BIOACTIVITY OF AQUEOUS AND N-HEXANE NEEM LEAF EXTRACTS AGAINST THE COWPEA WEEVIL, Callosobruchus maculatus (F)

The incessant problems encountered with overreliance on synthetic insecticides have necessitated the search for alternative control strategies which are safe, effective and affordable. This study investigated the bioactivity of neem (Azadirachta indica A. Juss.) leaf extracts against the storage pest of cowpea, Callosobruchus maculatus (F.), under laboratory conditions. The weevils were reared on cowpea seeds treated with aqueous and n-hexane leaf extracts at concentrations 0.5, 1.0, 2.0 and 3.0 mg/ml to compare toxicity, anti-oviposition effect, adult emergence and adult longevity. After 24 h of exposure, observed mortality was 63%, 73% and 97% for weevils poisoned with 3.0 mg/ml aqueous, 2.0 mg/ml n-hexane and 3.0 mg/ml n-hexane leaf extracts, respectively. The 24-h median lethal concentrations (LC50) obtained for aqueous and n-hexane extracts were 5.28 ± 1.25 mg/ml and 2.27 ± 0.13 mg/ml, respectively with corresponding median lethal time (LT50) values of 36.14 ± 3.73 h and 21.69 ± 3.17 h. Female weevils reared on treated cowpea seeds laid significantly fewer eggs and adult longevity was significantly shortened. Adult emergence was also reduced significantly in a concentration-dependent manner and there was no offspring emergence from seeds treated with high concentrations of n-hexane extract. However, leaf extracts did not alter the natural sex ratio of the insect pest. The results from this study showed that neem leaf extract is effective in controlling C. maculatus and could, therefore, serve as an alternative to synthetic insecticides in controlling the storage pes