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3.2.1 Ecology. Many species of insects are associated with stored produce. Of these, only some directly damage the produce. The produce can support an insect community including scavengers, predators and parasites as well as the primary pests. Each species will show different behaviour, tolerances and preferences with respect to:
For a particular locality, commodity and storage method there are usually only a few species which are important pests.
3.2.2 Identification. It is important to identify the main pest species in order to:
(a) assess whether the insects found are likely to cause serious damage and to merit control; and
(b) choose an appropriate control technique, since many treatments are selective in their action.
3.2.3 Biology. The effectiveness of control measures may be greatly increased by applying an elementary knowledge of the biology of pest species. For example, what is the likely source of infestation? Does the pest have a resistant phase? It is mobile enough to reinfest? What are its tolerances?
3.2.4. Reference collections. The majority of storage pests are small and difficult for non-specialists to identify. The accompanying notes are only an introduction.
Field-workers can identify insects most easily by comparing them with a prepared reference collection of the main pests to be found in their particular area.
The most common pests should be collected and sent for specialist identification. Larvae, caterpillars and pupae should be preserved in 70-percent ethyl alcohol. Adults should be submitted as found. Specimens should be labelled with the locality in which they were collected, the date and the produce concerned.
In the descriptions that follow the title of each pest in sections 3.3. 1-3, the lettered notes correspond to the following system:
(a) recognition; (b) commodities attacked; (c) pest status.
3.3.1 Major primary pests
Of cereals
1. Maize weevils/rice weevils: Sitophilus spp. (Col., Curculionidae).
(a) distinguishable from all other common storage pests by the long beak (or rostrum) characteristic of all the weevils, 2.5 to 4 mm long, dark brown, sometimes with four lighter soots on the wing cases
(b) maize, rice, sorghum, wheat;
(c) the most important primary pests of cereals in the humid tropics, attacks undamaged grain; often infests before harvest. The larvae develop within the grain, leaving a characteristic round hole on emergence.
2. Angoumois grain moth: Sitotroga cerealella (Lep., Gelechiidae).
(a) a small cream- or fawn-coloured moth sometimes with a small black spot on the forewing, the wings are very narrow and fringed with long bristles; the sharply pointed tip of the hindwing is characteristic;
(b) sorghum, maize, wheat, rice
(c) Sitotroga replaces Sitophilus as the main pest in the more arid areas. Damage may be very serious in maize stored on the cob damage is more limited with shelled grain, as the moths do not penetrate more than a few centimetres from the surface. The developing larvae cause all damage, as the adults do not
Figure 3.2 Sitotroga cerealella
3. Lesser grain borer: Rhizopertha dominica (Col., Bostrychidae).
(a) a small, almost cylindrical beetle, with the head "tucked" under the thorax so that it is invisible from above, the thorax has a prominent pattern of tubercles, as shown. Dinoderus spp. (also Bostrychidae) are also sometimes found in maize, they have a similar cylindrical shape but are markedly shorter and stouter;
(b) sorghum, maize and other cereals; cassava;
(c) a major primary nest in drier tropical regions. The Bostrychidae are adapted to boring into hard substances such as wood and are capable of attacking previously undamaged grain where they can cause serious damage. Bostrychidae may also sometimes be found attacking the timber of storage structures.
Figure 3.3 Rhizopertha dominica
4. Larger grain borer: Prostephanus truncatus (Col.,Bostrychidae).
(a) very similar but slightly larger than the lesser grain borer. Larvae and adults cause damage to various commodities;
(b) maize and other cereals, cassava and groundnuts
(c) this pest was accidentally introduced from South or Central America into Africa, where it causes heavy damage, especially to maize stored on the cob and dried cassava. Weight losses caused by this pest are 3 to 5 times higher than those caused by the normally occurring pest. Countries infested at present are the United Republic of Tanzania, Kenya and Togo. It is expected that this pest will spread to other countries.
