Management of Selected Crop Pests in Tanzania. Sokoine University of Agriculture Morogoro, Tanzania

Transcription

1 Management of Selected Crop Pests in Tanzania Sokoine University of Agriculture Morogoro, Tanzania Prof. Rhodes H. Makundi - Editor Pest Management Centre Tanzania Publishing House (TPH), Dar es Salaam, Tanzania ISBN

2 CHAPTER 10 Storage and Protection of Durable Food Crops and Their Products in Tanzania R.H. Makundi, A.W. Massawe and H.S. Laswai Introduction Tanzania is mainly agricultural, with the majority of the population of more than 30 million people living in rural areas, where most of the cereals and food legumes are produced. Cereals and food legumes are the most widely cultivated crops for human food and animal feed. As food grains, they are the main staple for the majority of the population. Cereals and food legumes constitute the major components of the daily diet. The major cereals are maize (Zea mays L.), sorghum (Sorghum bicolor (L), rice (Oryza sativa L.), wheat (Triticum sp. L.) and millets. Food legumes are an important source of protein for the majority of the population. Among the widely cultivated pulses are: Beans (Phaseolus vulgaris L.), broad beans (Vicia faba), chickpeas (Cicer arientinum L.), cow peas (Vigna unguiculata L. and V. sinensis L), pigeon peas (Cajanus cajan L.). Groundnuts (Arachis hypogaea L.), peas (Pisum sativa L.) and bambara groundnuts (Voandzeia subterranea) are also widely cultivated. In some regions, roots and tubers, particularly cassava, sweet potatoes and round potatoes are also cultivated and are some of the staple food crops. The production of cereals and legumes also involves local processing, mainly at home, with the excess produce being traded. However a large proportion of the produce is retained at home, mainly in traditional stores, until the next harvest. Invariably, the storage period can range from a few months up to one year. Although certain levels of losses occur at different stages in the production of the commodity, deterioration during storage can be dramatic particularly in warm and humid conditions in Tanzania. Climate plays an important role in determining the amount of losses particularly because it influences biological activity within the commodity, breeding and multiplication of pests. Proper storage at village level aims at reducing food losses with subsequent increases in the quantity and quality of food. To achieve this objective requires that we minimize the effect of the biological and physical agents which cause deterioration of the stored commodities. 185

3 A considerable quantity of stored produce is lost through attacks of insect pests. These flourish and reproduce rapidly because of the favourable climate, particularly the warm and humid conditions in most parts of Tanzania. The crops are often infested in the field before they are harvested and this source of future infestation is carried into the store. This necessitates carrying out control measures. However, some of the traditional storage methods generally make it very difficult to control insect pests, particularly the larger grain borer, Prostephanus truncatus (Horn). Several factors increase the risks of infestation and losses in traditional storage structures in Tanzania. They include: - Extreme variations in temperature, particularly within the stores - Structures which easily permit easy entry of insects and other pests - Easy re-infestation after treatment - Unimproved structures which do not allow enough ventilation and which are difficult to carry out proper inspection and pest control operations. At the village level, safe storage of crops is largely dependent on the kind of store. The aim of this chapter is to discuss the storage practices and protection of durable foodstuffs, particularly cereals and pulses. Storage of commodities at farm level in Tanzania Grain storage is carried out for varying duration and for several purposes. These include - future supply of food at family level - for sale later on when prices are favourable - reserves for food security at national level - keeping the grain safe before processing or while on transit The duration of storage, therefore, will depend on the purpose of storage, and to some extent may also determine the type of storage structure used. Due to the large area of the country, climatic differences between regions obviously occur. Tanzania, therefore, has several agro-ecological zones, which influence the farming systems (for details see Rwamugira, 1991, from whom the following description was adopted). The agro ecological conditions determine the types of crops stored in an area, the kind of storage structures and to some extent the distribution and occurrence of pest species. Along the coastal belt (Tanga, Coast, Dar es Salaam, Lindi, Mtwara) the altitude is low; these areas are humid and warm for most times of the year. In this zone, infertile sandy soils are not suitable for extensive cultivation of cereal crops. However, cassava growing is predominant. The central area of the country (Maasai steppe, Shinyanga, part of Morogoro, Dodoma, Singida) is either arid or semi-arid and drought tolerant crops are cultivated. In the highlands (northern highlands - Mts Meru and Kilimanjaro; southern highlands- covering part of Iringa and Mbeya; southwestern highlands- Ufipa and Sumbawanga; western highlands - Kigoma and Kagera; and the isolated 186

