INTRODUCTION AND LITERATURE REVIEW
The maize weevil (Sitophilus zeamais) Motschulsky (Coleoptera: Curculionidae) is a major pest of stored maize in the tropics and temperate regions of the world (Adedire, 2001). Its infestation causes severe post harvest losses of staple food crops in Nigeria leading to major economic loss (Oni and Ileke, 2008). The pest also infests other stored cereal grains as its alternative hosts. Notable among its secondary hosts is wheat that is one of the staple foods in Africa for combating malnutrition in young children. The destructive activities of insects and other storage pests have been adequately subdued by chemical control methods comprising fumigation of stored commodity with carbon disulphide, phosphine or dusting with malathion, carbaryl, primiphos methyl or permethrin. These chemicals have been reported to be effective against stored product pests (Ogunwolu and Idowu, 1994; Adedire et al., 2011). In developed countries, conventional fumigation technology is currently being scrutinized for many reasons such as ozone depletion potential or methyl bromide and carcinogenic concerns with phosphine (Adedire, 2002; Adedire et al., 2011). The problems of many synthetic insecticides include high persistence, poor knowledge of application, increasing costs of application, pest resurgence, genetic resistance by the insects and lethal effects on non-target organisms in addition to direct toxicity to users (Okonkwo and Okoye, 1996; Akinkurolere et al., 2006; Oni and Ileke, 2008).
Over the years, there is a steady increase in the use of plant products as a cheaper and ecologically safer means of controlling insect pest infestations of stored cereal and grains especially in the tropics (Lale, 1992; Adedire and Ajayi, 1996). Currently, attention is being given to the use of edible plant materials as grain protectants (Adedire and Lajide, 2003; Akinkurolere et al., 2006; Adedire et al., 2011). The tropics is well endowed with these plant species, some of which are also used for medicinal purposes. A fundamental knowledge of the biology of S. zeamais is a prerequisite for devising methods of efficient control. To do this, a sound knowledge about its response to the effects of environmental and biological factors is essential. Insect oviposition behaviour is an important contributor to the fitness of insects because of the consequent effect on the number and quality of offspring (Smith, 1986; Honek, 1993; Stejskal and Kucerova, 1996). Oviposition behaviour varies according to insect species and strain, population density, environmental conditions, food, age and size of the individual (Stejskal and Kucerova, 1996). Despite the importance of S. zeamais, there is need to have quantitative data describing its life history traits over ranges of environmental conditions. An understanding of the biology and behaviour of the maize weevil in relation to gain quantity will assist in the development of improved management practices for the control of the pest.
1.1.1 Maize as a staple food in Africa
Maize (Zea mays Linnaeus) belongs to the grass family Gramineae. It is an essential component of global food security. It is the third most important cereal grown in Nigeria next only to sorghum and millet (Adegbola, 1990). Maize is one of the most important cereal crops grown in the world and it forms one of the major diets of millions of people. In Africa, maize is primarily grown by small-scale farmers for use as both human food and animal feed. Its cob is consumed in different ways. For example; it could be grilled, boiled, roasted or milled into various products (Polaszek and Khan, 1998). Industrially, maize is used to produce alcohol, starch, pulp abrasive, oil and bio-fuel (Morris, 2007; Acharya and Young, 2008; Tom, 2008). In Zimbabwe, maize is used for beer brewing and as a medium of exchange for goods and services (Stanning, 1989). The principal producers of maize in sub-Saharan Africa are Kenya, South Africa, Tanzania, Ethiopia and Nigeria (Kfir, 1998; Seshu Reddy, 1998). Increased productivity in staple food, such as maize, is critical to raising rural incomes and stimulating broad-based economic growth (Eicher and Byerlee, 1997). The demand for maize in developing countries, unarguably, surpasses the demand for both wheat and rice. This is as a result of the growth in meat and poultry consumption, which consequently, have led to the rapid increase in the demand for maize as livestock feed. Thus, the exploding demand for maize presents an urgent challenge for most developing countries (Pingali and Pandey, 2000).
