3.4 The search for other alternatives
Farmers, through a long history of battle against stored product pest, have learnt to exploit natural resources, or to implement accessible methods, that would lead to a degree of population suppression of pests. Traditional methods usually provide cheap and feasible ways of post-harvest handling of the crops. Basically, farmers should be fairly aware that hygienic practices are essential for successful storing, i.e. thorough cleaning of bins or granaries, avoidance of mixing infested grains with healthy ones, burning crop residues after-harvest, sealing cracks and holes in muddy structures and any other practices that insure that the crop is stored in a clean and uncontaminated environment. During storage, some traditionally used materials are often added to the product, which contribute to the reduction of pests activity (Dakshinamurthy, 1988). Inert dust, for example, is added in variable amounts to the stored product. Friction of dust particles with insect's cuticle leads to desiccation and hampers the development of the pest (Golob et al., 1997). Grahn & Schmutterer (1995) showed that hydrophobic amorphous silica dusts resulted in efficient control of Callosobruchus chinensis, as no beetles survived after 48 hours at a concentration of 0.1 percentage. A similar effect can also be achieved through treatment with wood ash, collected from burnt tree wood or a farmer's stove. Some farmers may also add fine sand to hinder the pests activity, in which the high proportion of quartz causes damage to the sensitive cuticle of the newly hatched larvae (Kroschel & Koch, 1996). In an experiment in India, pre-treatment of Vigna radiata seeds with inert clay resulted in 100 percentage adult mortality of Callosobruchus chinensis within 24 hours. Seeds maintained over 80 percentage germination for up to 12 months of storage under ambient conditions (Babu et al., 1989). In Bangladesh, covering potato tubers with rice husk, sand or wood ash resulted in good control of Phthorimaea operculella in store (Das et al., 1995). Botanical insect deterrents or seed protectants may also be applied to products by some farmers with varied degrees of success. Plant products such as neem powder, leaves of hoary basil (Ocimum spp.), mint (Mentha spp.) or black pepper (Piper spp.), showed some positive results in limiting insect infestation. Neem tree has been exploited widely and neem extracts showed good results in reducing damage by certain pests (see Ishrat et al., 1994). In the Sudan, spraying potato tubers with neem and then placing them in jute sacks reduced post-harvest loss caused by Phthorimaea operculella (Siddig et al., 1987). Other extracted oils, such as coconut, maize or ground nut oil, have been recognised as toxicants or growth inhibitors of bruchids (Ramzan, 1994; see also Reddy et al., 1994).
3.1.1 Control using sunlight
Exposure to sunlight, or exposure followed by sieving of the grains, is a well known technique among farmers in sub-Saharan Africa, specially against the different pests of beans (Chinwada & Giga, 1996). In this method, grains are spread on a dark paper or a black polyethylene sheet and left exposed to sunlight for at least seven hours. After sunning, grains are sieved using a 5 mm sieve. The process may be repeated every three to four weeks depending on the size of production and availability of labour. This method proved to be quite effective in reducing bruchid infestation with no, or minimal, effect on grain quality or germination (Songa & Rono, 1998; see also Chinwada & Giga, 1996; Lale & Sastawa, 1996). In an experiment in Costa Rica, Leal & Zeledon (1994) showed that periodic sieving of stored maize decreased populations of adult pests by up to 99 percentage in 24 weeks.
3.1.2 Control by drying
Farmers, as mentioned earlier, may use bush dryers, solar dryers or light fire underneath the crop, to reduce the water contents and to deter or kill the different insect stages. In Cameroon, for example, the use of a 50 kg capacity solar heater eradicated infestations of Callosobruchus maculatus from cowpea seeds. It was also demonstrated that temperatures of up to 850C did not adversely affect seed germination (Ntoukam et al., 1997). Other traditional methods include mechanical removal of insects, infested grains or cobs. Winnowing, shaking and restacking the grains led to the disturbance of insects and a reduction of their activity.
