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Cowpea (Vigna unguiculata (L.) Walpers) originated in the savannah region of west and central Africa. It belongs to the family leguminosae and subfamily papilionacae. Cowpea is one of the most ancient crops known to man .Its origin and subsequent domestication is associated with pearl millet and sorghum in Africa. It is now a broadly adapted and highly variable crop, cultivated around the world primarily for seed, but also as a vegetable (for leafy greens, green pods, fresh shelled green peas, and shelled dried peas), a cover crop and for fodder. It is an important crop for many subsistence farmers in tropical areas, especially in Africa. It is a food security crop in the seminarial zone of west and central Africa which ensure farm household subsistence food supply even in years. Production areas are also spread all around the world, Central, South America and Asia, making cowpea a global crop.
Nutritionally, cowpea contains about 24%protein,62% soluble carbohydrate and small amounts of other nutrients.Thus,most of its nutritional value is provided by proteins and carbohydrates. Its chemical composition is similar to that of most edible legumes. Cowpea is a major source of protein in the diet of many people in sub-Saharan Africa. Cowpea has been found to be an important food grain crop for approximately 200 million people in the dry savannah of tropical west and central Africa (Bressani, 1985).
In the field, the crop is susceptible to many pests (Singh et al., 1990). The dry, ripe seeds, however, before harvest or in storage are vulnerable to only few pests of which the cowpea weevil, Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) is the most important insect pest. Infestation by this beetle commences in the field (Prevett, 1961), but most damage is done during storage. Over 90% of the insect damage to cowpea seeds is caused by C. maculatus (Caswell, 1981). Infestation may reach 100% within 3-5 months of storage ( Singh, 1977). The germination of the beans is adversely affected due to the beetles' emergence holes ( Baier and Webster, 1992). Damaged seeds lose weight and market value and they are generally unacceptable for human consumption ( Javaid, I. and Poswal, M.A.T., 1995. Evaluation of certain spices for the control of Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae) in cowpea seeds. African Entomology 3, pp. 87-89. View Record in Scopus | Cited By in Scopus (2)Javaid and Poswal, 1995).
To protect the stored beans against C. maculatus, many methods can be used. Traditionally the beans, if treated at all, are mixed or covered with materials that are available locally. They can be mixed with ash, sand, or other dry fine substances that fill up the space between the beans and provide a barrier to insect movement (Golob and Webley, 1980). Fresh, dry or processed plant materials can be applied as insecticides or to repel the pest insects. These methods, in combination with the natural occurrence of parasitoids, should keep the beetle infestation as low as possible. Nowadays, methods such as storage in airtight plastic or steel containers, application of chemical insecticides, gamma irradiation, freezing the beans or heating them, are some of the additional possibilities. However, most of these methods require high inputs, often unavailable and unaffordable for subsistence farmers. As for ash and sand, the main disadvantage is that to be effective they have to be applied in such large quantities that they are practical only for small amounts of beans ( Wegmann, 1983). With the introduction of - often subsidised - chemical pesticides much of the traditional knowledge of the use of plant materials as pesticides is diminishing ( Koné, 1993). Moreover, the development of synthetic pesticides has limited the more widespread use of botanical insecticides ( Delobel and Malonga, 1987). Meanwhile C. maculatus continues to destroy stocks of beans that could prevent famine.The estimated global post-harvest losses caused by insect damage, microbial deterioration and other factors are of the order of 10-25% (Matthews, G.A., 1993. Insecticide application in stores. In: Matthews, G.A. and Histop, E.C., Editors, 1993. Application Technology for Crop Protection, CAB International, London, pp. 305-315.Matthews, 1993). The bruchid beetle Callosobruchus maculatus (F.) has been associated with dried legumes for thousands of years (Mitchell and Messina). Synthetic chemical pesticides have been used to protect grains but their widespread use has led to the development of pest resistance ( Champ and Dyte, 1976; Zettler and White). Alternatives to synthetic chemical insecticides are highly desirable ( Xie et al., 1995). Plant volatile oils have been used in the protection of stored grains ( El; Saxena; Risha and Gbolade). .This is an important pest that mainly attacks beans of various species, specifically cowpea and can alternatively attack other pulse crops (see Lienard & Seck, 1994). 