Abstract:
The botanical insecticides are generally pest-specific, relatively harmless to non-target organisms and biodegradable. Unlike conventional insecticides which are based on a single active ingredient, plant-derived insecticides comprise of an array of chemical compounds which act concertedly on both behavioural and physiological processes. Thus the chances of pest developing resistance to such substances are less likely. Essential oils are good example of plant compounds with such properties. Toxicity of a large number of essential oils and their constituents has been evaluated against several stored-product pests. Consequently, determination of the extraction yield of the essential oils and comparison of the chemical composition of essential oil of different plant species and chemotypes is an important step in promoting and understanding their use as insecticides. This study involved characterisation of essential oils of Lantana camara, Lantana trifolia, Ocimum americanum and Tephrosia vogelii, which have known repellent, insecticidal and antifeedant activities against various insect pests. The fresh leaves of L. camara, L. trifolia and T. vogelii were collected and hydro-distilled for 3h in a modified Clevenger apparatus. The essential oil from the leaves of L. lantana and T. vogelii was obtained in a yield of 0.63% w/w and 0.056%, respectively, after drying over anhydrous Na2SO4. The leaves and flowers of L. trifolia gave 0.060% and 0.064% of essential oil respectively. The leaves of O. americanum were collected, air-dried under shade to constant weight and ground to a fine powder. The essential oil obtained by steam distillation using a modified clevenger apparatus gave 4% based on dry weight. The main components of the essential oil from L. camara were β-caryophyllene (21.35%), α-humulene (8.73%), sabinene (9.15%), 1, 8-cineole (5.54%). L. trifolia essential oil had germacrene D as the main component in leaves (23.14%) and flowers (24.88%). Other components in essential oil from the leaves' were cis-ocimene (7.27%), ß-caryophyllene (6.32%); and ß-caryophyllene (11.93%) and α- humulene (4.65%) in flowers. In O. americanum essential oil the major components were
terpinen-4-ol (43.21%) and 1, 8-cineole (16.13%) while in T. vogelii essential oil nerolidol (7.73%), β-caryophylene (6.89%), caryophyllene oxide (6.22%) and germacrene D (6.20%) were the main components. A number of compounds in the oils are known to exhibit insecticidal and pharmacological properties, and are used in the cosmetic, flavour and fragrance industries