Due to the significant increase in the human population, and the consequent increase in the amounts of food and grains produced, many small scale farmers adopted the use of pesticides as a means of pest control. Dusting and fumigation of grains are the most commonly used chemical methods among small-scale farmers (see Rai et al., 1987; Gwinner et al., 1996). Dusting, in particular, is an easily applied method, and can be implemented with very cheap tools such as small perforated metal cans or jute bags. For small amounts of grains, dust can be mixed with grains using a shovel. Dust should be mixed thoroughly and distributed evenly all over the produce. Dusters can also be used as a surface treatment to treat the bags, sacks or the whole granary. For larger amounts of grains or when storing maize cobs, a "sandwich method" is applied, whereby dust is spread lightly inside the granary, covering the bottom and walls with a thin layer, then the produce is entered in to make a layer of 20 cm, followed by another layer of dust, and so on until the granary is full.
The most commonly used insecticide dusts among farmers belong to two main groups of chemicals: (1) organophosphorus compounds, such as chlorpyrifos-methyl, fenitrothion, malathion, methacrifos and pirimiphos-methyl, and (2) pyrethroids, such as cyfluthrin, deltamethrin, fenvalerate and permethrin (see Table 2 a, b).
Fumigants are low molecular weight chemicals, highly toxic and volatile, that are used during storage to kill all insect stages residing in the produce. Fumigation is a widely used method all over the world on small as well as large storage scale. The method can be applied at the farm level in gas-tight granaries or silos, under gas-tight sheets carefully covering the product or at a large scale storage as in large warehouses. Fumigants are commercially available in a solid, liquid or gaseous state. Phosphine (PH3), for example, is a formulated fumigant commercially available as either tablets, pellets, bags or plates. Methyl bromide (CH3Br), on the other hand, is gaseous in form and packed in a liquid form in pressurised steel bottles. At temperature above 40C it takes a gaseous state, thus, once the container is opened, the gas is released and starts to act as a fumigant. The two compounds are the most widely spread fumigants in use. However, a problem of human toxicity due to inadequate application of the method is considered a drawback regarding this industry, specially in the developing countries, where inappropriate handling of such toxicants is widespread. Another problem with the use of fumigants has recently aroused, which is the developing of resistance from insects against fumigants. The problem started as a result of improper application of the chemicals in use, i.e. application of incorrect doses, fumigation in non gas-tight containers or insufficient exposure time. Recently, fumigation has been highly discouraged at a small-scale level. moreover, the use of methyl bromide has been strongly restricted in industrialised countries because of its ozone-depleting potential. However, fumigation is still the most widely operated method as an essential large scale post-harvest practice.
3.4 The search for other alternatives
Trials have been conducted on the use of carbon dioxide as a fumigant to replace methyl bromide in the control of insects and mites damaging stored products (Newton et al., 1993). The use of Co2 rich atmospheres showed promising results in disinfesting food commodities in small storage facilities (Krishnamurthy et al., 1993). A relatively new technique used by the Indonesian National Logistic Agency (Bulog) for milled rice is to seal bag sticks into large plastic enclosures flushed with carbon dioxide (Hodges & Surendro, 1996).
Treatment with high-pressure carbon dioxide under different temperatures may result in different rates of mortality, for example, at 150C, 95 percentage mortality of Lasioderma serricorne was observed after 38.5 min of treatment, while the same level of control was achieved within 1 minute at 450C (Ulrichs, 1995). Corinth & Rau (1990) showed that each tonne of grain requires about 19-27 kg carbon dioxide to achieve complete mortality of Oryzaephilus surinamensis, Tribolium castaneum and Sitophilus granarius in 4-6 weeks.
The use of "Biogas" as a fumigant, with methane and carbon dioxide as its main components, may achieve good results in the control of stored pests. Subramanya et al.(1994) showed that biogas significantly reduced infestations and loss in stored pigeon pea infested with Callosobruchus chinensis. Gursharan et al. (1994) recorded up to 100 percentage mortality of Sitophilus oryzae, Rhyzopertha dominica, Trogoderma granarium and Tribolium castaneum after six days' exposure to biogas in PVC bins. Another method for the control of insects in industrial premises was developed, where a Gas Operated Liquid Dispensing system was used to mix separate sources of carbon dioxide and insecticide concentrate. The system, given the name Turbocide GOLD, produces a fine insecticidal aerosol that was reported to give excellent control of Tribolium castaneum, T. confusum and Lasioderma serricorne (Groome et al., 1994).
Several studies have focused on developing post-harvest technologies as a key role in ensuring food security. Consumers are now aware of the danger in the use of chemical pesticides to protect stored products. This, and the world-wide trend to minimise the use of toxic substances applied on food products, have led scientists to seek less dangerous alternatives. Fumigation, for example, has become an endangered technology due to pressures regarding environmental contamination and health concerns (Banks, 1994). The following is a list of some of the most recent trials to use natural products in place of synthetic pesticides. Materials listed here are only the ones that showed a significant degree of success, and can be widely used in stores with confidence against certain storage pests.