1.1.2 Constraints of maize production
Despite the worldwide increase in the demand for maize, its production is constrained by various biological, physical and chemical factors. These include the problems of insect attack, weeds and pathogen infestation, soil fertility and climate (Sanchez et al., 1997). In addition, the substitution of traditional cultivars by high-yielding varieties has raised the specter of massive maize failure because of increased susceptibility of the later to diseases and pests (Plucknett and Smith, 1982). Amidst other constraints of maize production, insects constitute a major threat. Insect pests destroy approximately 14% of all potential food production, including maize, despite the yearly application of more than 300 million kilogram of pesticides (Pimentel, 2007). Losing crops to insect pests constitutes a great constraint to the realization of food security for the ever increasing world population, it is necessary to address the issue of maize grain loss to insect pest damage (Berenbaum, 1995).
1.1.3 Justification of the study
The devastating loss of stored products to insect attack has necessitated the use of various measures to control maize weevils. Maize grains treated with certain materials such as wood ash, plant oils and plant powders have proven to be effective in the control of S. zeamais infestation (Lale, 1992). However, the formulation of these plant products into dosage and their adoption of their use in large scale storage had not been adequately addressed. Synthetic chemical insecticides are commonly used by maize farmers to protect grains from infestation. However, the widespread use of insecticides for the control of stored product insect pests is of global concern with respect to environmental health hazards, insecticide resistance development, chemical residues in food, side effects on non-target organisms and the associated high costs (Cherry et al., 2005; Adebe et al., 2009).
To this effect, the development of alternative control strategies such as the use of botanicals like Cymbopogon citratus Stapf (powder) in the control of S. zeamais is of paramount interest. The essence of this research work is to therefore, provide environmentally friendly and safer means of controlling S. zeamais in stored maize where sophisticated pesticide and insecticide of grains are not affordable especially among peasant farmers.
1.1.4 Objectives of the study
The overall aim of this research work was to evaluate the use of Cymbopogon citratus as insecticide against S. zeamais in stored product by studying the oviposition, mortality and longevity of S. zeamais treated with C. citratus extracts. The specific objectives were to:
- determine the phyto-chemical constituents of citratus,
(ii) assess the toxicity of C. citratus powder against S. zeamais,
- determine the efficacy of different extracts of citratus against S. zeamais in maize and
- determine the effect of the extract of citratus on natality, repellency and longevity of S. zeamais.
1.2 Literature Review
Maize weevil (Sitophilus zeamais) (Fig.1) is a small snout beetle which varies in size 3 – 3.5 mm long, dark brown-black in colour and shiny and pitted with numerous punctures. The punctures on the thorax are in irregular pattern, while those on the elytra (wing cases) are in lines. The elytra also usually have four pale reddish-brown or orange- brown oval markings (Youdeowei, 1993). The maize weevil has fully developed wings beneath its wing covers and can fly readily. The maize weevil has a characteristic rostrum (snout or beak) and elbowed antennae of the family Curculionidae (weevil family). The antennae has eight segments and are often carried in an extended position when the insect is walking (Fenemore and Prakash, 2006).
Host range: The maize weevil commonly attacks standing crops, in particular, maize before harvest and is also commonly associated with rice (Nardon and Nardon, 2002). It infests raw or processed cereals such as wheat, oats, barley, sorghum, rye and buckwheat. It can breed in crops with a moisture content of a much wider range than Sitophilus oryzae L. and has been found in fruit such as apples during storage. Although the maize weevil cannot readily breed in finely processed grains, it can easily breed in products such as macaroni, noodles, and milled cereals that have been exposed to excessive moisture (Mason, 2003).
Distribution: S. zeamais occurs throughout warm, humid regions around the world, especially in locations where maize is grown.