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 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). A similar effect can also be achieved through treatment with wood ash, collected from burnt tree wood or a farmer's stove. Botanical insect deterrents or seed protectants may also be applied to products by some farmers with varied degrees of success..Exposure to sunlight, or exposure followed by sieving of the grains, is a well known technique among farmers in sub-Saharan Africa, especially against the different pests of beans (Chinwada & Giga, 1996). 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. 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.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 large scale storage as in large warehouses. Fumigants are commercially available in a solid, liquid or gaseous state. However, a problem of human toxicity due to inadequate application of the method is considered a drawback regarding this industry especially 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; recently, fumigation has been highly discouraged at a small-scale level. Moreover, the use of methyl bromide has been strongly restricted in industrialized countries because of its ozone-depleting potential. However, fumigation is still the most widely operated method as essential large scale post-harvest practices. 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. 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). Eugenol (C10H12O2), is an allyl chain-substituted guaiacol. Eugenol is a member of the phenylpropanoids class of chemical compounds. It is a clear to pale yellow oily liquid extracted from certain essential oils especially from clove oil, nutmeg, cinnamon, and bay leaf. It is slightly soluble in water and soluble in organic solvents. It has a pleasant, spicy, clove-like aroma. Methyl eugenol (derivative of eugenol)is a natural constituent in many types of plants and fruits consumed by humans and animals, (e.g., anise, nutmeg, mace, pixuri seeds, basil, blackberry essence, bananas, walnuts, citrus, black pepper). Methyl eugenol is also used as a flavoring added to ice cream, cookies, pies, puddings, candy, cola soft drinks, chewing and bubble gum, etcObjectives
The main objective of study is to:
Determine the biological activity of methyleugenol against cowpea bettle.
The specific aims are:
To assess the efficiency of insecticidal properties of methyleugenol against major insect pest of cowpea.
To determine the concentrations that have effects on insects habits and behavior.
To assess the level of toxicity of methyl eugenol against cowpea bettle.
Cowpea (Vigna unguiculata L. Walp.), an annual legume, is also commonly referred to as southern pea, blackeye pea, crowder pea, lubia, niebe, coupe or frijole. Cowpea originated in Africa and is widely grown in Africa, Latin America, Southeast Asia and in the southern United States. It is chiefly used as a grain crop, for animal fodder, or as a vegetable. The history of cowpea dates to ancient West African cereal farming, 5 to 6 thousand years ago, where it was closely associated with the cultivation of sorghum and pearl millet. The Cowpea (Vigna unguiculata) is one of several species of the widely cultivated genus Vigna. Four cultivated subspecies are recognised:
Vigna unguiculata subsp. cylindrica Catjang
Vigna unguiculata subsp. dekindtiana
Vigna unguiculata subsp. sesquipedalis Yardlong bean
Vigna unguiculata subsp. unguiculata Black-eyed pea
Cowpeas are one of the most important food legume crops in the semi-arid tropics covering Asia, Africa, southern Europe and Central and South America. A drought-tolerant and warm-weather crop, cowpeas are well-adapted to the drier regions of the tropics, where other food legumes do not perform well. It also has the useful ability to fix atmospheric nitrogen through its root nodules, and it grows well in poor soils with more than 85% sand and with less than 0.2% organic matter and low levels of phosphorus. In addition, it is shade tolerant, and therefore, compatible as an intercrop with maize, millet, sorghum, sugarcane, and cotton. This makes cowpea an important component of traditional intercropping systems, especially in the complex and elegant subsistence farming systems of the dry savannas in sub-Saharan Africa.
