ABSTRACT
A study was conducted to determine the efficacy of extracts of Cymbopogon citratus (Stapf) as post harvest protectants of maize seeds against Sitophilus zeamais Motsch, field to store insect pest of maize. For contact toxicity, 1 g each of acetone, chloroform, methanol and aqueous extracts was re-suspended in 5 ml of de-ionized water and used to impregnate filter papers, ten insects were introduced into the filter papers and observed daily for 28 days, the set up was replicated four times. Also, aqueous extracts of C. citratus from dose rates of 1.0 g, 2.0 g, 3.0 g, 4.0 g, 5.0 g and 6.0 g / 50 g of grains infested with 10 pairs of sexed S. zeamais for 28 days were tested for insecticidal properties and compared with experimental control and the synthetic conventional insecticide [Coopex (0.25 g)] against S. zeamais under laboratory conditions of temperature and relative humidity (28 ± 2oC; 75 ± 5% RH) and laid in a completely randomized design. Parameters assessed were effect of C. citratus extracts on weevil mortality (toxicity test), natality, repellency and longevity of S. zeamais treated with C. citratus extracts. Data obtained were analyszed using Analysis of Variance (ANOVA) and means were separated using New Duncan Multiple Range Test at 5% level of significance. Results showed that the insecticidal compounds found in C. citratus extract were flavonoids, phenolic, alkaloids, saponins, to which the insecticidal properties were attributed. Also, results showed significant (p<0.05) increase in mortality of over 50% tested at 1g of the extract, and at higher treatment of 6 g, 90% mortality was recorded as the extract suppressed adult emergence compared to the experimental control. The results also revealed that the efficacy of C. citratus extract on the weevils were dose-dependent with higher doses providing greater protection of the maize grains. Progeny development was significantly (p<0.05) inhibited as fewer adults emerged from treated grains when compared with the control. The insecticidal potential of C. citratus extracts and its ready availability made it an attractive biopesticide in upgrading traditional post-harvest protection practices. The outcome of this study showed some promise in ensuring a steady supply of good quality maize grains, hence further investigation should be done to know the active ingredients that caused the actual mortality of the insect pest. The plant used in controlling this weevil is harmless and so, efforts should be intensified toward their cultivation, packaging and application on a large scale as botanical insecticides.
CHAPTER ONE
INTRODUCTION AND LITERATURE REVIEW
1.1 Introduction
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.