4 eastern-arc mountains - Uluguru, Udzungwa, Usambara and Pare Mountains) the conditions are characterized by very wet rainy seasons and mild temperatures. In the lake zone (Kagera, Mwanza, Mara) a markedly warm and humid conditions are experienced. These variations in climate have a strong influence on the storage practices and structures, the incidence of pests, the duration of storage, and the kind of crops cultivated and stored. Therefore, before storage in some areas, the grain must be properly dried before it is introduced to the store for long duration storage. For example, in the central areas of Tanzania (Dodoma, Singida, Tabora) adequate drying of the crop takes place and it may not require further drying before storage. However, in some areas like the lake regions, the coastal zone, and some of the southern and northern highlands further drying after harvest is necessary to prevent fungal infestation. Causes of losses Insect pests Insects are the most important causative agents of grain losses in storage in Tanzania. Although more than 100 species of insects are responsible for damage of stored commodities in the tropics, only about 20 species are major pests (De Lima 1987). In Tanzania, the majority of species, which are stored product pests belong to two orders, the Lepidoptera (moths) and the Coleoptera (beetles). Table 1 summarizes the most common species of storage pests in Tanzania. These pests can also be grouped according to the principal commodities, which they attack as shown in Table 2. Table 1: Major insect pests of stored food in the tropics (after De Lima, 1987) Common name Scientific name Products attacked Rice weevil Sitophilus oryzae Rice, maize, paddy, wheat, sorghum Maize weevil Sitophilus zeamais Maize, wheat, sorghum. Granary weevil Sitophilus granarius Wheat Lesser grain borer Rhizopertha dominica Wheat, maize, paddy, sorghum Larger grain borer Prostephanus truncatus Maize, dried cassava Khapra beetle Trogoderma granarium Wheat, maize, sorghum, rice, pulses, oil seeds and oil seed cake. Red flour beetle Tribolium castaneum Wheat, wheat flour, maize flour, cocoa, ground nuts, animal feed Confused flour beetle Tribolium confusum Wheat, wheat flour, milled Cereal products, dried fruits. 187

5 Saw toothed grain Oryzaephilus surinamensis Cereals, cereal products, beetles dried fruits, oilseed, oil seed cake, copra Saw toothed grain beetles Oryzaephilus mercator beetles Cereals, cereal products, dried fruits, oilseed, oil seed cake, copra Pulse beetles Callossobruchus spp.} Acanthoscelides obtectus} Zabrotes subfasciatus} Caryedon serratus} Cowpeas, grams, beans, mung beans, groundnuts Flat grain beetles Cryptolestes spp.} Rice, wheat, flour, Laemophloeus pusilus} groundnuts, milled cereal products, maize Tobacco beetle Lasioderma serrocorne Cocoa, cassava, dried tobacco leaves, and their products Copra beetle Necrobia rufipes Copra, fish meal Hide beetle Dermestes spp. Dried fish, animal feed Coffee bean weevil Araecerus fasciculatus Coffee, cocoa, spices Indian meal moth Plodia interpunctella Rice, maize, groundnuts, dried fruits Mediterranean Ephestia küehniella Cereals, wheat flour, nuts Flour moth Tropical warehouse Ephestia cautella moth Maize, wheat, rice, groundnuts, sorghum, cocoa Rice moth Cocyra cephalonica Rice, maize, wheat, sorghum, groundnuts Angoumois grain Sitotroga cerealella Maize, wheat, paddy, moth sorghum 188

6 Table 2: Classification of insect pests according to the stored commodities attacked Commodity Cereal grains Pulses Flour and damaged cereal grains or their products Dried fruit Animal hides and dried fish Pest species Sitophilus spp., Prostephanus truncatus Rhizopertha dominica Cryptolestes spp Oryzaephilus spp. Trogoderma granarius Ephestia cautella Sitotroga cerealella Plodia interpunctella Acanthocellides obtectus Callosobruchus maculatus Zabrotes subfasciatus Caryedon serratus Tribolium spp Tenebroides mauritanius Ephestia kühniella Oryzaephilus spp Plodia interpunctella Ephestia cautella Cocyra cephalonica Dermestes spp Necrobia rufipes Characteristics of the major storage insect pests Some of these descriptions are adopted from World Food Programme (WFP) (1970) Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) This is a primary pest which feeds and breeds on sound kernels of maize and dried cassava roots. The beetle is native to Tropical Central America where it is not a serious pest. In sub Saharan Africa including Tanzania, it became introduced in the early 1980's and is currently the most serious threat to farm stored maize and cassava (Makundi 1987). Life history: The female lays eggs inside the grains or in the dust produced by adult's tunneling into the grain or cassava. The larvae feed internally or in the dust produced by the adults. Development from egg to adult takes about 35 days in maize under optimum conditions (32 0 C and 80% relative humidity) (Shires 1980). Many environmental factors influence the development and rate of increase of P. truncatus and these have influence on the level of damage caused by this pest. 189