Cowpea seed is a nutritious component in the human diet, as well as a nutritious livestock feed. Nutrient content of cowpea seed is summarized in Table 1.
Table 1. Nutrient content of mature cowpea seed (average of eight varieties).
1From Bressani R. Chap. 28 in Cowpea Research, Production and Utilization, Wiley and Sons.
The protein in cowpea seed is rich in the amino acids, lysine and tryptophan, compared to cereal grains; however, it is deficient in methionine and cystine when compared to animal proteins. Therefore, cowpea seed is valued as a nutritional supplement to cereals and an extender of animal proteins.
Cowpea can be used at all stages of growth as a vegetable crop. The tender green leaves are an important food source in Africa and are prepared as a pot herb, like spinach. Immature snapped pods are used in the same way as snapbeans, often being mixed with other foods. Green cowpea seeds are boiled as a fresh vegetable, or may be canned or frozen. Dry mature seeds are also suitable for boiling and canning.
In many areas of the world, the cowpea is the only available high quality legume hay for livestock feed. Digestibility and yield of certain cultivars have been shown to be comparable to alfalfa. Cowpea may be used green or as dry fodder. It also is used as a green manure crop, a nitrogen fixing crop, or for erosion control. Similar to other grain legumes, cowpea contains trypsin inhibitors which limit protein utilization.
III. Growth Habits:
Cowpea is a warm-season, annual, herbaceous legume. Plant types are often categorized as erect, semi-erect, prostrate (trailing), or climbing. There is much variability within the species. Growth habit ranges from indeterminate to fairly determinate with the non-vining types tending to be more determinate. Cowpea generally is strongly taprooted. Root depth has been measured at 95 in. 8 weeks after seeding.
Cowpea seed ranges in size from the very small wild types up to nearly 14 in. long and the number of seeds per pounds range from 1600 to 4300. Seed shape is a major characteristic correlated with seed development in the pod. Seeds develop a kidney shape if not restricted within the pod. When seed growth is restricted by the pod the seed becomes progressively more globular.
The seed coat can be either smooth or wrinkled and of various colors including white, cream, green, buff, red, brown, and black. Seed may also be speckled. mottled, or blotchy. Many are also referred to as "eyed" (blackeye, pinkeye purple hull, etc.) where the white colored hilum is surrounded by another color.
Emergence is epigeal (similar to common bean, and lupin) where the cotyledons emerge from the ground during germination. This type of emergence makes cowpea more susceptible to seedling injury, since the plant does not regenerate buds below the cotyledonary node.
The trifoliolate leaves develop alternately. Leaves are smooth, dull to shiny, and rarely pubescent. Commonly, the terminal leaflet is longer and larger than the lateral leaflets. There is a wide range in leaf size and shape.
Cowpea generally is day neutral. Flowers are borne in multiple racemes on 8 to 20 in. flower stalks (peduncles) that arise from the leaf axil. Two or three pods per peduncle are common and often four or more pods are carried on a single peduncle. The presence of these long peduncles is a distinguishing feature of cowpea and this characteristic also facilitates harvest. The open display of flowers above the foliage and the presence of floral nectaries contribute to the attraction of insects. Cowpea primarily is self pollinating.
Cowpea pods are smooth, 6 to 10 in. long, cylindrical and generally somewhat curved. As the seeds approach the green-mature stage for use as a vegetable, pod color may be distinctive, most commonly green. yellow or purple. As the seeds dry, pod color of the green and yellow types becomes tan or brown.
IV. Environment Requirements:
Cowpea is a warm-season crop well adapted to many areas of the humid tropics and temperate zones. It tolerates heat and dry conditions, but is intolerant of frost. Germination is rapid at temperatures above 65°F; colder temperatures slow germination.