Material Used as Plant / Product pest Country Reference
________________________________________________________________________________________________________________________________
Seed extract
of Ricinus communis Protectant Wheat grains S. oryzae Egypt Mahgoub & Ahmad, 1996
Leaves & stems of Repellent & Maize & sorghum S. zeamais Kenya Bekele et al., 1996
Ocimum suave protectant grains R. dominica
S. cerealella
Seed powder &
essential oils of
Dennettia tripetala
Piper guineense Seed protectants Maize & S. zeamais
Monodora myristica cowpea C. maculatus Nigeria Okonkwo & Okoye, 1996
Xylopia aethiopica
Dried & powdered tissues of
Dicoma sessiliflora Seed protectants Wheat grains S. oryzae
Neorautanenia mitis P. truncatus Malawi Chimbe & Galley, 1996
Citrus peel oils Fumigant Cowpea & cereal S. zeamais
grains C. maculatus Nigeria Don Pedro, 1996
Eugenol (essential oil of Repellent & grain Grains S. zeamais Ocimum suave) protectant S. granarius
T. castaneum Germany Obeng-Ofori &
P. truncatus Reichmuth, 1997
Plant powders of
Ricinus communis
Gaura coccinea
Larrea tridentata
Ribes ciliatum
Castilleja tenuiflora Grain protectants Stored maize S. zeamais
Alchemilla procumbens & beans A. obtectus Costa Rica Araya et al., 1996
Guazuma tomentosa
Acorus calamus oil Space treatment _ S. zeamais Egypt Risha, 1993
Dried plant material of
Cydista aequinoctialis
Ageratum conyzoides Graine protectants Maize seeds S. zeamais
Catharanthus roseus T. castaneum Philippines Vallador et al., 1994
Gliricidia sepium
Leaves of Post-harvest grain Maize & sorghum S. zeamais Kenya Jembere et al., 1995
Ocimum kilimandscharicum protectant grains R. dominica
S. cerealella
Essential oils:
Cassia oil Grain protectants Stored wheat T. castaneum
Illicium verum O. surinamensis China Xu, H. et al., 1993
Clausena dunniana R. dominica
Japanese mint Fumigant Stored sorghum S. oryzae China Singh et al., 1995
(Mentha arvensis)
Extracted oils of
Nigerian Indigenous plants:
Mondora tenuifolia
Lippia adoensis
Cymbopogon citratus Natural insecticides Maize grains S. zeamais Nigeria Odeyemi, 1993
Petiveria alliacea
Leaf powder of
Lagundi (Vitex negundo) Seed protectant Maize grains S. zeamais Bangladesh Buiyah
& Quiniones, 1990
Extracts of Feeding deterrent Food stuffs S. granarius
Helenium aromaticum R. dominica Poland Bloszyk et al., 1990
Neem (Azadirachta indica) oil
Copra oil Seed protectant Stored maize S. zeamais Ghana Cobbinah & Appiah, 1989
Palm kernel oil
Neem oil Natural insecticide Stored maize S. zeamais Benin Kossou, 1989
Acetone extract of
Dill (Anethum graveolens) seeds Seed protectant Wheat grains S. oryzae USA Su, 1989
Oils of
castor (Ricinus communis)
radish (Brassica campestris) Grain protectants Stored wheat S. oryzae India Ran et al., 1988
Garlic (Allium sativum) Insect repellent Stored maize Sitophilus sp Brazil Sasaki & Calafiiori, 1988
Groundnut oil Carriers for Wheat grains R. dominica
Sesame oil pyrethrin T. castaneum India Trivedi, 1987
Alcohol extracts of:
Neem (Azadirachta indica)
Sweet flag (Acours calamus) Natural insecticides _ T. granarium India Pal et al., 1996
Chandani (Taberna montana coronaria)
Imli seeds (Tamarindus indica)
Asriple (Lantana camara)
Oils from:
Citrus (Citrus limon)
Garlic (Allium sativum)
Pondia (Mentha spicata) Natural insecticides Sorghum grains T. granarium India Sudesh et al., 1996 a
Powdered leaves of:
Mentha longifolia
Thymus vulgaris Natural insecticides Wheat grains T. granarium Egypt Mostafa, 1993
& powdered seeds of:
Piper nigrum
(-)-Homogynolide: Isolated Insect Antifeedant Different grains T. granarium
from Homogyne alpina S. granarius Japan Mori &
T. confusum Matsushima, 1995
Extracts of
African plants:
Entandrophragma sp. Insect Antifeedants _ S. granarius
Kaaya sp. T. confusum Zaire Szafranski et al., 1993
Quassia sp. T. granarium
Vernonia sp.