7 Among them are temperature, the relative humidity, type of maize cultivar, the moisture content of the grain and whether the grain is on the cob or is shelled. In surveys carried out in Tanzania maize with moisture content of 10.6% and even 9% was infested by P. truncatus (Golob and Hodges 1982). This renders the drying of maize before storage a less effective control measure against P. truncatus as compared with the other stored product pests. It also implies that P. truncatus is able to get established in the more arid areas of the country. Rhizopertha dominica (the lesser grain borer) (Coleoptera: Bostrichidae) Rhizoperta dominica is a primary pest of sound cereal grains, but feeds also on similar starchy products. Wheat is the most favourable cereal for multiplication of R. dominica, but it also flourishes on rice and causes heavy losses after long periods of infestation. Other small grains like sorghum are usually highly infested by the lesser grain borer. Life cycle: The duration of the life cycle of R. dominica is influenced by several factors including temperature, relative humidity, interspecific competition and the moisture content of the commodity. The nature of the commodity also influences the life cycle. Under optimum conditions (28 0 C and 70% R.H.) the duration of the life cycle from egg to adult on whole meal wheat flour takes about 40 days (Howe 1950). On sorghum, the female produced up to 326 eggs (Makundi 1996). Females of R. dominica also showed great variations in the number of eggs produced on different cultivars of sorghum, ranging from 148 eggs to 384 eggs (Makundi and Wilkins 1997). Adult longevity also varies. On sorghum variety Serena, the female beetles had a mean longevity of 144 days (Makundi 1996, Makundi and Wilkins 1997). Sitophilus zeamais, S. oryzae, S. granarius (Coleoptera: Curculionidae) The three species of weevils are second to the larger grain borer as the most destructive pests of stored cereals in Tanzania. They are primary pests and are morphologically very similar. The head of the adult is prolonged into a well defined rostrum or "snout' and therefore are easily recognized and distinguished from the other groups of stored product pests. The three common species may be distinguished as follows (WFP 1970): S. granarius: The adult has a uniform chest-brown colour; hind wings are absent and the prothorax has punctures distinctly oblong or oval. Adults are mm long. S. oryzae: The elytra have four reddish spots. The prothorax is very densely set with round or irregular punctures. The hind wings are present. Adults are mm long. S. zeamais: This has similar characteristics as for S. oryzae. Adults are mm long. Life history: Under optimum conditions, S. granarius produces about 200 eggs. Before oviposition the female bores a small hole in the food grain with her 190

8 mouthparts, deposits the egg in it and then seals the hole with a drop of gum like secretion. The larvae complete development inside the grain. Several larvae can develop successfully in large seeds like maize. S. granarius is wingless and therefore it does not attack crops in the field. Sitophilus zeamais has preference for feeding and laying eggs on maize, rather than wheat and rice. S. oryzae has preference for feeding on rice and wheat rather than maize. Humidity rather than temperature is more important in egg laying for both species. The number of eggs laid at 70% R.H. is greater than at 50% R.H. Females of the two species lay about eggs in a lifetime. Tribolium confusum (confused flour beetle) and T. castaneum (rust red flour beetle) (Coleoptera: Tenebrionidae). These are the commonest species in Tanzania. They are prevalent in cereal products and grains, which have been damaged by primary pests. They also infest oil seeds and animal feeds. Life history: The eggs are laid singly over a period of many months and can reach 450 eggs by a single female. The larvae grow to reach 5 mm in length and later pupate in the food. The adults are long lived. T. confusum adults live up to 600 days. Cryptolestes spp.(flat grain beetles) (Coleoptera: Cucujidae) These are small-flattened beetles. Many species live under the bark of trees, but a few species are grain feeders and have become pests of stored products. Cryptolestes ferrugineus (the rust red grain beetle) are minute insects (2-3 mm long). The proliferation of the species is encouraged by local damp spots in the grain which enable them to spread and spoil bulk foodstuffs. Life history: The eggs are laid in cracks in the grain or are dropped loose in the grain. Under optimum conditions, the development period from egg to adult is 23 days. Adults live for 6-9 months. Oryzaephilus spp (the saw toothed grain beetle) Two species, O. surinamensis and O. mercator are main pests of stored commodities. Both species are very small, active brown insects. They attack cereals and cereal products and are serious pests of packaged foods. O. surinamensis causes problems in farm stored grains. Life history: The eggs are laid in crevices or cracks in the grain. Up to 400 eggs are laid by the female under warm conditions. The larvae are yellowish with flacks of brown and a brown head. Pupation occurs in a little cell constructed of bits of food and debris. The adults are long-lived (6-10 months). Trogoderma granarium (the khapra beetle) (Coleoptera: Dermestidae) 191

