Utilization of Ruta graveolens and Azadirachta indica leaf powders and their binary combinations for the management of Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) in stored maize in Sri Lanka
Maize weevil (Sitophilus zeamais, Motsch.) is a ubiquitous and cosmopolitan field-to-store pest of maize of economic importance in tropical and subtropical regions of the world. The current renewed interest to replace synthetic insecticides has served as the impetus for the evaluation and intensification of eco-friendly anti-weevil measures such as the use of plant based bio-insecticides. This study was therefore, designed to investigate the insecticidal and repellent potency of leaf powders of Ruta graveolens and Azadirachta indica in single treatments and combinations of various proportions under ambient laboratory conditions against Sitophilus zeamais in both contact and vapour forms. For both plants, 100 g each of maize grains were mixed with leaf powders at different doses of 1.0, 3.0, 5.0, 7.0 and 10.0 g. The bio-potential of three combinations of 1:1, 1:2 and 2:1 of R. graveolens and A. indica leaf powders incorporating into the dosage level of 10.0g was also evaluated. Maize grains treated with R. graveolens leaf powder caused 100 % contact and 68 % fumigation mortalities within 24 hours of exposure whilst repelling 97 % and 96 % weevils within an hour in both contact and vapour forms respectively. A. indica powder was less efficacious compared to that of R. graveolens causing significantly lower repellency within an hour and mortalities after 10 days of storage. Application of 2:1 combination of R. graveolens and A. indica produced respective levels of 100 % and 73% contact and fumigation mortalities within 24 hours and highly significant repellent effects on S. zeamais, offering greater potential for large-scale storage since such combinations are relatively cheaper than using either of the botanicals singly. Leaf powders of the two plants under present investigation revealed their improved grain protecting capacity when used individually and in mixtures as bio-rational leads in large-scale protection of stored maize.
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