Of grain legumes
Bruchids (including the bean weevil): Callosobruchus maculatus (Col., Bruchidae).
(a) rather stout beetles; active, with long legs and antennae; antennae not "clubbed"; wing areas often mottled, spotted or otherwise marked; last segment of abdomen just visible beyond wing cases. The various genera and species in this group are difficult for nonspecialists to identify;
(b) preferred commodity varies with species. Callosobruchus species feed on cowpeas, pigeon peas and green gram; Acanthoscelides on Phaseolus beans; Caryedon serratus on groundnuts (c) several species are major pests on their characteristic crops, especially Callosobruchus maculatus on cowpeas. Infestation often commences in the field before harvest; larvae develop hidden within the bean.
Figure 3 .5 Callosobruchus maculatus
3.3.2 Locally important pests
Pyralid moths: Ephestia spp., Plodia interpunctella, Corcyra cephalonica (Lep., Pyralidae).
(a) all have the general outline shown, with broader wings than Sitotroga and a shorter fringe of bristles. Ephestia spp. have dark forewings, sometimes indistinctly banded, and paler hindwings; Corcyra is uniformly dark grey-brown, Plodia has fobewings cream at the I>ase and red-brown on the outer half;
(b) various species on cereals milled cereal products, groundnuts, dried fruit;
(c) can be major primary pests, and important on flour and other products, larvae, which are free-living caterpillars, spin silk as they move (which is both a problem in itself and often the first visible sign of infestation).
Flour beetles: Tribolium, Gnatocerus, Palorus, etc. (Col., Tenebrionidae).
(a) elongated, reddish-brown beetles, active; Gnatocerus spp. are recognizable from the small, upward pointing horns on the head of the male;
(b) cereals (especially after damage by primary pests), groundnuts, milled cereal products;
(c) can attack intact grain via the embryo, but infestation is usually more serious on damaged or milled products, where they can be major nests.
Khapra beetle: Trogoderma granaria (Col., Dermestidae).
(a) dark brown, "marbled" with lighter bands; very finely hairy; 3 mm long; larvae with lone conspicuous bristles. Easily confused with other dermestids; if an infestation is suspected, identification by an expert should be sought;
(b) groundnuts, cereals, grain legumes;
(c) major pest in drier areas; important partly because larvae can enter a resistant resting phase (lasting up to several years); difficult to eradicate.
Figure 3.9 Trogoderma granaria
Merchant and saw-tooth grain beetles: Oryzaephilus spp. (Col., Silvanidae).
(a) active, dark-brown beetle, about 4 mm long, markedly elongated and flattened in shape; recognizable by the six prominent teeth on each edge of the thorax;
(b) cereals (especially rice), cereal products, oil-seeds;
(c) secondary pests; can be particularly important on milled products.
Figure 3.11 Lasioderma serricorne
Tobacco beetle: Lasioderma serricorne (Col., Anobiidae).
(a) reddish-brown, finely hairy, 2 to 4 mm long head curved under thorax
(b) tobacco and cocoa; secondary on cereals and grain legumes
(c) can be important on any of the above commodities.
Coffee-bean weevil: Araeocerus fasciculatus (Col., Anthribidae).
(a) gray-brown, usually slightly mottled; similar in shape to a bruchid, with the tip of the abdomen exposed; distinguished from bruchids by the loose, three-segmented club on the antennae;
(b) coffee, cocoa; secondary on cereals;
(c) not usually very damaging but its presence in export consignments can lead to their rejection.
Figure 3.12 Araeocerus fasciculatus
Dermestids (skin beetles): Dermestes spp. (Col., Dermestidae).
(a) larger (5- to 10-mm) beetles, usually black or black and white, larvae bristly as in Trogoderma;
(b) animal products;
(c) major pests of driedfish, dried meat, hides.
Copra beetle: Necrobia ruppes (Col., Cleridae).
(a) 4- to 7-mm, metallic green body, reddishash legs;
(b) copra, palm kernels, animal products;
(c) serious pest only on mouldy produce; can be common on dried fish, etc.