9 T. granarium is a little oval, dark brown beetle with a small head. Adults are mm in length and are densely covered with hairs. They are most serious pests of grains stored in hot dry conditions. Life history: The eggs are laid singly either scattered among grain or sometimes in the groove of the grain. Females lay between 50 and 80 eggs. The larvae are hairy. The development from egg to adult takes 25 days under optimum conditions. Under unfavourable conditions, the mature larvae enter a resting stage (diapause) in crevices in the store. They may remain hidden for periods of up to four years. Development is resumed when the food supplies become available and the temperature is favourable. In the resting stage the larvae are extremely difficult to control by fumigants or contact insecticides. Dermestes spp. (hide beetles) (Coleoptera: Dermestidae) Dermestes maculatus: They attack hides, skins and dried fish. Life history: The eggs are laid in the produce and the larvae feed freely on it. Under optimum conditions the development from egg to adult takes about 30 days. Adults are long lived. Lasioderma serricorne (the cigarette beetle) The adults of this species are small, oval or mainly globular, mm long. Life history: Female lay about a 100 eggs. The eggs hatch after 5-6 days. The developmental period is strongly influenced by the food. The adults live for 2-4 days and do not feed. The pulse beetles (Bruchids)(Coleoptera: Bruchidae) The bruchids breed in leguminous seeds, which may be attacked in the pod in the field. Others will feed on dried stored seeds. The most common species are Acanthoscelides obtectus (the American seed beetle), Callosobruchus maculatus and C. chinensis (the cowpea beetles) and Zabrotes subfasciatus. Life history: The eggs are laid among the dried beans. The first stage larvae are active with well-developed legs. They bore into the beans and eat them from the inside. The second and third stage larvae grow to fat and sessile grubs. They pupate in small oval cells excavated immediately under the seed coat from where the adults emerge. The adults do not feed and they have a short longevity of about a week at 35 0 C and 3-4 weeks at 18 0 C. Lepidoptera: Moths Ephestia cautella (Pyrallidae) (the Tropical warehouse moth) This moth attacks almost any stored food commodity. Life history: The female lays up to 300 eggs on the food commodity during a life span of about 14 days. The larvae feed and move freely in the produce causing contamination with webbing and frass. They enter a wandering phase and produce 192

10 a fine thread used to spin a cocoon. Large numbers of larvae occur in stacks and these may be covered completely by dense webbing over the surface of bags. Under optimum conditions (28 0 C and 70% R.H.), the development from egg to adult takes about 25 days. Sitotroga cerealella (Gelechiidae) (the Angoumois grain moths) The adults of this moth are small pale yellowish in colour. The forewings have one or two black dots. The hind wing has obvious fringe of long hairs. Life history: The eggs are laid soon after harvesting on maize, rice sorghum and wheat. The larvae burrow into the grains, develop and pupate inside. The adults do not feed. Cocyra cephalonica (Pyralidae) (the rice moth) The adults of C. cephalonica are similar in size or slightly larger than E. cautella. The forewings are uniformly coloured pale buff-brown without spots, but with the veins slightly darkened. Life history: Adults lay eggs and the larvae move over the food spinning dense and strong webbing. They also spin strong pupal cocoons. The developmental period from egg to adult is days under optimum conditions. Plodia interpunctella (Pyralidae) (the Indian meal moth) The adults have one third of the forewings with pale yellowish buff colour and the remainder is reddish brown. The spoilage of the food stuff is caused mainly by the webbing and frass produced by the larvae. Life history: The female lays as many as 500 eggs. The developmental period takes about 26 days at 30 0 C and 70% R.H.. Vertebrates pests: Rodents (Class: Rodentia) Several species of rodents are pests of field crops, but three species, referred to as "commensal" rodents are the main pests of stored produce. The term commensal refers to animals that live as tenants of humans and share their food, a reflection of the life habits of the three important pest species. The three species of commensal rodents are: Rattus rattus (the roof rat), Rattus norvergicus (Norway rat) and Mus musculus (the house mouse). Another species, Mastomys natalensis, is not strictly commensal. This species is the most serious field pest of cereal crops in Tanzania, but when there are no crops in the fields they invade stores and cause substantial damage. Life history: The three commensal species are poly-oestrous. They undergo repeated 4-day sexual cycles until they become pregnant. The gestation period lasts days in the house mouse, and 3 weeks in the roof rat and Norway rat. The young are born at a very early stage of development with no hair on their skin and with closed eyes and ears. They grow very fast and mature rapidly, reaching sexual maturity at weeks for the Norway and roof rat and 6-8 weeks for the house mouse. The average number of young in a litter is usually 193

11 5-6 for the house mouse and 7-8 for the Norway and roof rat. The commensal rodents can survive on a variety of feed, including seeds, grain, fruits, animal feeds and food of animal origin. In stores, the commensal rodents cause substantial losses to grain by feeding directly on the grain or grain products. Losses also occur as a result of contamination by faeces, urine and hairs. Self-heating Grain infestation by insects may result into self-heating, which is a self-destruction process in grain storage. Insects produce a certain amount of heat as a result of respiration. Since grain is a poor conductor of heat, some pockets of infestation in bulk storage tend to become warm. This warmth accelerates the respiration and is also favourable for multiplication of insect pests. This leads to development of "hot-spots" in the grain, which may spread throughout the bulk. As the temperature increases in the hot spots, some of the insects are killed but others move out. The hot spots also lead to moisture movement to the top of the grain or other cooler regions within the bulk. This moisture usually condenses and results to damp regions where moulds grow and sprouting takes place. Mites (Acari) Mites belong to the subclass Arachnida. They are much smaller than insects and in grain they are usually detected when they are present in large numbers. They can appear as "dust" on the surface of the grain, the sacks or the stacks. The commonest mites that are pests of stored grain are the Astigmata, which generally have a shiny body and legs. Mites are usually a very serious problem when they infest grain, grain products and other foodstuffs with a high moisture content under suitable temperature. These conditions enable them to multiply rapidly and form a dense population, which can cause severe damage, losses and food deterioration. A heavy infestation by mites causes tainting. In cereals, they acquire a characteristic musty or sour smell, which is a serious problem in grain products such as flour. Any food prepared from the flour that is infested with mites has a sour taste and poor odour. Acrid mites can cause dermatitis in people handling the infested products. These skin reactions affect sensitive people who come into contact with a large number of mites (Busvine 1980). Pest species - The flour mite, Acarus siro, is the commonest of the mite pests of stored food. It also occurs on all kinds of products, like cheese, hay, grain, etc. - The cheese mite, Tyrophagus casei, is found on cheese and various other stored food, grain, damp flour, etc. - The dried fruit mite, Carpoglyphus lactis, is commonly associated with dried fruits and jam. 194

