Toxicity and repellant potency of Hyptis spicigera extracts on Sitophilus zeamais motschulsky (Coleoptera: Curculionidae)

Abstract

Bush mint, Hyptis spicigera has been used traditionally as a cereal and legume protectant against storage insect pests. The objective of this study was to evaluate the toxic and repellant activity of H. spicigera extracts against Sitophilus zeamais, a major insect pest of stored cereal grains in the tropics. Essential oils and solvent extracts were tested against the maize weevil in fumigation, contact and repellence bioassays. Results obtained showed that essential oils had strong fumigant toxicity against S. zeamais with a LC50 value of 48.11 µL/l air at 48 h. Contact toxicity assay gave more than 50% mortality at 48 h of exposure for methanol extracts and not hexane extracts. Hexane extracts had higher repellant activity than the methanol extracts at the same dosage. GC-MS revealed that essential oils is composed of at least fifteen mono- and sesquiterpenoids, with the major constituents being α-pinene (27.4%), β-pinene (17.6%), L-phellandrene (12.2%), α-thujene (12.2%), isocaryophyllene (9.2%), and limonene (9.4%). Bush mint may be exploited for the development of botanical insecticides to be used in weevil management.