Extract of
Water Hyacinth Insect Antifeedant Rice C. maculatus India Rani & Jamil, 1989 (Eichhornia crassipes)
Extracts from dried fruits of Grain protectants Rice T. castaneum
Star anise (Illicium verum) S. zeamais Singapore Ho et al., 1995
Oil of:
Cinnamomum micranthum Grain protectant Different grains T. castaneum China Xu et al., 1996
Isolates from leaves of:
Nicotiana tabacum Antifeedant _ T. castaneum India Archna et al., 1995
Dried plant materials of
Gliricidia sepium Grain protectant Stored maize seeds T. castaneum
Cosmos caudatus S. zeamais Philippines Vallador et al., 1994
Seed extracts of:
Aphanamixis polystachya Insect repellents Wheat flour T. castaneum UK Talukder & Howse, 1995
Extracts of:
Chenopodium ambrosioides Natural insecticides Grains T. castaneum
Convolvulus arvensis S. granarius Egypt Abdallah et al., 1988
Conyza dioscoridis
Eucalyptus powder Insect toxicant Rice S. cerealella India Dakshinamurthy, 1988
Leaf extracts of:
Lantana inica Insect toxicants Grains S. cerealella India Ranganath, 1993
Mentha sp.
Extracts of Piper nigrum Insect toxicant Grains S. cerealella Brazil Boff et al., 1995
Dried meant leaves
(Mentha spicata)
Powdered seeds of custard
apple (Annona squamosa) Protectants Stored wheat R. dominica India Patel & Valand, 1994
Leaf powders of:
Chromolaena odorata
Calotropis procera Natural insecticides Rice grains R. dominica India Jacob & Sheila, 1993
Datura metel
Azadirachta indica
Essential oils of Goldenrod Insect toxicant R. dominica
(Solidago canadensis) & repellent Grains S. granarius Poland Kalemba et al., 1990
T. confusum
Plant extracts of:
Origanum vulgare Natural insecticide Kidney beans A. obtectus France Regnault & Hamraoui, 1995
Essentials oils of:
Rosmarinus officinalis
Thymus vulgaris Fumigants, ovi-&
Thymus serpyllum larvicides Kidney beans A. obtectus France Regnault &
Ocimum basilicum Hamraoui, 1994
Cinnamomum verum
Soybean oil
Black pepper (Piper nigrum) Natural insecticides Stored beans A. obtectus Brazil Faroni, 1995
Sunflower oil Natural insecticides Stored grains A. obtectus Cuba Roche & Simanca, 1987
Ground powder of Grain protectant Wheat grains R. dominica
Fenugreek (Trigonella foenum) O. surinamensis Egypt Afifi et al., 1988
S. oryzae
Extracts of:
Pongamia glabra
Jatropha cureas Seed protectants Potato seeds P. operculella India Shelke et al., 1987
Ipomoea carnea
Powdered leaves of
Lantana aculeata Tuber protectants Potato tubers P. operculella India Lal, 1987
Eucalyptus globulus
Dried foliage of:
Eucalyptus globulus
Lantana camara Tuber protectants Stored potato tubers P. operculella Peru Raman et al., 1987
Minthostachys sp.