12 Fungi The fungi most important in grain storage are the moulds. They are microscopic and require water, oxygen and suitable temperature to develop. Their optimum temperature for development and reproduction is above 20 0 C. The stored crops or their products form ideal substrates for the growth of fungi when other conditions are favourable. Growth and reproduction of fungi The simplest structure of a fungus is the hypha, a thread like structure, which grows inside the host material. Several hyphae produce a mat like structure, or mycelium. The mycelium develops within and on the surface of the grain, in a form of network. From the mycelium, asexual reproductive structures known as sporangiophores grow out and extend beyond the surface of the host material or substrate. At the end of the sporangiophores a sac like structure, called a sporangium, is formed. This structure contains the spores. Mature spores will usually be dispersed and geminate to produce new hyphae and mycelia when conditions are favourable. Pest species Pre-harvest fungi There are several groups including Alternaria spp., Fusarium spp, Cladosporium spp. and Helminthosporium spp. Post-harvest fungi These usually invade the crop during storage. They include the following species. Aspergillus flavus: These are common on cereals and oil seeds. They cause deterioration of oils, proteins and starches. In inadequately dried oil seeds and cereals they produce aflatoxins. Aspergillus niger: These also infest cereals and oil seeds, but produce less toxic aflatoxins. Aspergillus glaucus: These are very common species of moulds which are able to invade stored products with low moisture content and high sugar contents. Penicillium spp. Fungi belonging to this group are most associated with fruit rots. Fusarium spp. These fungi are common on cereals causing discolouration of the grains, production of odours and bad taste, alteration of the normal composition of the grain including production of toxic materials. Infestation by fungi often lead to production of toxic metabolites called mycotoxins. Moulds growing in animal feeds produce a specific group of mycotoxins called aflatoxins. The aflatoxins are associated with poisoning of animals leading to death. Impact of outbreaks of the larger grain borer on maize storage in Tanzania. The introduction of P. truncatus in Tanzania (Dunstan and Magazini 1981) and its subsequent spread to most of the regions (Golob and Hodges 1982, Makundi 1987) has led to greater attention on the storage system particularly, the storage of 195

13 maize. Since maize is the main food crop in Tanzania, its safe storage is a vital element in storage practices. Before the outbreaks of P. truncatus in Tanzania, storage of cob maize was the common practice in most of the outbreak foci. The maize is usually stored with the cobs still with the sheath, although in some areas the sheaths are removed. Most storage structures are designed for the storage and drying of the maize through natural ventilation. This has some implication for the control of P. truncatus since the beetle develops and multiplies more successfully on cob maize than on shelled maize. Therefore, a major change has to be made in the traditional storage system to accommodate the storage of shelled maize in the absence of effective control measures for P.truncatus on cob maize. However, this is also complicated by the fact that S. zeamais, the other major storage pest in Tanzania, multiplies more successfully on shelled maize than on unshelled maize. Traditional storage structures in Tanzania Existing storage structures, unless they have been improved, do not protect the commodities from infestation by pests, being insects, rodents or fungal infection. The common traditional storage structures and practices in Tanzania include: - Containers: these are unfixed and are of various sizes, usually kept in the house. They include woven sacks, baskets, pots, "vilindo" (made of tree backs, animal skins, or reeds). The capacity is usually up to 300 kg of the commodity (shelled maize, paddy, sorghum beans, etc.) - Cribs ("Vihenge"): These are raised, usually round, free-standing structures, with or without a roof of thatch. Their construction is variable depending on local circumstances. Some are constructed with interwoven sticks or bamboo splits sometimes plastered with mud, clay or dung. Cob maize, finger millet, paddy, beans etc. can be stored in them (Mphuru and Maro 1975). They also vary in size. - Storage in the house. A storage structure is usually built as an extension to the house. This structure, "chanja" is usually raised above the ground and the area beneath is used either as a kitchen or goat pen. Another type of house storage is placing the produce in a loft (particularly maize) and is subjected to heat and smoke from the fire underneath. - Open air storage: Maize cobs or other crops are stored outside the house on racks or platforms. Bundles of maize hung on trees are also another form of open-air storage. Identification of sources of infestation and their control An important consideration before attempting to control storage pests is to identify the potential sources of infestation in the new crop. A proper management of storage pests also requires an understanding of the cycle of infestation of the respective crop. Fig 1 shows this cycle, which is particularly relevant for insect pests of stored commodities. 196

