Effects of essential oils of lantana camara and two ocimum species on bean weevil (acanthoscelides obtectus) and their chemical compositions
Syombua, Muinde Esther
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Crude plant products have been widely used by different communities in Africa for pest control. Synthetic insecticides have a number of drawbacks, such as development of genetic resistance, eco-toxicity, negative health effects to users and high cost of application. However, not enough scientific research has been conducted on the efficacy of traditional methods in weevil control, the chemical profiles of plant products and their safety to the users. The bioactivities of the powedered products and essential oils of three plants, namely Ocimum gratissimum, Ocimum americanum and Lantana camara, growing in Eastern Kenya against common bean weevil (Acanthoscelides obtectus) were determined. Actellic Super and untreated bean grains were used as positive and negative control, respectively. The powdered plant materials (dried leaves) induced relatively low mortality, with the most effective O. americanum (Kitui) causing 52% mortality at dose level of 30% w/w after 28 days. The essential oils of all the three plants were found to have significant repellency to the bruchid. The most repellent essential oil was that of L. camara (Kitui) (RD50 0.26 μl per cm2), followed by O. gratissimum (Kitui) (RD50 0.33 μl per cm2), then O. americanum (Kitui) (RD50 0.44 μl per cm2). However, the repellencies of the essential oils of the same species of plants from Machakos region were lower [L. camara (Machakos) RD50 0.62 μl per cm2; O. americanum (Machakos) RD50 0.75 μl per cm2 and O. gratissimum (Machakos) RD50 0.92μl per cm2]. The essential oils of the three plants also showed significant mortality, with LD50 of that of O. americanum (Kitui) being 5.28μl per cm2, that of O. gratissimum (Kitui) being 5.39 μl per cm2 and that of L. camara (Kitui), 14.55 μl per cm2. The essential oils of the plants from Machakos region gave the following mortalities: O. americanum (Machakos) LD50 34.20 μl per cm2, O. gratissimum (Machakos) LD50 89.93 μl per cm2 and L. camara (Machakos) LD50 19.48 μl per cm2. The oil from O. americanum (Kitui) gave the best mortality. The essential oils were analysed by gas chromatography (GC) and gas chromatography-linked mass spectrometry (GC-MS) and the major constituent compounds identified by comparison with MS database and GC co-injections with authentic samples. The relative amounts of different constituents of oils of the plants from the two regions varied sharply. Subtractive bioassays were carried out to characterize the constituents in the essential oil blends responsible for mortality. The most effective essential oil was O. americanum whose constituent compounds were 1,8-cineole with mortality of 51%, α-terpineol 39% and lastly linalool 28%. The results provide a baseline for the use of O. americanum and L. camara essential oils as bio-organic post-harvest pest control agents.