Calamus oil of Contact toxicant & Wheat & cowpea S. oryzae
(Acorus calamus) seed protectant seeds C. maculatus
T. confusum
L. serricorne USA Su, 1991 a
Chenopodium oil from
Chenopodium ambrosioides Toxicant & repellent Wheat & cowpea S. oryzae
C. maculatus
L. serricorne USA Su, 1991 b
Coconut oil
Groundnut oil Seed surface protectants Chickpea seeds C. chinensis India Singal, 1995
Mustard oil
Dust and ether extracts of
Brown pepper (Piper guineense) Natural insecticide _ C. maculatus Nigeria Mbata et al., 1995
Seed powder of neem
Sweetflag (Acorus calamus)
Custard apple (Annona squamosa)
Black pepper (Piper nigrum)
Rhizome powder of turmeric
(Curcuma longa) Seed protectants Red grams C. chinensis India Shivanna et al., 1994
Neem kernel powder
Melia azedarach
Solanum incanum Seed protectants Faba bean seeds C. maculatus Yemen Al-Hemyari, 1994
Acacia wood stove ash
Margosan (neem based pesticide)
Saponin (extract of
Castanospermum australe)
Juliflorine (extract of Natural insecticides _ C. analis Pakistan Rahila et al., 1994
Prosopis juliflora)
Coconut oil Grain protectant Green gram,
bengal gram, C. chinensis India Jacob, 1994
green peas and cowpeas
Leaf poders & volatile oils of
Lippia adoensis
Cymbopogon citratus
Lantana camara Grain protectants Cowpea seeds C. maculatus Nigeria Adebayo & Gbolade,
Eugenia uniflora 1994
Oils of
mustard
Groundnut
Soyabean Grain protectants Chickpea C. chinensis India Singal & Singh, 1990
Rapeseed
Seed oil of
Neem
Brown pepper (Piper guineense) Grain protectants Cowpea seeds C. maculatus Nigeria Ivbijaro, 1990
Castor oil Grain protectant Cowpea seeds C. rhodesianus Zimbabwe Giga & Magnets, 1990
Olive oil
Sesame oil Grain protectants Cowpea seeds C. chinensis Egypt Zewar, 1988
Olive oil
Mustard oil Grain protectants Green gram seeds C. maculatus Pakistan Ahmed et al., 1988
Ether extract of
Annona squamosa Contact toxicant _ C. chinensis Japan Ohsawa et al., 1990
Leaves of Beguina
(Vitex negundo) Grain protectant Pulse grains C. chinensis India Prakash & Rao, 1990
Maize oil
Coconut oil Grain protectants Stored cowpea seeds C. maculatus Egypt El-Sayed et al., 1989
Oils from:
Navel orange
Sweet orange
Grapefruit Grain protectants Wheat grains S. granarius Egypt El-Sayed et al., 1989
Citronella oil T. castaneum
(Extract of Cymbopogon nardus) Insect repellent _ C. chinensis India Saraswathi & Rao, 1987
Himalayan cedarwood oil
(Cedrus deodara) Natural insecticide _ C. analis India Singh & Agarwal, 1988
Neem oil
Karanj oil (Pongamia pinnata) Natural insecticides _ C. chinensis India Khaire et al., 1987
___________________________________________________________________________________________________________________________________
Temperature is a crucial environmental factor that influences the development of insects. There is always a minimum, optimum and a maximum range of temperature in which insects can survive. Insects differ in their tolerance to either low or high temperatures, however, a general pattern of population increase is shown on Figure 5. Most stored product pests would follow the same pattern of survival under a different range of temperatures. As temperature approaches zero, insect development, activity and movement decline to a minimum. Gradual increase in temperature will increase insect activity up to a certain range that differs among different species. Further increase in temperature above the optimum range will lead to increase in insect mortality and crashing of the population.
The use of high temperature is a well known technique to control stored product pests. For example, temperatures of above 400C are lethal for most stored food pests (Gwinner et al., 1996). Adult emergence of Sitotroga cerealella, Sitophilus oryzae and Rhyzopertha dominica can be totally suppressed after exposing their pupae to 450C for 72 hours (Sharma et al., 1997). However, low temperature treatment of grains may also provide a degree of control. Evans et al. (1987) stated that cold treatment, in combination with drying, is more useful for protecting grain from attack and deterioration than for disinfestation. In the USA, a prototype grain chiller was tested to determine its efficacy as a pest management tool in stored popcorn. Fewer Plodia interpunctella were trapped in the chilled aeration bins compared to the traditionally managed popcorn bins. Costs of chilled aeration (0.11 cents/kg) were competitive with the costs of conventional pest management practices such as fumigation and ambient aeration (Mason et al., 1997).