14 Fig 1: Cycle of infestation of crops before and during storage RESIDUAL INSECT POPULATION IN STORAGE STRUCTURE PRE-HARVEST INFESTATION OF MATURE CROPS IN FIELD INFESTATION OF NEWLY CROSS INFESTATION HARVESTED CROPS INFESTED STORED COMMODITIES (OLD STOCKS) DEVELOPMENT OF FULL INFESTATION CROP DAMAGE AND LOSSES; FOOD CONTAMINATION The common sources of infestation are: - Field infestation in the pre-harvest period. At the time of physiological maturity of the crop low levels of infestation may occur as a result of insects flying from nearby infested stores. Initially, this infestation is low and it may not easily be detected. However it increases with time and gets further distributed away from the store. Pre-harvest field infestation can be reduced or prevented by (a) harvesting the crop as soon as it is mature and reasonably dry, (b) applying an insecticide or sanitation and hygiene measures in and around the store in the pre-harvest period. - Residual population of the pest. This population has its origin in the previous crop. Insects and mites survive on crop debris and in wall and floor cracks in the storage structures. In the case of the larger grain borer, P. truncatus, the 197

15 wood and thatching materials used in the construction of the structure could also harbour a residual population of the pest. Old sacks are also a potential source of infestation. The Khapra beetle, Torogoderma granarium, can hibernate in cracks in the storage structure for several years and attacks the newly introduced crop. Practical measures for management of the residual pest population include: - (a) Strict hygienic measures involving cleaning and removing all the previous crop residues. - (b) Burning or burying the debris after thorough cleaning to kill the adults, larvae and eggs. - (c) Removal of containers such as bags and immersing them in hot water to kill potential pests and their eggs. Other measures include plastering the walls of the storage structures and dusting or spraying them with a recommended and effective insecticide. Cross infestation of crops. Cross infestation of crops by pests can be defined as the transfer or movement of a pest from a previously infested stock to a fresh stock within the same store building or another building. The insects either fly or simply cross over by crawling into the new stocks. Cross infestation can be prevented by removing and disinfesting the old stocks followed by thorough cleaning and treatment of the walls and floor of the storage building before the new crop is introduced. In addition, the new stocks must also be treated with an insecticide or should be fumigated followed by dusting with an appropriate insecticide. Protection of durable crops from pest infestation Improving storage management To successfully combat infestation of stored crops, we need to improve both the storage structures and practices since most losses will occur at farm level. This approach requires that we introduce changes to improve the existing traditional structures. These improvements must be flexible enough to incorporate any new control measures found useful against the pests. The following practical management measures have potential to reduce infestation and crop damage before and during storage: Harvesting Many of the primary storage pests (Sitophilus spp., P. truncatus, etc.) usually begin infesting the crop in the field before harvesting. Sometimes, delayed harvesting is a form of temporary storage in some areas, but its immediate consequence is that the pre-harvest infestation starts in the field. Farmers should therefore be encouraged to harvest their crop early, as soon as it is reasonably dry. Storage hygiene - The focus should be on storage premises at farm level. Farmers must be instructed to clean their stores thoroughly before introducing the new crop. Debris removed from the store may harbour surviving pests and therefore it must be burned or buried. 198

16 - Sacks or other kinds of bags for storing the grains can be an important source of new infestation and they should be dis-infested. At the very least, they should be thoroughly brushed out as soon as they are emptied, but where possible, insecticide treatment should be done. - When the infestation is low, farmers tend to ignore it. If alternative control measures are not available, any of the crop showing signs of infestation should be sorted out when the grain is introduced to the store. However, low infestations are difficult to detect in bulk grain, and therefore, the pest may be introduced inadvertently into the store. It may be two or more months before the pest becomes established and is readily detectable, particularly for P. truncatus. Therefore, the regular checking of the commodity in the store is mandatory. As far as inspection is concerned, there is really no substitute for the regular removal of samples of the crop from the store and close examination of the grain. For P. truncatus, a large amount of dust is produced which is relatively easy to detect. However, pheromone trapping has been developed to facilitate the detection of the pest at low levels of infestation. State of the grain during storage -Most storage pests perform poorly at moisture levels below 13%. However, P. truncatus has been found infesting maize with moisture content as low as 9% (Golob and Hodges 1982). Therefore, thorough drying of grains will reduce the extent of infestation, but this may not be a substitute for other control measures. - In the absence of insecticide treatment, "sunning" may increase mortality of exposed eggs, larvae and adults, but in order for this to be effective it must be repeated many times at regular intervals. - Initial processing of the crop may lead to higher infestation particularly where mechanical damage of the grains occur. For example, Cowley et al. (1980) showed that P. truncatus causes more weight loss in loose artificially damaged maize grains than loose undamaged grains. Similarly, the grain becomes more vulnerable to infestation by secondary storage pests when damaged. The storage structure - Modified storage structures are necessary for safe keeping of grains but can only provide s a short-term solution to the pest problems during storage. For example, modified cribs suitable for storage of grains, particularly maize, have been demonstrated and are suitable for long term storage when the grain has been treated FOCAL Project no. 9, Progress Reports 2005). - The idea of using sealed metal drums for the storage of grain in Tanzania was first proposed by Pattinson (1968). However, large metal drums could be difficult to obtain for storage of large quantities of grains (>1 ton). Plastic drums are a better alternative. They are cheaper, do not corrode, are available and there is a great variation in volume that can store from small quantities of grain (100kg) to large quantities exceeding 5 tonnes. The light weight of the plastic containers makes them easy to handle. Provided that the grain is dry, 199

