Chemical composition of ocimum kilimandscharicum and ocimum americanum and their bioactivities against sitophilus zeamais
The maize weevil, Sitophilus zeamais (Motchulsky), the lesser grain borer, Rhyzopertha dominica (Fabricius), and the Angoumois grain moth, Sitotroga cerealella (Oliver), are serious pests of stored grain in Kenya. The control of these insect pests relies heavily on the use of synthetic insecticides by better off farmers; but there is an increasing cost of application and erratic supply in developing countries. In addition, small-scale farmers cannot afford these commercial products and there increasing cases of resistance development by the pests. In contrast with synthetic pesticides, the traditional uses of plants-derived pesticides are usually simple, eco-friendly, cost–effective and accessible to communities with minimal external input. The major challenge is lack of sufficient research on the efficacy of these products, epigenetic (chemotypic) differences associated with a given plant species growing in different environments. Also there is lack of effective downstream promotion of promising local plants and their products. In this study, essential oils of Ocimum kilimandscharicum growing in Limuru, Ngong, Kasarani, Kakamega and Trans-Nzoia regions and Ocimum americanum from Machakos region were screened for their repellence and fumigant toxicity against maize weevils. The aerial parts of the plants were hydrodistilled using Clevenger apparatus. Analyses of the essential oils collected, and identification of their constituents was performed with Gas Chromatograph (GC) with FID detector and GC-linked Spectrometer (MS). Comparison of mass spectra of individual constituents and with NIST data led to the identification of prominent constituents. From the analyses, 9 constituents were identified in both O. kilimandscharicum essential oils and 9 from O. americanum. Camphor was the major constituent in O. kilimandscharicum growing in all areas. 1,8-Cineole, which was a minor component in O. kilimandscharicum, was the major component in O. americanum. Subtractive blend mortality bioassays showed that camphor is responsible for 82% activity of O. kilimandscharicum oils, while 1,8-cineole contributed 18% of the activity, with the other compounds contributing less than 5%. Probit analysis showed that essential oils of plants collected from Limuru and Ngong regions were more toxic against the maize weevils. Plants from Kakamega region had the lowest activity both in fumigation and damage assays. Essential oils of O. kilimandscharicum from Trans-Nzoia and Ngong regions had the highest repellence at 93% against the maize weevils. O. kilimandscharicum essential oil was more repellent to maize weevil than O. americanum. The germination of the maize seeds was not affected when the seeds were treated with the essential oils of both O. americanum and O. kilimandscharicum. Maize grains treated with higher dosage of O. kilimandscharicum had the highest germination rate at 90%. The ground powdered materials provided less protection against S. zeamais. Essential oils could be used by small scales farmers as maize protectants against maize weevils. This study lays ground for using essential oils by small scales farmers to control post-harvest pests.