In Belgium, work has been done on using a combination of controlled atmospheres with an ice-forming preparation from a bacterium (Pseudomonas syringae), which is able to reduce freeze-resistance in insects (Desimpelaere, 1996). The combination resulted in 100 percentage mortality of Sitophilus granarius after exposure to 100 p.p.m. solution combined with -10 0C for 24 hours (see also Mignon et al., 1995). Similarly, Plodia interpunctella larvae are known to be freeze susceptible. In winter, they avoid freezing by lowering their supercooling point. Feng et al. (1996) showed the possibility of elevating the supercooling point of the larvae using bacterial ice nucleators.
The search for other alternatives to pesticides is still going on, with the hope that one day, a competitive and economic method, or an integrated group of methods that can be widely applicable, will emerge. In search for other methods, Pradzynska (1982) treated different stages of Sitophilus granarius with ultrasonic waves. Treatment for 5 min at 14.5 W/cm2 resulted in 100 percentage mortality within 2 min when treated outside the grains and within 4 min in the kernels. Tests in Mexico, exposing of adults of Tribolium confusum, T. castaneum, Sitophilus zeamais, Prostephanus truncatus and Oryzaephilus surinamensis to argon or helio-neon lasers, resulted in shortening of the life span; anorexia and dehydration, melanization and sclerotization and reductions in the size of F1 generation adults which were sterile. Chemical analysis showed the nutritive value of the laser-exposed maize flour remained unchanged, and the germination of exposed grains was not adversely affected (Ramos et al., 1983). In Iran, an electrohydrodynamic (EHD) system which generated air ions within a strong electric field was used for the control of Tribolium confusum. Negative ions resulted in maximum mortality of pupa due to the body fluid losses caused by the electric wind of the system (Shayesteh & Barthakur, 1997).
In Iraq, releasing cytoplasmically incompatible males of Ephestia cautella reduced infestation rate of stored dates when the release ratio was 80:1:(incompatible males strain: wild strain) (Ahmed et al., 1994). While in an interesting experiment in the USA, half-filled kidney beans containers were rolled or tumbled every 8 h or 2-3 times/day for 2 weeks, which resulted in disturbing the alignment between stable bracing sites of Acanthoscelides obtectus and target beans, and prevented the larvae of from completing entrance holes. Populations of A. obtectus in all rolled or tumbled containers were reduced by about 97 percentage compared with stationary controls (Spencer et al., 1991).
The followings are different applicable methods that might provide potential alternatives for the wide use of pesticides. Though their application is still rather limited, however, an intensive amount of research is carried out to facilitate the use of each method, and to achieve a plausible degree of integration among the different methods.
Biological control may provide a useful and safe alternative for the control of crop pests. However, the use of biological control against stored product pests is still limited, though recently gaining ground due to increasing health concerns. On a small scale, the use of natural enemies may become available as a degree of "filth" or a small number of insects can be tolerated. Moreover, natural enemies may minimise the number of insect pests carried to the store at the end of the season. The following is a brief listing of some promising trials against certain post-harvest pests through the use of natural enemies.
B.C. Agent Used against Ref Remarks
------------------------------------------------------------------------------------------------------------------
Uscana lariophaga C. chinensis Alzouma, 1995 The parasitoid was (Trichogrammatidae) present in stores throughout the dry season. Laboratory studies showed that it was an effective parasitoid of bruchids
in Niger.
Anisopterolamus calandrae C. chinensis Islam & Nargis 50 released pairs Pteromalidae (1994) resulted in almost 100% control in red lentil debris in Bangladesh
R. dominica Ahmed, 1996 Percentage parasitism reached almost 60% in the field, Saudi Arabia.
S. zeamais Wen & Brower Percentage 1994 suppression reached over 90% in maize stored in drums following parasitoid release.
Trichogramma evanescens E. Kuehniella Scholler et al., Females were able to (Trichogrammatidae) (1995) parasitize eggs at 55 cm depth in wheat.
Lyctocoris campestris O. surinamensis Trematerra & A fairly wide host Anthocoridae T. castaneum Dioli, 1993 range larval predator E. elutella recorded from wheat S. cerealella stores in central Italy.
Teretriosoma nigrescens P. truncatus Boye et al.,1995 releasing the predator Histeridae in the field resulted in quicker dispersal than in stores.
P. truncatus & Rees, 1991 Use of predator R. dominica reduced weight loss in maize caused by the bostrichids.
Dinarmus basalis C. chinensis Alzouma, 1995 The parasitoid (Pteromalidae) B. atrolineatus showed efficiency in the control of the two bruchids.