17 this type of airtight storage can be very successful in preventing insect and rodent infestation although the cost is high initially. Even metal drums that are open at the top can be successfully used in this way if a piece of polyethylene bag is tied tightly over the top. However, the drum and polyethylene must be undamaged if air tightness is to be maintained. - Essentially, different types of commodities should be equally treated or they should be stored separately to prevent one becoming the source of infestation for the other. Protection of durable crops during storage Many farmers and extension officers have the wrong notion that pesticides are the ultimate solution to pest problems during storage. In fact, pesticides should only be used when they are needed to complement other pest control measures like good storage hygiene. Pest management in durable crops should focus on economically feasible and practical approaches. Before implementing control measures, the farmers have to take certain initial steps to reduce initial losses of the crop. These include: - Timely harvesting. This prevents pre-harvest infestation by insect pests and damage by rodents and birds. - Proper drying. This reduces fungal infestation and it may also kill insect eggs and larvae as well as some adults. - Threshing. Infestation by some insect pests (e.g. the larger grain borer, P. truncatus) is higher in un-threshed maize still on the cob. - Hygienic measures. This should focus mainly on the storage structure. They should involve thorough cleaning of the storage structure to remove debris from the previous crop and sanitation in the surroundings of the store. An integrated approach for management of storage pests There is no single integrated approach package common to all storage pest situations in Tanzania. However, in developing an IPM package the following considerations are important. - Accurate identification of the pest species and the sources of infestation. The crops being attacked and the levels of losses with time should also be known. - A clear understanding of the biology and ecology of the pest species/species complex of the storage system under consideration. - Establishing the shortcomings, which enhance the pest status/losses in the storage system. It is important to establish criteria such as the time of harvesting (early or late), sanitation and hygiene status around and within the storage structure, etc. - How farmers store their commodities and the general storage and pest management practices. (i) are the storage structures improved? Do they have rodent proofing? what is the hygiene status in the stores? Are they properly repaired and plastered? etc. 200

18 - An evaluation of the potential practical approaches: Bio-control, synthetic insecticides, physical techniques (air tight or hermetic storage), use of inert materials (clays, etc), use of botanicals, fumigation, resistant cultivars, etc. There are some fundamental practices, which need to be incorporated in formulating an IPM package. These practices are the basis for safe storage of durable crops, particularly cereals and pulses or their products. They include: Timely harvesting of crops. Harvesting depends on the type of crop, and the prevailing climatic conditions at crop maturity. Early harvesting is recommended for some crops, but the decision to harvest early is dependent on the agro-climatic conditions. Early harvesting may reduce the risks of field infestation by insects, crop damage by rodents and fungal infection, particularly when the wet conditions become extended. However, early harvesting may not be appropriate if the crop is not yet adequately dry when the dry season starts. Some farmers often leave the crop in the field for a long period, which is takes the form of temporary storage, but it allows the crop to dry properly before harvesting and storage. It is important to consider these facts before advising farmers to harvest their crops. Late harvesting has the advantage that the crop dries in the field without farmers spending time and effort to enhance drying. However, among the disadvantages include pilferage, field infestation by pests including damage by rodents. Drying: Proper drying of the crop has several advantages, which include reduction or prevention of fungal infection and retardation of development of insect pests. Crop drying reduces the moisture content to a safe level for storage. The safe moisture content level depends on the type of crop and the purpose (which determines the duration of storage) for which the grain is stored. The drying can be done in a variety of ways, including: - Natural drying, which occurs largely by natural ventilation of the grain either in the field or in storage. The process of drying is dependent on the differences in atmospheric air moisture outside the grain and the air surrounding the grain. When the moisture content of the air surrounding the grain is higher than that of the atmospheric air, the grain will lose water. If there is more moisture in the atmospheric air surrounding the grain than within the grain, the grain will absolve moisture until an equilibrium is reached. This type of equilibrium determines whether deliberate efforts should be made to dry the grain further or not. - Deliberate exposure of the grain to drying conditions, which may involve mechanical measures for drying. Sunning is widely practised while mechanical driers may be applied, particularly for large consignments of grain. The mechanical driers utilize mechanical devices, to push heated dry air through the grain until the desired moisture content is reached. Drying should, in principle, not be too slow or too rapid. Too slow drying may lead to fungal 201