3.3.3 Widespread minor pests
Flat grain beetles: Cryptolestes spp. (Col., Cucujidae).
(a) very small (1 to 2 mm), flattened, red-brown in colour;
(b) cereals, cereal products, cowpeas, cocoa
(c) can become very abundant, especially in flour or damaged grain.
Sap-feeding beetles: Carpophilus spp. (Col., Nitidulidae).
(a) small, active beetles which may be brown or black in colour; sometimes with orange-brown patches on the wing cases distinguishable from other storage pests by the last two segments of the abdomen, which are not covered by the wing cases and are clearly visible from above. In the related Brachypeplus species, three abdominal segments are visible;
(b) damp grain, palm kernels, copra, cocoa;
(c) very widespread and sometimes abundant; common at harvest in cereals; only damaging on stored grain where produce is already damaged or mouldy.
3.3.4 Other arthropods
Ants (Hymenoptera, Formicidae) and termites (Isoptera).
Can be abundant in farm stores; usually act only as scavengers and so rarely need controlling; termites may cause severe damage to timber structures.
Ants can be controlled with insecticidal dusts applied on the (usually distinct) trails to the communal nests, timber may be protected from termites (and fungal rots) by regular application of old engine oil.
Parasitic wasps (Hymenoptera).
Very tiny insects (most less than 2 mm long) usually with four clear wings. They are beneficial, parasitizing the eggs and larvae of various moth and beetle pests. Can help to reduce pest increase in some situations.
Mites.
Mites belong to the class of Arachnida (subclass Acarina) and may be distinguished from insects by the possession of eight legs and an apparently unsegmented body. Those found on stored products are extremely small 0.2 to 1 mm in length, and are easily overlooked.
Some species are predators on the eggs of moths or on other mites, but many are serious pests of flour and other processed foods. The types are difficult to distinguish, but the pest species are smaller than the predatory ones (a hand lens is usually required to see them) whitish in colour and slow-moving. Their importance as pests of stored products in the tropics has not been properly investigated, but if found in very large numbers they should be regarded as pests.
Phosphine fumigation kills mites but other insecticides may be less effective. If there is a problem with mites it is important to choose a chemical (e.g. an acaricide) that specifically states that it is effective against them and approved/recommended for use on stored products.
Food-loss assessments provide the basis of programmes for reducing postharvest losses. Assessments may be made by surveys of both traditional methods and improved methods, and be followed by quantitative, technical and financial comparisons. Trials may be conducted to determine the acceptability of improved storage structures or methods of operating. A distinction should be made between loss surveys and field studies or trials; but both may seek to compare traditional and improved methods for reduction of losses.
Many of the terms used in loss assessment measurements are given in Section 1.1.
Three types of loss surveys can be identified.
4.3.1 General survey. A preliminary examination of specific problem points, and on-site appraisal of measures likely to reduce losses. This type of survey should be conducted before any loss-reduction project commences. As a result, the post-production system will be fully understood, the point at which significant losses may arise identified, and the causes of these losses suggested. (In addition, all relevant available data from other sources on losses should be collected and collated. If possible, the expected losses should be roughly estimated. )
4.3.2 Pilot survey (non-randomized survey). In this type of survey, a quantitative approach is based on established sampling techniques, but a completely scientific sampling design is not followed.
The sampling techniques adopted will determine the reliability of the survey results (see Section 2.5). A completely randomized survey is very costly, although difficulties arising from lack of cooperation by farmers and inaccessibility of sites are usually the decisive factors in choosing villages and farms for the sample. A statistically valid sampling of stored unshelled produce is rarely possible unless the storage container is emptied. This is often unacceptable to the farmer; it also disturbs the pest populations and stratifications that exist in the container. This storage container and its contents cannot then be used later for loss assessment measurements. Therefore, in pilot surveys on storage losses, random samples are usually taken from the produce which can be reached.