Cowpeas are grown under both irrigated and non-irrigated regimes. The crop responds positively to irrigation but will also produce well under dryland conditions. Cowpea is more drought resistant than common bean. Drought resistance is one reason that cowpea is such an important crop in many underdeveloped parts of the world. If irrigation is used, more vegetative growth and some delay in maturity may result. Application rates should insure that the crop is not overwatered, especially in more northern latitudes, as this will suppress growth by lowering soil temperatures. The most critical moisture requiring period is just prior to and during bloom.
Cowpea performs well on a wide variety of soils and soil conditions, but performs best on well-drained sandy loams or sandy soils where soil pH is in the range of 5.5 to 6.5.
V. Cultural Practices:
A. Seedbed Preparation:
Soils should be cultivated deeply enough to insure that no barrier to penetration of the soil by the taproot (such as a hardpan) exists. Cowpea may be adversely affected by soil crusting under certain soil and environmental conditions.
B. Seeding Date:
Cowpea should not be planted until soil temperatures are consistently above 65°F and soil moisture is adequate for germination and growth. Seeds will decay in cool, wet soils.
C. Method and Rate of Seeding:
Traditionally, cowpea in the United States has been seeded in rows spaced 30 to 36 in. apart with seeds spaced 2 to 4 in. in the row. Recently, higher plant populations achieved by using narrow rows 12 to 20 in. have been used in commercial plantings. For forage purposes, the crop may be seeded in rows or broadcast (solid-seeded). Seed should be planted 1 to 1 1/2 in. deep and good seed-soil contact is important. The amount of seed to sow per acre depends on seed weight, germination percentage, and plant spacing. Recommended field seeding rates range from 18 to 22 lb/acre for viney, indeterminate types to 40 to 50 lbs for large-seeded determinate types. Optimum plant spacing depends on vine type. Highly determinate types may be planted 2 to 3 in. apart. Viney indeterminate types require more space, and a final stand with 8 to 9 in. between plants in 30 in. rows is considered to be a minimally acceptable population.
D. Fertility and Lime Requirements:
Cowpea, like all legumes, forms a symbiotic relationship with a specific soil bacterium (Rhizobium spp.). Rhizobium makes atmospheric nitrogen available to the plant by a process called nitrogen fixation. Fixation occurs in root nodules of the plant and the bacteria utilize sugars produced by the plant. Although cowpea Rhizobium is normally widespread, seed inoculation with Rhizobium specific to cowpea would be beneficial in areas where it is not present. Always use Rhizobium of the cowpea type.
Excess nitrogen (N) promotes lush vegetative growth, delays maturity, may reduce seed yield and may suppress nitrogen fixation. The plant will perform well under low N conditions due to a high capacity for N fixation. A starter N rate of around 27 lb/acre is sometimes required for early plant development on low-N soils.
Cowpeas have been grouped into the following market classes based on seed type and color:Black eye and purple eye-The immature pods shell easily because the hull (pod wall) is pliable and the seeds come out of the pod clean and free. The shelled peas are attractive, mild flavored and suitable for processing. The white hilum is surrounded by black, pink, or light-red.
Brown eye-Pods vary in color from green to lavender and have a wide range of lengths. The immature seeds, when cooked, are a medium to dark brown color, very tender, and have a delicate flavor.
Crowder-Seeds are closely crowded in the pods and tend to be globular in shape.
Cream-Seeds of these types are generally cream colored and have no noticeable "eye" (the hilum is inconspicuous).
Clay-These are generally older varieties that are medium to dark brown in color and kidney shaped. They are no longer commonly grown.
White acre-The peas are kidney shaped with a blunt end. This type is a semi-crowder, generally tan in color and somewhat small. Pods are quite stiff.
Weed Control: Adequate weed control is necessary for good growth and high yields.
1. Mechanical: Use of the rotary hoe and row cultivator in cowpea is similar to that of soybean. One or two rotary hoeings followed by timely cultivation should be done when no herbicides are used. One or more cultivations should also be done when herbicides are used.