------------------------------------------------------------------------------------------------------------------
In addition to arthropod natural enemies, other bacterial and protozoan agents may also become applicable. McGaughey et al.(1987) reviewed the use of the entomopathogenic bacterium Bacillus thuringiensis against pests of stored grain and seed. B. thuringiensis proved to be ideally suited for use on stored grain and seeds, being compatible with other protectants and available in different formulations for convenient application. In bulk stores, dressing a 10 cm deep surface layer with B. thuringiensis at 125 mg/kg controlled both Plodia interpunctella and Ephestia cautella. B. thuringiensis retained its activity for up to 2 years in stored grains, where it was not exposed to ultraviolet radiation, but P. interpunctella developed resistance levels of over 100-fold in 15 generations on a B. thuringiensis-treated diet in the laboratory.
Kroschel & Koch (1996) treated potato tubers with a mixture of B. thuringiensis and fine sand. Good results were obtained against the potato tuber moth Phthorimaea operculella. While Raman et al. (1987) showed that a dust formulation of Bacillus thuringiensis applied to potato tubers was most effective.
Figure 14:
Nosema sp. can also be used for the control of certain pests. One example is the use of spores against Lasioderma serricorne. (Ghosh et al., 1995). Insect death is caused by severe damage to gut epithelial tissues and fat bodies.
Moreover, the use of transgenic plants is currently gaining ground in different parts of the world. (Bt) can be introduced to plant tissues and serve as protectants against infestation by certain pests. In a storage bioassay in Belgium, selected potato line tubers carrying the Bacillus thuringiensis CryIAb6 insecticidal crystal protein gene gave 100 percentage control of P. operculella damage (Jansens et al., 1995).
The use of varied short wave doses of about 0.2-0.5 Kilogray (kGy) provides another alternative in the control of pests in store. Combined treatments of radiation and carbon dioxide produced a higher mortality in T. confusum than did either treatment alone (Omar et al., 1988). This method has the advantage of leaving no residues in the product, though it might not be feasible due to the high costs involved in application. Other experiments involved the use of microwave energy against stored product pests. A special microwave unit was developed with a variable speed conveyor belt and tested for insect control in stored milled rice. Results indicated that Tribolium confusum and Cryptolestes pusillus can be killed economically with microwave energy (Langlinais, 1989).
The use of pheromones is one of the most promising techniques aimed at the control of stored product insects that may lead to a drastic reduction of chemical treatments against crop pests (Trematerra, 1997). Pheromone traps can be used to monitor the dynamics and occurrence of different stored product pests, such as Phthorimaea operculella (Trematerra et al., 1996). In Peru, mixtures of the pheromone components (4E,7Z)-4,7-tridecadien-1-ol acetate (PTM 1) and (4E,7Z,10Z)-4,7,10-tridecatrien-1-ol acetate (PTM 2) were evaluated for its attractiveness to Phthorimaea operculella. Ratios of PTM 1:PTM 2 of 9:1 or 1:15 gave the highest captures. ass trapping showed the feasibility of direct control in the field and stores. Microencapsulated pheromone sprays resulted in significant reduction of larval infestation in stored tubers (Raman, 1988).
In addition, certain compounds extracted from insect bodies may serve as attractants, repellents or arrestants to other insects of the same species. For example, hexane wash of Lasioderma serricorne females reduced egg laying by conspecific females in treated tobacco leaf disk stacks, therefore it may have use as an oviposition deterrent (Howlader & Ambadkar, 1995). In another experiment on Callosobruchus chinensis, crude extracts of females captured more than 60 percentage of males of in a laboratory culture using a pitfall trap, resulting in lower adult infestation levels in the following generation (Islam, 1994). In Japan, two arrestants of Oryzaephilus surinamensis, 13-oxo-(Z)-octadecenoic acid and 15-oxo-(Z)-11-icosenoic acid, were synthesised for the first time (Nakajima et al., 1997). The two compounds were previously extracted from wheat flour infested by this pest (Nakajima et al., 1996).
There is a wealth of information regarding the selection of resistant plants through intensive breeding programmes. Though host plant resistance is a promising strategy in pest control, insect populations are able to develop biotypes that can attack formerly resistant varieties, and there is evidence that improved varieties tend to perform poorly under low input conditions (CIMMYT, 1992). However, this strategy may result, along with other control methods, in a significant degree of pests population regulation.