In spite of these limitations, pilot surveys on storage, even when they may not be statistically sound, can provide valuable data on the losses occurring and allow their progression over time to be monitored.
The aim of a pilot survey is to establish an estimate of the losses and provide data on their causes. Improvements may then be introduced, which need to be monitored and adjusted as more information becomes available.
4.3.3 Reliable survey (randomized survey). In this type of survey the objective is to obtain statistically reliable quantitative data on losses at village, regional or national level. A stratified random sampling programme and statistically acceptable method of sample analysis are essential. Such procedures are costly and require a large number of personnel specially trained for the job.
Such surveys are best suited to the processes of harvesting, threshing, drying and processing. They are less suitable for evaluating losses during storage because of problems arising from the processes of biological deterioration.
Field trials are frequently used to compare the losses that occur with traditional and improved practices.
Three types of trials may be conducted.
4.4.1 Equipment testing. Newly developed or purchased equipment must be tested for its suitability to harvest, thresh, dry or process the locally produced crops. Once a certain type of equipment has been found to be effective, it is essential to determine how it performs when used by farmers.
4.4.2 Storage simulation teals at research stations. Great caution must be exercised when interpreting results from this type of trial because the situation at research stations is very different from that on the farms.
4.4.3 Trials at farm level. These trials are conducted to meet two objectives. Alternative post-production practices can be evaluated for their effect on the level of losses. The trials are conducted with the cooperation of farmers in their own fields and villages or, if this proves impossible, at local research stations or experimental farms; these results, however, should be treated with due caution.
Potential improvements are introduced and evaluated when used by a group of farmers, without previously being fully tested by research stations. This type of trial is extremely useful in evaluating methods which have already proved successful in other regions or countries. During the trial, adjustments may be made to improve the performance of the equipment. Suitable practical training programmes are arranged for the farming community.
The result of surveys and evaluations is only valid for the conditions under which they were conducted. There is almost always a seasonal effect, which can only be determined over a number of years, on the level of losses.
A more practical solution to this problem is to conduct a survey to identify where in the system the losses are heavy and the magnitude of these losses. Improvements are introduced and are intensively monitored and evaluated. The improved and unimproved methods are compared, from the viewpoint of reducing losses, in the same season and same locality.
Such long-term surveys, although providing reliable information, are very costly and do not, of course, lead directly to a loss reduction.
4.6.1 Loss during harvesting, in-field drying, stacking, transport, threshing, drying and cleaning. During these activities, all losses caused by biological agents should be adjusted to a dry-weight basis. Other losses are usually expressed in terms of weight of material at 14-percent moisture content.
Both these methods of presentation require calculations that may lead to confusion in interpreting the results; but if the losses are expressed as percentages (rather than adjusted weights), and the basis of presentation is stated, the results can be compared with other loss-assessment figures.
Losses in harvesting, in-field drying and stacking operations are expressed as a percentage of yield. The yield is defined as the obtained yield, which is the maximum quantity of clean grain minus the losses being assessed. During threshing and cleaning, losses are expressed as a percentage of the grain input to the operation.
Farmers' practices in time of harvesting, duration of in-field drying, and stacking have a marked effect on the level of losses. Harvesting losses generally increase with any delay in the harvest beyond the time considered best by the farmer.
Losses during in-field drying, transport, sun-drying and cleaning tend to be low, while threshing losses depend very much on the methods used and the time of harvest.
4.6.2 Loss during storage at farm and village level due to insects and moulds. The loss in weight during storage must always be related to the quantity in store at the time of assessment. There are three methods of assessing losses during storage, and others are being developed.
In the standard volume/weight method, the dry weight of a standard volume of grain is measured by a standard method at the beginning of the storage period and is compared with the dry weight of the same volume of grain after a certain storage period. The principle underlying the method is that the main storage pests develop inside the grains. The shape of the grains will remain intact and the damaged grains will occupy the same volume but will weigh less. There are also, however, surface-feeding insects. Their feeding may affect the volume of the grains, so that more grains may fit in the same volume, leading to unreliable results.