2. Chemical: The term " cowpea" is not found on most herbicide labels. Rather, the crop is referred to as blackeyed peas, southern peas, pinkeyed peas or crowder peas. Farmers planning on producing cowpeas should check with their State Agricultural Extension Service for advice on chemical weed control.
Diseases and their Control:
Root rot and damping off are caused by three different fungi. Symptoms vary and include rapid death of young succulent plants, discoloration of taproots, longitudinal cracks of the stems, stunting, wilting and poor yields. Complete control of root rot and damping off is difficult, and no variety of cowpea is resistant to root rot. Persistent damp weather prior to development of the first true leaf and also the crowding of seedlings due to poor seed spacing may increase damping off. The following control practices help reduce losses from these diseases:
Fungal and viral diseases can be reduced by:
treating high quality seed with fungicides labeled for cowpeas.
applying cowpea-labeled fungicides in the furrow.
avoiding throwing soil against plant steins during cultivation.
a four or five year rotation with other crops.
seeding into warm, well-prepared soils.
planting certified seed of resistant varieties.
the removal of virus-affected plants.
Southern blight is caused by a fungus that attacks roots and stems of cowpeas. The occurrence of southern blight is not restricted to the South. The first visible symptom of southern blight is a progressive, yellowing and wilting of the foliage beginning on the lower leaves. The plant dies within a few days after the rust symptoms appear. A brownish vascular discoloration inside die stem may extend several inches above the soil line. During warm, moist conditions, the coarse, white mycelium of the fungus makes characteristic fan-shaped patterns of growth on the stem at the soil line. In this white-mat of the fungus, numerous smooth, round, light-tan to dark-brown mustard seed-like bodies called sclerotia are formed. In addition to the cultural practices listed above, bury previous crop debris and the sclerotia, at least 6 in. deep as far ahead of planting as possible.
Several viruses can attack cowpea. A characteristic symptom of the mosaic virus disease is an intermixing of light and dark-brown areas. Mottled areas are irregular in outline and may follow the main veins. Infected leaves are generally smaller than healthy ones, and often there is a slight puckering and curling of leaf edges. Infected plants usually are more dwarfed and bushy and yields are reduced. Mosaic diseases can also result in malformed pods. Plants infected during seedling stages may be barren and fail to produce. The best way to prevent large yield losses from virus diseases is to grow tolerant varieties.
Fusarium wilt usually causes the lower leaves on one side of the plant to turn yellow. Infected plants usually are stunted and wilted as the organism develops in the food and water conducting tissues. Brick red tissue can be observed in the stem when it is split lengthwise. The best control of Fusarium wilt is the use of resistant varieties. When resistant varieties are not used, it is important that root-knot nematode control practices be followed since nematodes increase plant susceptibility to Fusarium wilt.
Insects and Other Predators and Their Control:
Root-knot nematodes cause the root to appear knotted and galled. Above ground nematode symptoms appear as nutrient deficiencies, with stunting and often wilting because the root system is incapable of absorbing adequate amounts of water and nutrients. Do not confuse nematode root symptoms with the nodules of nitrogen fixing bacteria. Nodules are attached to sides of roots, and galls are within the roots. Root-knot nematodes can also be harmful to the cowpea because root injuries make the plants much more susceptible to attack by Fusarium wilt. In addition to detecting the presence of nematodes by observing galled roots, they can be detected by a soil test for nematodes. If nematodes are present certain practices help reduce nematode populations. These practices include crop rotation, fallowing, sanitation, weed control, and planting resistant varieties.
Cowpea curculio is a small weevil that causes blister-like spots on the surface of the pod. These spots result from adults puncturing the pod to feed on or to lay eggs. Punctures from feeding result in small malformed peas, and the results of egg laying are many legless grubs that destroy developing peas.
