Biological effects of plant extracts against the rice weevil Sitophilus oryzae in stored maize
Toxicity and protectant potential of chloroform extract of the leaves of the siam weed Chromolaena odorata (L.) and mahogany tree Khaya senegalensis (Ders.) against the rice weevil Sitophilus oryzae (L.) were assessed in the laboratory using contact toxicity, progeny production, damage assessment and repellency assays. The extract of C. odorata was moderately toxic to S. oryzae but that of K. senegalensis was highly toxic to the weevils, evoking 100% mortality in maize treated with the highest dosage after 72 h of exposure. Grains treated with extract of the two plant materials caused a highly significant reduction in the number of progeny produced by S. oryzae and the higher dosages completely inhibited progeny production by the weevils. Leaf extracts of both plant species were highly repellent to S. oryzae and repellency was dose-dependent. The extract of C. odorata and K. senegalensis lost their toxicity after only 24 h following application against the weevils. Phytochemical screening of the plant extract indicates that K. senegalensis contains chiefly saponins, alkaloids, cardiac glycosides and some anthraquinones. C. odorata, on the other hand, contains mainly alkaloids and flavonoids. The potential practical use of leaf extracts of the two plant species as grain protectants in farm-stored grains in rural communities in Ghana is discussed.
JOURNAL OF THE GHANA SCIENCE ASSOCIATION Volume 2 No. 2 (2000) pp. 62-69