Larvicidal activity against Anopheles gambiae Giles and chemical composition of essential oils from four plants cultivated in Cameroon

Larvicidal activity against Anopheles gambiae Giles and chemical composition of essential oils from four plants cultivated in Cameroon. The chemical composition of the essential oils obtained by hydrodistillation of dry leaves from Cymbopogon citrates (DC.) Stapf, Ocimum canum Sims, Ocimum gratissimum L. var 'gratissimum' L. and Thymus vulgaris L. cultivated in Cameroon were analyzed and their larvicidal activity against fourth instar larvae of Anopheles gambiae Giles were determined. The yields of extraction indicate that T. vulgaris (0.95%) was richer in essential oil than C. citratus (0.67%). O. canum and O. gratissimum have approximately the same content in volatile constituents (0.59% and 0.60%, respectively). The analyses by GC and GC/MS showed that these oils are monoterpenic (86.8-97.4%). Oxygenated monoterpenes predominate in C. citratus and O. canum (81.6% and 68.9%, respectively) while O. gratissimum oil contains a majority of monoterpene hydrocarbons (61.0%). T. vulgaris is characterized by the same proportion of monoterpene hydrocarbons (45.6%) and oxygenated monoterpenes (48.9%). The main compounds found in the essential oil of C. citratus are acyclic monoterpenes such as geraniol (15.6%), geranial (39.3%), neral (21.9%) and myrcene (14.0%). The essential oil of T. vulgaris is characterized by p-menthane structures given by thymol (40.1%), p-cymene (23.4%) and gamma-terpinene (15.1%); p-cymene (32.1%) and thymol (24.3%) were also the constituents quantitatively important in O. gratissimum essential oil while linalool (56.3%) and limonene (10.9%) were predominant in O. canum. Bioassay test done by the World Health Organization standard protocol revealed that these essential oils have remarkable larvicidal properties as they could induce 100% mortality in the larvae of A. gambiae at the concentration of 100 ppm for C. citratus, 200 ppm with T. vulgaris, 350 ppm for O. gratissimum and 400 ppm for O. canum. Their LC50 and LC80 show the same reactivity order. The essential oil of C. citratus was found to be the most efficient, with respective values of: LC50 = 18 ppm and LC80 = 25 ppm.