Aphids are small, green, soft-bodied insects that feed by piercing the plant tissue and withdrawing plant juices. Infestations of this pest develop on leaves and the fruiting stems. Their feeding, especially on the fruiting stem reduces the amount of plant nutrients available for pod and pea development. Infested foliage turns yellow and dies. Aphids excrete large quantities of a sugary substance called honey dew which supports the growth of sooty mold. Sooty mold, a fungus, is dark in color, which reduces the amount of sunlight that reaches the leaf. Mild damp weather favors development of aphid populations.
Green stink bugs cause damage by puncturing the pods and feeding on developing peas. In the Southern States, the lesser cornstalk borer and possibly other borers may be a problem, especially where cowpeas border fields of maturing corn or sorghum. Lesser cornstalk borer damage may be significantly reduced by clean cultivation at least two weeks prior to planting. In more northerly areas, some damage may be experienced from the European corn borer.
Cowpea can be harvested at three different stages of maturity: green snaps, green-mature, and dry. Depending on temperature, fresh-market (green-mature) peas are ready for harvest 16 to 17 days after bloom (60 to 90 days after planting). Harvest date for green snap pods is normally specified by the processor. Mechanical harvest requires the use of a snap bean or green pea harvester. Most domestic cowpea production is mechanically harvested, however, hand harvested cowpeas suffer less damage and the harvest season may continue over a 1 to 3 week period. One person can hand harvest 12 to 20 bushels of cowpea pods per day. Cowpea pods are packed, 25 pounds net, in bushel hampers or mesh bags (not burlap sacks).
Mature green cowpeas are normally harvested mechanically by some type of mobile viner. Dry cowpeas may be windrowed to facilitate drying or straight combined using a small grain or soybean combine.
J. Drying and Storage:
Harvested green cowpeas will "heat" resulting in spoilage unless kept cool. Post-harvest, provide shade and adequate ventilation is necessary on the way to the cooler. Cowpeas cooled below 45¡F may show chilling injury.
Dry cowpea seed is cleaned, graded, fumigated and packed in small plastic bags for sale to consumers.
The Cowpea Weevil: Callosobruchus maculatus (Fabricius)
This is an important pest that mainly attacks beans of various species, and can alternatively attack other pulse crops (see Lienard & Seck, 1994). This species originated in Africa but is now found all over the tropics and sub-tropics. Adults are 2-4 mm, brownish with black markings. They have a short life span of about 12 days and do not feed. Two forms of this species have been identified; the active (flying) form and the flightless form. The flying form disperses and colonises cowpea fields.
Bean beetles, Callosobruchus maculatus (Coleoptera: Bruchidae), are
agricultural pest insects of Africa and Asia that presently range throughout the
tropical and subtropical world. This species also is known as the southern
cowpea weevil. The larvae of this species feed and develop exclusively on the
seed of legumes (Fabaceae) hence the name bean beetle. The adults do not
require food or water and spend their limited lifespan (one - two weeks) mating
and laying eggs on beans. The systematic placement of bean beetles is as
follows: Callosobruchus is one of the genera in the family Bruchidae (seed
weevils) that is in the superfamily Chrysomeloidea. This group is part of the order
of beetles, Coleoptera (from Greek "sheath-winged" referring the stiff outer, first
pair of wings (elytra) that protect the membranous second pair of flight wings).
The Coleoptera is largest of the orders that comprise the class Insecta. Insects
are the largest and most diverse (750,000 described species) of all the animal
classes that are found in all but marine environments.
Bean beetles exhibit two adult forms (morphs), a sedentary (flightless) form and
a dispersal (flying) form. The dispersal morph is induced by high larval density in
stored beans or laboratory cultures, and is caused by density dependent microhabitat
temperature increases (Utida 1956, 1972). Induction of the dispersal
morph allows individuals to move to new, higher quality habitats. These two
(2) morphs have very different life history characteristics such as longer adult
lifespan in the dispersal morph and significantly reduced fecundity compared to
the sedentary morph (Utida 1956, 1972). In the sedentary form, the sexes are
highly dimorphic and readily distinguished but sex differences are very subtle in
the dispersal form. A very short adult life span and a larval stage in which
most or all life-time feeding occurs is not unusual in insects (for example the
Order Ephemeroptera, mayflies), but this life cycle seems strange compared to
the dominance of adult stages in familiar birds and mammals.Ecologically, bean beetles are
herbivores that have specialized on seed consumption. They are a part of food
webs in that eggs and larvae are prey for parasitoid wasp species (Boeke et al.