- zeamais belongs to the order Coleoptera and the family Curculionidae…
Life cycle: Female chew into grains where they lay their eggs throughout most of their adult life of up to one year, although 50% of their eggs may be laid in the first 4 – 5 weeks. Each female may lay up to 150 eggs in her life time. Eggs hatches few days into soft, white, legless, fleshy grub which feeds on the interior of the grain kernel. The grub changes to a naked white pupa and later emerges as an adult beetle (Proctor, 2010). The rate of development is slightly slower for the maize weevil than the rice weevil. A minimum of 30 days is required for passing through the egg, larval and pupal stages (Mason, 2003). Development time ranges from 30 days under optimal conditions to over 110 days in unfavourable conditions. Eggs, larval and pupal stages are all found within tunnels and chambers bored in the grain and are thus not normally seen on the outside (CABI, 2010). The larval stages feed on the internal parts of the grain and as such, it is difficult to detect infestations early. Adult emerges from the grain and can be seen walking over the grain surfaces. Adult emergence holes are large with irregular edges. Females release a sex pheromone which attracts males (Fenemore and Prakash, 2006).
Damage and detection: Early detection of infestation is difficult. As S. zeamais larvae feed on the interior of individual grains, often leaving only the hulls, a flour-like grain dust, mixed with frass is evident. Infested grains contain holes through which adults have emerged. A possible indication of grain infestation is that grain when placed in water, floating to the surface (Meikle et al., 1999). Ragged holes in individual grains, similar to damage caused by rice weevil and granary weevil, may indicate infestation. In large stores of grain, an increase in temperature may be detected. The most obvious sign of infestation is the emergence of adults. One study recorded, 5 weeks after infestation, the emergence of 100 adults per kg per day (Meikle et al., 1999). Damages caused are formation of heat cavities in the grain mass, pollution of the grain mass, dissemination of molds in the grain, depreciation of the product and low per germination (Oduor et al., 2000). In Africa, subsistence grain production supports the livelihoods of the majority of the population (Udo, 2005). Insect infestation of maize grains leads to the reduction in both quality and quantity of harvested crops and in most cases pre-disposes the stored grains to secondary attack by disease causing pathogens (Evans, 1987). Grain losses as high as 80% have been reported in developing countries (Pingali and Pandey, 2000; Tapondjou et al., 2002). Post-harvest losses due to S. zeamais have been recognized as an important constraint to grain storage in Africa (Markham et al., 1994; Oduor et al., 2000).
1.2.2. Cymbopogon citratus
Cymbopogon citratus (Fig. 2) belongs to the order Poales and the family Poaceae. Cymbopogon popularly known as citronella grass or lemon grass comprises approximately 500 genera and 8,000 herb species (Barbosa et al., 2008). Lemon grass is a tufted perennial grass growing to a height of 1 m with numerous stiff leafy stems arising from short rhizomatous roots. It has an economic life span of about 5 years (Carianne de Boer, 2005). The leaf blade is linear, tapered at both ends and can grow to a length of 50 cm and width of 1.5 cm. The leaf sheath is tubular in shape and acts as a pseudostem. Leaves are long, glacous, green, linear tapering upwards and along the margins. The rhizome produces new suckers that extend vertically as tillers to form dense chumps (Tajidin et al., 2012; Karkalla and Bhushan, 2013).
Cymbopogon citratus is an economically important aromatic perennial plant that has been used to extract essential oils. It is grown around the world and has a century-long record of extensive therapeutic applications in traditional and ayurvedic medicine in a number of countries (Aftab et al., 2011; Tarkang et al., 2012). It is used in herbal medicine for a wide range of applications based on its antibacterial (Wannissorn et al., 2005), antifungal (Nakagawa et al., 2003; Irkin and Korukluoglu, 2009); antiprotozoal (Holetz et al., 2003), anti-carcinogenic (Puatonachokchai et al., 2002), anti-inflammatory (Abe et al., 2004), antioxidant (Masuda et al., 2008; Garg et al.,