The dry weight of a standard volume of grain depends on moisture content and variety. A standard base-line curve for the dry weight of a fixed volume of grain as moisture content changes must be prepared at the beginning of the storage period for all varieties for which losses are being assessed. This is a major problem of the method. The method has been used successfully without base-line data in very dry climates where moisture content changes are small during the storage period.
In the count and weight method, the damaged and undamaged grains in a sample of 100-1 000 grains are counted and weighed. The weight of the sample is compared with the weight it would have registered in the absence of damage. The basic equation is:
where
U = weight of undamaged fraction in sample
N = total number of grains in sample
Ua = average weight of one undamaged kernel
D = weight of damaged fraction in sample
The percentage weight loss must be adjusted to 14-percent mcwb, or the moisture content should be stated.
The percentage weight loss can also be calculated as:
Let Nu = number of undamaged grains
Nd = number of damaged grains
U + D = as above,then
This formula does not require the value of the mean weight of undamaged grain.
The main disadvantages of this method are that:
(a) insects may show a preference for grains of certain dimensions, composition or moisture content, and consequently the mean weight of insect-damaged grains before damage may be different from the mean weight of grains in the undamaged sample; and
(b) a visibly undamaged grain may have a hidden infestation which leads to an underestimate of the loss.
Because of these possible sources of error, negative weight-loss figures may be obtained at low infestation levels.
In the converted percentage damage method, the percentage damage is converted into weight loss by multiplying by a factor which is constant for a particular commodity. The method is of limited reliability and is only used when the other two methods cannot be used.
Before attempting to apply control measures it is essential to identify the pest concerned and to understand how it is a threat to the safe storage of the commodity.
Preventing infestation is always preferable to controlling infestation that has assumed serious proportions. The potential sources of infestation must be known so that the build-up of pests during storage can be controlled more easily and economically.
The type of storage structure influences the susceptibility of its produce to pest development. It also affects the method of control that will be most economical. A brief description of storage structures is given in Section 5.6.
Figure 5.1 Potential sources of infestation
5.2.1 Weight loss. As they develop on a commodity, insect pests feed continuously. Estimates of the resultant loss vary widely with the commodity, the locality and the storage practices involved. For grains or grain legumes in the tropics, stored under traditional conditions, a loss in the range of 10-30 percent might be expected over a full storage season.
5.2.2 Loss in quality/market value. Infested produce is contaminated with insect debris and has an increased dust content. Grains are holed and often discoloured. Food prepared from infested produce may have an unpleasant odour or taste.
Prices in traditional markets are relatively insensitive to pest-damage. Centralized marketing and distribution of produce usually depend, however, on a grading system which penalizes infested produce.
Export crops such as coffee, cocoa and groundnuts call for particularly high quality standards.
5.2.3 Promotion of mould development. Insects, moulds, and the grains themselves produce water in respiration, i.e. a breakdown of carbohydrate substrate. In humid conditions, without adequate ventilation, mould development and "caking" can spread rapidly, causing severe losses.
5.2.4 Reduced germination in seed material. Damage to the embryo of the seed will usually prevent germination; some storage pests prefer to attack the embryo.
5.2.5 Reduced nutritional value. Removal of the embryo by storage pests will tend to reduce the protein content of the grain.
5.3.1 Survival from last season. Insect pests are able to survive from one season to the next in a variety of situations, as follows:
(a) infested residues from the previous year, stored at home or on the farm;
(b) the structure of the store itself, in:
(c) natural habitats, such as:
Figure 5.2 Survival of insect pests
5.3.2 Infestation of fresh produce. Such produce can be infested by:
(a) active migration to the crop maturing in the field, from home, farm-store, warehouse or "bush"; or
(b) contamination when material is put into a store that is already infested.
