has built up during the first few months.

Price of Grain and Insect Control

The economics of insect control are difficult to accurately calculate due to the difficulty of projecting the estimated grain and financial tosses due to expected insect infestation. On the other hand, the cost of one or repeated treatments per ton of grain, including labor expenses, can be readily calculated. This cost may then be compared to the equivalent quantity of grain of the same value. Farmers will often find this to be the most persuasive argument in favor of insect control, as the cost of insecticide per ton may be very favorable. It is certainly worth comparing the cost of various alternative insecticides ides to insure that farmers are using the most economical chemical, keeping in mind the relative effectiveness of each chemical.

Insect Control Measures

Except for the mixing of clean grain with infested grain or the presence of nearby materials literally crawling with insects, infestations usually start at a comparatively low level. Even under favorable temperature and moisture conditions, a second generation of insects takes about a month to become active and start multiplying. The essence of preventing insect-caused losses to grains is to prevent the infestation from the start. Note, however, that complete exclusion is almost impossible in most countries today.

Good Housekeeping

Sanitation on the farm in dryers and silos will do much to prevent infestation and to reduce losses. It is essential that clean, insect-free, and weatherproof storage be provided from the very beginning and that nearby sources of infestation be eliminated.

Concrete bins and metal bins with tight seams are readily cleaned. Wooden bins or loose metal sheathing around sacked grain need to be cleaned as thoroughly as possible and then sprayed with an appropriate insecticide. Refer co one Peace Corps/VITA Small Farm Grain Storage Manual for information on insecticides for the treatment of storage sites and bulk grain. When cleaning and before spraying or dusting! remove all waste grain and feedstuffs. Be sure to clean out any machinery no matter how simple or complicated. Remove all residue from the premises, use it for feed or burn or fumigate it.

Dockage, which includes chaff, broken stems, husks, grain dust, and other impurities, greatly encourages the development of insects in stored grain. Dockage also tends to absorb and retain moisture and prevent aeration. Thus, it must be screened or sifted out before storage.

Making the Storage Container Weathertight

Any holes in the roof should be repaired to prevent rain from entering. Windows, vents, and evacuation shoots should be provided with means for closing during periods of rain or designed to prevent rain from entering. Doors should likewise be protected. Good drainage should be provided around the exterior of the building, away from the storage container, particularly at the joint between the walls and the floor to prevent seepage from beneath the floor.

Areas surrounding storage facilities should be maintained free of accumulated debris, grain and grain product residues, and equipment. This should be a part of the routine housekeeping program. Furthermore, at least one meter around the entire storage facility should be kept free of vegetation. Such measures will also help control rat infestation.

Bags should preferably be stored up off the floor on wooden supports or be hung from the ceiling or roof. They too need to be cleaned and treated with insecticide before use. The Peace Corps /VITA Small Farm Grain Storage e Manual gives details on the construction or sack storage pallets. Sanitation in a warehouse means stacking sacks 60 cm away from the walls so that inspections and cleanings can go around the entire bulk of grain and so that fumigation tarps ran be properly sealed. Sanitation in warehouse storage means cleaning floors whenever a lot is moved and before another is stored in its place. It means a constant cleaning and policing of the building and of the surrounding area so that the problems due to insects from nearby residues and debris will be kept to a minimum.

Insecticide Treatment of Storage Facilities and Grounds

Insecticide treatment of storage facilities and grounds will be of little help in insect control unless the areas are first cleaned so that the chemicals can reach the structures and grounds themselves. Walls and other structures covered with dust and debris can be treated with pesticide chemicals only by applying them in excessive amounts.

Insecticides can be brought into contact with insects in two ways. First, insecticide sprays or dusts may be mixed with grain or applied to room areas and surfaces such as walls, floors, etc. The insects walk over the surface and thus contact the insecticide. This type of application is commonly referred to as residual spraying or dusting, especially if the specific contact insecticide used remains effective for a long period of time. Secondly, an insecticide may be released into the atmosphere as a gas. This method is referred to as fumigation.

Many insecticides are available though relatively few are safe for use on stored grain and it is important that the appropriate ones be used. This is true not only from the standpoint of effectiveness in controlling the insects, but also from the standpoint of human safety. All pesticide chemicals are potentially dangerous, especially those related to food product storage. Therefore, the least hazardous chemical should be chosen.

