GC-MS analysis of pesticidal essential oils from four Kenyan plants
Essential oils are complex mixtures of odours and steam volatile compounds which are deposited by plants in the subcuticular space of glandular hairs, in excretory cavities and canals or exceptionally in heart wood. Essential oils have been found to have no specific biological functions in plants, but constitute many compounds that are insect repellents or act to alter insect feeding behavior, growth and development, ecdysis (moulting) and behavior during mating and oviposition. Others possess antifungal, insecticidal and antiseptic properties. Essential oils of leaves of Tagetes minuta L. (Asteraceae), Fuerstia africana T.C.E. Friers (Labiaae), Tephrosia vogelii Hook. f. (Leguminosae) and Sphaeranthus ukambensis were obtained by steam distillation using the Clevenger apparatus (Guenter, 1949). Compounds in the essential oils were identified by gas chromatography-mass spectrometry (GC-MS). The characteristic volatiles isolated from the four plants were identified as follows: major constituents of the essential oils from F. africana included the following: limonene (39.1%), (Z)-β-ocimene (30.5%), targetone<dihydro> (26.2%), E -targetone (59%), λ-terpinene (26.2 %), peripitenone 23.7%, ocimene allo (22.8%) and Z-targetone (100%). The compounds with the highest relative abundance value in the essential oils included Z-targetone, E-targetone and (Z)-β-Ocimene. Analysis of T. minuta also revealed a number of compounds which included mycene (20.1%), verbinene (8.0%), Z-ocimenone (35.2%), E-caryophylene (25.5%), α-Humelene (11.8%), germacrene D (21.4%) and camphene (2%). The major constituent of T. minuta was Z-ocimene, caryophylene E and mycene. Essential oils from T. vogelii as revealed by the GC-MS analysis were pinene α (32.7%), limonene (35.1%), copaene α (22.9%), β -elemene (36.7%), Z-nerolidol (77.7%), δ-cadinene (67.6%), α humelene (69.6%) and -4-α-ol-(β)copaene (65.7%). The highest proportion of the essential oils constituted Z- nerolidol δ-cadinene, α-humelene and 4-α-ol-β. Copaene. The compounds in essential oils obtained from S. ukambensis were α-copaene (23.8%), β-bourbonene (25.5%), α-gurjunene (14.3%), cymene<2, 5-dimethoxy-para (87.7%), α -humelene (100%), λ- muurolene (17.9%), λ-cadinene (77.3%), caryophylene oxide (54.7%) and δ-cadinene (61.9%). The major compounds contained in essential oils from S. ukambensis were α-humele, λ-cadinene, δ-cadinene and cymene<2, 5-dimethoxy-para. The compounds which were common in the four test plants included: α-pinene, α-humele, ocimene allo and (E) β-ocimene. Each of the test plant secreted essential oils constituting numerous volatiles known to exhibit acaricidal, insecticidal and/or arthropod repellent properties. These plants may be useful sources of compounds for use in the control of arthropods of medical, veterinary and agricultural importance.
Key words: Essential oils, GC-MS, Tagetes minuta, F. africana, T. vogelii, S. ukambensis