ABSTRACT
This study evaluated the efficacy of ethanolic extracts of Allium sativum (Woodwill) cloves, Aloe vera (Miller) leaves and their mixtureagainst Sitophilus zeamais (Motschulsky) under laboratory conditions. Experiments were carried out to check the toxicity, adult mortality, oviposition inhibition, repellent and anti-feedant effects of these plant extracts against S. zeamais. The experiments were laid out in a completely randomized design of ten treatments replicated three times. The adult weevils were exposed to maize grains treated with three concentrations (200, 250 and 300mg/ml) of A. sativum and A. vera and equal proportion (50:50) of their mixture while the control was treated with only the solvent. Probit analysis revealed A. sativum had lower LC50 than A. vera and therefore was more toxic than it. All treatments recorded higher significant (p<0.05) mortality than the experimental control. Most mortality occurred within 24hr of treatments showing that the extracts had no residual toxicity. Among the extracts used, the mixture recorded highest adult mortality (43.3%) followed by A. sativum (35%) and A. vera (30%). The plant extracts showed varying levels of repellency to the weevils with the highest repellency (66.7%) recorded by the mixture followed by A. sativum (44.4%) and A. vera (26.7%). The extracts were able to lower progeny production with the mixture having the highest percentage inhibition rate (61.5%) while the control recorded the lowest (19.1%). Percentage grains weight losses was significantly higher in the untreated check than in the treated grains and are recorded as follows: mixture (3.5 %), A. sativum (6.1%), A. vera (14.1%) and control (29.3%). The overall results show that the mixture of the two plant extracts performed better in all bioassays than individual plant extracts. The efficacy of the treatments were in the order: mixture > A. sativum > A. vera. Results also proved that A. sativum was a better biopesticide than A. vera. The effectiveness of the extracts was dose-dependent with the severity of their effects increasing with increase in concentration (200 < 250< 300mg/ml). The results in the present study shows that the ethanolic extract of A. sativum cloves, Aloe vera leaves and their mixture are toxic to S. zeamais and can suppress progeny development of the weevils by having oviposition inhibition and anti-feedant effects, thus resulting in better protection of maize grains from pest damage.
CHAPTER ONE
INTRODUCTION AND LITERATURE REVIEW
1.1 Introduction
Insects are a problem in stored grain throughout the world because they reduce the quantity and quality of grain. Grains constitute the most important staple foodstuff for the ever-growing population in the tropics. Grains are small, hard, dry seeds (with or without hull or fruit layers attached) harvested for human food or animal feed (Babcock, 1976). Agronomists also call the plants producing such seeds ‘grains’ or ‘grain crops’. Grains include cereals such as maize, rice, wheat, oats, barley, sorghum, millet etc; legumes or pulses such as soybean, cowpea, pigeon pea, peanut etc and oilseeds such as mustard seeds, canola, sunflower seeds etc. Cereal grains are all members of the grass family and they contain much starch, a carbohydrate that provides dietary energy. Legumes have higher protein than most other plant foods. They also contain starch or oil (Vaughan et al., 1997). Harvested, dry grain has advantages over other staple foods such as the starchy fruits (e.g., plantain, breadfruit) and roots/tubers (e.g., sweet potato, cassava, yam) in being easy to store, handle and transport. In particular, these qualities have allowed mechanical harvesting, transporting by rail or ship, long-term storage in grain silos, large-scale milling or pressing and industrial agriculture in general. Thus, major commodity exchanges deal with soybean, rice, wheat, maize and other grains but not in vegetables, tubers, or many other crops.
Grain storage has often resulted in quantitative and qualitative losses due to physical, chemical, and most importantly biological factors such as pests which may be birds, rodents, fungi, or insects (Oke and Muniru, 2001). The most important among storage pests are insects because apart from their direct damage they create conditions that allow secondary infestation by rot organisms mainly fungi (Lale, 2002).
Conservation of reserved food grain stocks is necessary to ensure a continuous supply at stable prices (Talukder, 2005). Losses are the most serious problem from insect infestation in grain storage, particularly in the developing countries, where poor sanitation and use of inappropriate storage facilities all encourage insect attack. It was estimated that over 20,000 species of field and storage pests destroy approximately one-third of the world’s food production, valued annually at over $100 billion, among which the highest losses (43% of potential production) occur in developing Asian and African countries (Jacobson, 1982; Ahmed and Grainge, 1986). Control of these pests relies on the widespread use of various synthetic chemical insecticides and fumigants. However, the application of various synthetic insecticides and fumigants to stored grains over the years has led to a number of problems such as environmental pollution, pesticide residue in food grains, development of insecticide resistance and toxicity to non-target organisms (Yusof and Ho, 1992; Cosimi et al., 2009).
The increasing public concern over pesticide safety and possible damage to the environment has resulted in increasing attention being given to natural products for the control of stored grain pests (Rajendran and Sriranjini, 2008). In this context, many plant products have been evaluated for their toxic properties against different stored grain pests, especially in the form of essential oils (Regnault-Roger, 1997; Rajendran and Sriranjini, 2008).