Residual insecticides are commonly available in two forms: liquids and dusts. The dusts may be in the form of powders, either directly applied to surfaces or mixed with water and sprayed on. The liquids are usually diluted with water or highly refined oils for application. They are generally applied by either hand-operated pressure sprayers or motor-drive pump units. For vertical surfaces they should be diluted to the point when the lieu d would begin to run down the vertical surface. On horizontal surfaces the spray should not from puddles or pools.

In treatment of storage facilities such as bins and warehouses and the surrounding areas, the following materials and procedures are recommended.

Pesticide Chemicals

1) 1/2 ricer 57% malathion emulsifiable concentrate in 16 liters of water, or

2) 1 liter 25% methoxychlor emulsifiable concentrate in 8 liters of water, or

3) 1-1/3 liters of 6% pyrethrin with 60% piperonyl butoxide in 16 liters of water.


1) Spray 8 liters of diluted insecticide per 100 square meters of surface area.

2) Spray the inside of cleaned walls of building to at least a height of 2 to 2-1/2 meters, or higher if easily reached with the sprayer. This is especially important if walls are rough-textured or have numerous cracks or joints in them.

3) Spray cleaned floors of storage areas giving special attention to the areas along wall-floor junctures and cracks of joints in the floors which may harbor insects.

4) Spraying on the exterior of the building should include:

a) the grounds, to a distance of about 2 meters from the building.

b) the pillars (or supports), if the building is raised off the ground, and an area of a few feet around the underside or the floor at the support.

c) the entire underside of raised wooden or concrete (if in poor repair) floors.

Frequency of Treatment

1) Metal bins with caulked seams can be cleaned so as to leave almost no food or insect residues. Spraying with malathion or methoxyclor about two weeks to prior placing new grain crop in the bins should be sufficient treatment for the structure. From then on, watch the grain.

2) Wood, mud, or thatch bin cleaning is more difficult and is often less complete. Spraying of the bin with malathion or methoxychlor should be accomplished each time before the bin is filled with grain.

3) Warehouses should have walls, floors, and overhead areas thoroughly sprayed once each year prior to receiving the new crop. In addition, whenever stacks of grain are fumigated, the floor and mall areas surrounding the stacks should be sorayed immediately prior to fumigation. When the fumigation tarp is removed, insects harbored in and on the walls and floor will otherwise cause reinfestation problems. Whenever a storage bag or area is emoptied, the area should be thoroughly cleaned and sprayed using an appropriate insecticide be ore an,, other grain is stored in the area.

Insecticide Treatment of Bags (Sacks)

A major problem in many regions is the handling of grain, feeds, and cereals in used bags. All woven bags, whether burlap Or polypropylene, will harbor insects. Plastic bags will be less of a problem than burlap, but they too hold grain debris between and among the fibers, in the weave, and at the seams, where insects live and feed.

It is impossible to clean burlap or polypropylene bags. When they are employed for grain, they should be treated with insecticides to kill the grain insects harbored in them. While it is true that grain insects can be killed by exposure to 60°C for ten minutes, real care is required to attain and maintain this temperature by the usual sun-heating procedures. In many rural areas a fairly good job is done by spreading used bags as well as grain in the sun on roofs, parched land, or patios, but usually not all insects are destroyed. In addition, some of them are merely driven into the soil or into the buildings under the roofs and remain in the area as a potential reinfestation problem. Consequently, the only completely reliable treatment for woven bags is by means of pesticide treatment or boiling the bags in water. Boiling has the possible disadvantage of weakening certain types of burlap or plastic weave bags. A few sacks should be pretested to determine any negative effects of the boiling before treating large lots of used bags.

Non-Chemical and Traditional Insect Control Methods

There are a variety of traditional methods to control insects in stored grain, including sunning, tine mixture of certain crushed or ground plants wit. the grain the mixture of sand or wood ash, smoking grain over cooking fires, storage in airtight containers, and the storage of unthreshed grain with the insect protectant husk left on. The Tropical Stored Products Centre in Britain has done extensive investigations into the use or such methods and is an excellent resource. Specific subject requests for information should be addressed to:

Tropical Stored Products Centre

Tropical Products Institute

London Road

Slough, Berks SL3 7HL, England

The Peace Corps/VI-- Small Farm Grain Storage Manual gives more detail in the use of the above traditional storage methods in Section 6, Part 1. Additional details may be found in Section. 8 of the FAO Handling and Storage of Food Grains in Tropical and Subtropical Areas

The function or various traditional insect control methods is not always clearly understood due to the unknown properties of the wide variety or local plants which may be mixed with stored grain. There is also a distinction between methods which actually kill insects and those which inhibit infestation by driving insects out of the grain or making it difficult for insects to attack the grain. Methods which actually kill the insects are generally preferable although infestation inhibitors should not be overlooked or discounted.