2003), and adults may be prey for birds, reptiles and amphibians, so they do
have the purpose of providing food for other organisms.
Adult beetles are small, about 2 to 3 mm long and somewhat teardrop and slightly elongate. They are pale to reddish brown with black and grey patches and two black spots near the middle on the wing cases. The posterior part of the abdomen is not covered by the wing cases. They do not feed on stored produce. They are very short-lived; usually they do not live longer than 12 days. During this time the females lay up to 115 eggs. The optimum temperature for egg-laying is about 30 to 35°C. The whole lifecycle takes about 4 to 5 weeks.
Once inseminated, adult females will lay (oviposit) single fertilized eggs on the external surface of a bean. Eggs are small (0.75mm long), clear, shiny and smooth, and oval or spindle shaped. They are firmly glued to the surface of pods and pulses. If the pods have opened, eggs are laid directly onto the seeds. Eggs are small, smooth, have domed structures with oval, flat bases. When newly laid they are translucent grey and inconspicuous. Upon hatching the empty eggs shells are white, and clearly visible to the naked eye. Eggs hatch within 5-6 days of oviposition.
The larvae are whitish and somewhat C-shaped with small heads. Upon hatching, they bite through the base of the eggs and bore into the seeds where they spend the whole lifecycle feeding on the seed. The larvae pupates inside the seed.
The larvae that hatches from the egg burrows from the egg through the seed coat and into
the bean endosperm without moving outside the protection of the egg. Once the
larva burrows into the bean, the remaining egg (shell) becomes opaque white
or mottled as it fills with frass (feces) from the larva. The larva burrows and feeds on the bean endosperm and embryo, undergoes a series of molts, and burrows to a position just underneath the seed coat prior to pupation
Although the seed coat of the bean is still intact, a round 1-2mm window is apparent at the location where the beetle is pupating .Pupation is the complete metamorphosis of the larval maggot to a winged adult. Pupation takes place in a chamber just under the testa of the seed. Pupation takes about seven days to complete.
. The adult that results from pupation chews through the seed coat and emerges from the bean . Adults are 2-3.5 mm long. The adults are fully mature 24 to
36 hours after emergence. Males seek females to inseminate ; and; females store viable sperm in their spermatheca (a structure in the female reproductive tract for storing sperm). Neither male nor female adults require food or water during their short adult lifetime (10-14 days).
Most bruchids are short, stout-bodied beetles with a short forewing not reaching the tip of the abdomen. Adults are characterised by their compact hairy bodies and relatively long antennae. Larvae of most species feed inside seeds and some develop in stored dry grains or legumes. All bruchids are phytophagous with most species able to avoid feeding on seed covers that contain toxins. This family contains several important field and stored crop pests.
Callosobruchus beetles are important pests of pulses. Infestation commonly begins in the field, where eggs are laid on maturing pods. As the pods dry, the pest's ability to infest them decreases. Thus, dry seeds stored in their pods are quite resistant to attack, whereas the threshed seeds are susceptible to attack throughout storage. Infestation may start in the pods before harvest and carry over into storage where substantial losses may occur. Levels of infestation in storage are strongly influenced by the type of storage structure employed and the variety of seed. Storage structures that maintain high levels of moisture in seeds are more prone to high levels of infestation. The value of dried pulses is strongly affected by levels of bruchid attack.
Cowpea bruchids are major pest of cowpeas, pigeon peas, soybean, green gram and lentils.