When crops mature in the field they may be infested with storage pests:
(a) maize, sorghum and other cereals can be infested by maize and rice weevils (Sitophilus spp.);
(b) cowpea and other grain legumes can be infested by bean weevils (Bruchidae); the larvae of these pests, developing within the seeds, will be carried into the store with the produce and will continue to breed.
The severity of field damage can be considerably affected by the crop variety and cultural practices.
In the humid tropics, conditions in stores may be highly favourable to the development of many species of storage pests. At 27-30°C and 70- to 90-percent relative humidity, on appropriate substrates, potential rates of increase are very high; examples include 25 times increase per month for the rice weevil (Sitophilus oryzae), 50 times per month for the bean weevil (Callosobruchus maculatus) and 70 times per month for the red flour beetle (Tribolium castaneum).
Competition, predation and parasitism may reduce the number of insect pests developing.
Dry conditions can significantly slow down the rates of development.
In general, a pest problem can be expected throughout the season in the more humid zones, but in semi-arid savannah areas pest activity usually subsides during the dry season.
At every level of storage there will be a choice of different storage methods:
The suitability of a particular method will depend on a number of factors, including:
Following are summary descriptions of each type of structure, with emphasis on the techniques for pest control that are possible within each system.
The methods described should not be taken as recommendations; instead, they seek to suggest a way of evaluating the various options available in a particular situation.
5.6.1 Traditional cribs. These are characterized by lower levels of ventilation than in the improved designs; maize is often stored in the husk, sorghum and millet on the head.
The crop must remain longer in the field to dry sufficiently to avoid moulding. Losses in the field due to birds, rodents and "lodging" will be severe, especially in humid areas.
The crib itself may not be secure against rodents.
Husk (for maize) may provide considerable protection against insects for traditional varieties. If an insecticide is used its efficiency may be reduced by the presence of the husks, although there is conflicting evidence; insecticides used are usually dusts.
In a "closed" type of crib (e.g. basketwork), insecticide may be relatively persistent; but since such cribs have poor penetration there is no scope for reapplication.
The capital cost of the structure is low but its life may be short.
5.6.2 Improved cribs. These cribs are well ventilated, allowing harvest at high moisture content. (Early harvest also reduces field loss.)
Such a crib can protect against rodents.
Ventilation more or less eliminates the mould problem but there may be superficial germination in very wet conditions.
Husks must be removed, because of the high moisture content, exposing grain to insect attack; insecticide treatment with dust or spray is therefore necessary in most localities.
Insecticides admixed initially will tend to break down rapidly, but can be reapplied, at least to the outside of the crib. Penetration to the inside is thus improved.
The capital cost is low to moderate, depending on the materials chosen. The durability of the cribs will also depend on materials. The recurrent cost is the pesticide.
5.6.3 Silos. Silos (traditional and improved) are unventilated structures storing bulk grain.
Produce must be very dry initially; artificial drying is obligatory in humid zones.
Rodent damage can be eliminated.
Moulding is very likely if condensation occurs; heating and cooling each day promote migration of moisture and local caking which can spread rapidly.
Silos require frequent inspection to guard against caking and may require artificial ventilation (which is not feasible at the rural level) or emptying and redrying.
Insect control in silos is potentially good. A suitable structure can be fumigated initially and then sealed against reinfestation; admixed insecticides (i.e. dusts) should prove comparatively persistent.
Low moisture content results in slow insect development.
With good management, silos are effective; but with poor management there is a risk of rapid and total loss of crop.
Bulk-handling equipment is required to install the larger types of silos.
The capital costs are high, and sometimes very high, depending on the materials used. The recurrent cost of drying may also be high, and considerable labour is required to collect fuel at harvest time.
5.6.4 Warehouses (and bag-stores in general). These require some initial drying, but have higher tolerance than bulk storage.
Rodent control is possible and bagged produce can be fairly well protected against pests.
For valuable produce it may be justifiable to fumigate and prevent reinfestation. Spraying is far more effective than in ventilated structures; insecticides will be comparatively persistent.
Handling in warehouses does not necessarily require specialized equipment.