Sunning grain kills some insects and drives others out of it. A 9% moisture content or less in grain (i.e., in very well-dried grain) will kill most insects or cause dormancy due to lack of adequate moisture for reproduction.

The mixture of certain local plants in ground, crushed or powdered form functions as an insecticide and/or an insect inhibitor. Others, though traditionally used, may have little appreciable effect. Research needs to be done in this area as it is conceivable that effective insecticides can be derived from these traditionally used plants. (It is suggested that the Peace Corps Volunteer actually try out the method in some grain kept under his/her control before recommending 'or general use.)

The mixture of sand or wood ash with grain acts as both an insecticide and an insect inhibitor. The sharp edges of the sand or wood ash scratches the waxy coating of the insect's body causing it to lose moisture and dehydrate. This method is only effective as an insecticide when used with dry grain at 9-10% moisture content or less. If used in more moist grain, the sand and wood ash function as inhibitors by filling the inter-granular spaces in the grain bulk, and slowing the movement of insects from one grain to another.

The smoking of grain stored above cooking fires is effective through the continual drying and heating of the grain, which are both insecticidal and insect-inhibiting. The smoke itself may be insecticidal in very heavy concentrations though more likely it is simply an insect inhibitor.

There are many forms of airtight containers, though they are traditionally rather small and often restricted to seed storage. Gourds- may be sealed with tar or resin. In some regions -he use of oil drums and kerosene tins is becoming increasingly more common as a form of airtight storage. Some underground pits may be airtight as well. Airtight containers are insecticidal, killing off all insects through gradual asphyxiation. The build-up of carbon dioxide and the depletion of oxygen is a gradual process which will depend on the number of insects in the grain, the grain moisture content and its respiration level, and the volume of the container not filled with grain. Generally, the asphyxiation of insects will occur after about four months.

The storage of unthreshed grain is common for both maize and rice, where the natural protection of the husk offers insect inhibiting advantages. Neither maize or rice husks completely shut out insect infestations although they are significantly more effective than storing husked grain without insecticide. The development of new maize varieties which have looser husks has brought about new storage difficulties as insects find much easier access to the grain kernels through the more penetrable husks.

Insect Control with Chemical Insecticides

There are many chemical insecticides which effectively kill insects. However, there are relatively few which are safe for use on stored grain or in association with any food product. As discussed earlier there are contact insecticides which may be in the form of liquids or dusts diluted before application, dusts applied directly, and fumigant gases. The number of insecticides which are locally available may-be large and varied though those which are suitable for grain storage will be much more limited. Insecticides not intended for use on food products are commonly missed in grain storage. Volunteers may and that this is a major local problem requiring immediate attention. The insecticide information sheets in Section 6 or the Peace Corps/ VITA Small Farm Grain Storage Manual gives application dosages, trade names and insects controlled by seven of the most common insecticides recommended for use in stored grain, in grain storage equipment, and in storage buildings and surrounding areas. Details are provided for a much wider variety of insecticides in Appendix C of the same manual. When in doubt as to the recommended dosage, application method, or safety of any insecticide, contact the local government office in charge of plant protection or grain storage. Do not rely solely on the judgment of private insecticide outlet salespersons as they may be misinformed or inclined to provide information which would increase their insecticide sales. Insecticides should always be clearly labeled witch dosage and application as well as their composition. Insecticide containers should never be reused for any the of food storage or preparation or water vessel. Insecticide dosages should e carefully respected, keeping in mind that increasing the dosage may not only be costly, but also dangerous.

Instructions for dusting, spraying, and admixing insecticides and for grain fumigation are included in the Peace Corps/VITA Small Farm Grain Storage Manual. Precautions for the treatment of poisin victims should he included in instructions to extension workers, farmers, and Volunteers.