19 infection while too rapid drying may result to shrinkage and breakage of the kernel coat, leading to easy infestation by primary and secondary pests. - Storage hygiene: This is a must in any storage structure or system. Good storage hygiene assures the following: No cross infestation occurs; the residual pest population is destroyed; fungal infection is prevented and the quality of the produce is maintained. It is necessary to insist that only clean grain is introduced to the store. The surroundings of the storage structure should also be maintained clean to reduce or prevent harbourage by rodents and other pests. - Good storage management: this should involve regular inspections to detect infestations, self-heating, etc. Records of stocks should be maintained. The first step in integrated management of storage pests should focus on the storage structure. All the potent weaknesses of the structure should be identified and improvements should be recommended. Improved structures should have the following incorporated: - A roof without leakages - Rodent proofing installed according to standard technical designs of barriers at about m above the ground on each of the supporting poles. - Plastering or non-plastering of the walls of the structure depending on the agroecological conditions of the area and the type of structure. - Means of loading and unloading, which do not enhance pest infestation. Dis-infestation and protection of grains Insecticide treatment To prevent infestation of the grain by insects, it is desirable to spray or dust the walls of the storage structure with a suitable insecticide. A score of insecticides are recommended after clearance for use in food stores. It is important to advise the farmer to use only the recommended insecticide. Prophylactic treatment of grain before storage can prevent infestations developing for up to a year. Low toxicity insecticides (eg Actellic Super Dust for maize treatment), in powder form, are mixed with the grain before storage. Regular inspection of the grain is necessary after treatment. Fumigation The control of internally feeding insects or infestation that are fully developed can be achieved by fumigation using fumigants such as aluminium phosphide. However, it is necessary to apply the insecticidal powder after fumigation to ensure continued protection of the grains for the rest of the storage period. 202

20 Bio-control of storage pests In Tanzania, interest on the biological control of storage pests only developed after outbreaks of the larger grain borer. The advantages of bio-control using natural enemies are: (i) the strategy is self perpetuating and farmers do not need to repeat the control measures (ii) it is much safer for the health of consumers since no toxic substance that could induce chronic toxicity are added to the stored commodity. (iii) in the long term, this approach is self sustaining, and under ideal conditions, damage does not reach the threshold level. Therefore, farmers may not need to apply alternative or supplementary control interventions. (iv) (v) the risk of inducing resistance in the pest is avoided, farmers do not have to change their storage practices, although improvement is recommended to reduce the other sources of losses. The only storage pest that has been targeted for biological control in Tanzania is the larger grain borer after identifying a predator, the histerid beetle, Teretriosoma nigrescens (Rees 1985). Field studies have been conducted in Tanzania and the beetle was found to be a voracious predator of the larger grain borer and is therefore capable of suppressing thee population of the pest. References Busvine, J.R Insects and Hygiene. Chapman and Hall. New York. 568 pp. Cowley, R.J, Horward, D.C. and Smith, R.H The effect of grain stability on damage caused by Prostephanus truncatus (Horn) and three other pests of stored maize. Journal of Stored Product Research 16: De Lima, C.P.F Insect pests and post-harvest problems in the tropics. Insect Science and its Application 8: Dunstan. W.R. and Magazine, I.A Outbreak and new record. Tanzania. The larger grain borer on stored products. FAO Plant Prot. Bull. 29, Focal Project No. 9. Progress Reports FOCAL Programme. Sokoine University of Agriculture, Morogoro, Tanzania. Golob, P. and Hodges, R Study of an outbreak of Prostephanus truncatus in Tanzania. Report of the Tropical Products Institute, G

21 Howe, R.W The development of Rhizopertha dominica (F.) (Coleoptera: Bostrichidae) under constant conditions. Ent. Mono. Mag. 85, Makundi, R.H Influence of resistance of kernels of sorghum cultivars on the biology of Rhizopertha dominica (Fab). PhD Thesis. University of Newcastle upon Tyne, England, UK. Makundi, R.H The spread of Prostephanus truncatus (Horn) in Africa and measures for its control FAO Plant Protection Bulletin 35(4): Makundi, R.H. and Wilkins, W.M Life table and demography of Rhizopertha dominica (Fab) (Coleoptera: Bostrichidae) on kernels of sorghum. In: R.H. Makundi and J.E. Mkenda (Eds.) Ecologically sound Pest Management for Sustainable Agriculture and Public Health. Proc. Third Annual Scient. Connf. Tanzania Ent. Assoc September Zanzibar, Tanzania. Mphuru, A. and Maro, M.A Grain storage and handling in Morogoro and Iringa regions of Tanzania. Trop. Stored Prod. Inf. 30: Pattinson, I. (1968) Crop storage problems in Tanzania. FAO/UNDP no. TA 2454, Rome. Rees, D.P Life history of Teretriosoma nigrescens (Lewis) (Coleoptera: Histeridae) and its ability to suppress populations of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) in the Yucatan Peninsula, Mexico. Tropical Science 30(2), Rwamugira, W Agricultural diversification and intensification study. Working paper no. 4. Classification of agro-ecological zones in mainland Tanzania. International Development Centre, Queen Elizabeth House, University of Oxford. 19 p. World Food Programme Food Storage Manual. Rome, Italy. 204