Phytochemical compositions and Adulticidal Effectiveness of Indian Borage, Coleus amboinicus Lour, against Maize Weevil, Sitophilus zeamais Motschulsky [Coleoptera: Curculionidae]

The use of synthetic chemical insecticides in the management of food grains pest have a serious adverse effect on consumer’s health and the environment. Public awareness of the adverse effects of the synthetic chemical insecticides has called for the urgent need to replace this method with safer and cheap method. This research evaluated the adulticidal effects of Coleus amboinicus leaf, stem and root powders and crude extracts against Sitophilus zeamais . The plant powders were tested at dosage 0.1, 0.3, 0.6, 0.9 and 1.5 g per 20 grams, while extracts were tested at concentration 0.1, 0.2, 0.3, 0.4 and 0.5 ml per 20 grams of maize seeds. The results revealed that Coleus amboinicus leaf powder at dosage 1.5g caused 63% mortality of adult S. zeamais after a day of treatment. This followed by C. amboinicus stem that evoked 56% weevil mortality. This research showed that C. amboinicus extracts were more toxic than powders of the tested Coleus amboinicus parts. The lethal concentration of leaf, stem and root extracts which caused 50% mortality in the population of S. zeamais response after 96 hours of exposure were 0.13 ml, 0.27ml and 0.48ml respectively. Coleus amboinicus leaf proved to be the most toxic to maize weevil, followed by stem powder and extract. The study established C. amboinicus leaf, stem, and root contain varying levels of phytochemicals, such as alkaloids, saponin, flavonoids, and tannin, which are responsible for the adulticidal toxicity to the maize weevil.