In the early stages of attack the only symptoms are the presence of eggs glued to the surface of the pulses. As development occurs entirely within the seed, the immature stages can normally not be seen. However, they can be detected after pupation takes place; although the seed coat of the bean is still intact, a round 1 to 2 mm window is apparent at the location where the beetle is pupating. The adults emerge through these windows leaving round holes in the grain that are the main evidence of damage.
3. Control Methods
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). Seeds maintained over 80 percentage germination for up to 12 months of storage under ambient conditions (Babu et al., 1989). 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). 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).
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).
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.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.
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), 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. 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.
Irradiation is the process by which an item is exposed to radiation. In common usage the term refers specifically to ionizing radiation, and to a level of radiation that will serve that specific purpose.Gamma irradiation is usally applied to grains such as cowpeas toprevent or control insects and pests infestation.
Pirimiphos-methyl is an organophosphate used as a pesticide. Pirimiphos-methyl can be applied as an interior surface paint additive, in order to achieve a residual pesticide effect.
Methyl eugenol is used in insect traps and lure products to attract certain fruit flies such as the Oriental fruit fly on affected food crops. It is naturally occurring in many plants and essential oils, and is a common flavoring ingredient added to foods, candy and gum.
Methods and material
The trials were conducted at the crop science department, university of Ghana ,legon , at the entomology laboratory
Callosobrochus maculatus were cultured in a controlled environment room at 27+ 2°C,60-70 relative humidity (r. h) at the entomology laboratory of the department of crop science. The food media used was whole cowpea grains.The insects
Methyl eugenol (4-allyl-1,2-dimethoxybenzene) is an alkyl chain-substituted guaialcohol. It is a member of the phenylpropanpids class of chemical compounds. It is clear to pale yellow oily liquid extracted from certain essential oils especially from clove oil. The chemical was diluted with analytical grade acetone to prepare series of concentrations. The concentrations of methyl eugenol were 0.1, 0 .01, 0.001, 0.0001g/ml, at 25°C. The insecticide (solvent) pirimiphos methyl was provided by the entomology laboratory of the crop science department.
Test arenas consisted of filter papers cut in half.Solutions of .1; .01;.001;.0001ml obtained from serial dilution of methyl eugenol (20-80%) were prepared using acetone as solvent.Each solution was applied to a half filter paper disc as uniformly as possible with a pipette. The other halves remained untreated .The treated and untreated filter papers were inserted into the petri dishes one aside the other. A control was set up using acetone only against non treated filter papers. After evaporation of solvent insects in group of 10 were then transferred to Petri dishes lined with treated and untreated filter papers and covered. Each treatment was replicated 4 times.The number of insects present on the treated strip with methyl eugenol and acetone for the control were recorded after 24hours exposure.Analysis of data were computed.
Contact toxicity by topical application
A stock solution of methyl eugenol (100%purity) was used to prepare serial dilutions of 0.1;0.01;0.001;0.0001 ml using acetone as a solvent. Insects were collected from their culture with the aspirator and transferred into Petri dishes lined with moist filter paper and chilled for 5 minutes to reduce their activity to enable application of treatment. Micro-applicator was used to apply 1.0µl of solution unto each insect thorax. Each Petri dish was filled with 10 insects each treated with each dose. Similar numbers were treated with acetone only as a control. Each treatment was replicated 4 times.Mortality was recorded every 24h for three days.The same process was followed with pirimiphos methyl as chemical(same concentration)
Contact toxicity on filter paper
Virgin filter paper was placed in Petri dish.Previous serial dilution preparation(100% methyl eugenol) made up of 0.1,0.01,0.001,0.0001 using acetone as a solvent was repeated and aliquot of it (1.0µl) was applied to the filter paper.The acetone was allowed to evaporate for 5 minutes.Then the insects (group of 10) were introduced in Petri dish. Treatments were replicated 4 times.Insects mortalities were recorded after 24 hours.