Essential oils of Cupressus lusitanica and Eucalyptus saligna as fumigants of stored product insect pests: The hope of the future for small-scale farmers

Laboratory studies were conducted in the Biotechnology Laboratory, Egerton University, Kenya to determine fumigant activity (space and grain fumigation) of essential oils of the Mexican cypress (C. lusitanica) and Sydney blue gum (E. Saligna) against major insect pests of stored food grains. Essential oils were obtained by hydro–distillation and evaluated for pesticidal activity against adult Tribolium castaneum, Acanthoscelides obtectus, Callosobrochus chinesis and Sitophilus zeamais. Bioassays were carried out under controlled temperatures (28±2oC) and relative humidity (65±5%) and laid out in completely randomised design with four replicates during the months of May to December, 2013. In the space fumigation test, essential oils were applied at five rates (0, 5, 10, 15 and 20 μl/L air) in the space fumigation chamber. On the other hand, in the grain fumigation test, 20 test insects in a metallic cage were placed in the grain fumigation chamber and assayed at 30, 50, 70 and 100 ìl/L air for 3-10 days. In both bioassays, the number of dead insects was recorded 24-168 h post- fumigation. Data on corrected percent mortality were homogenised using arcsine transformation before being subjected to analysis of variance (ANOVA) and means separated by Tukey’s HSD test. Results of both bioassays showed dose-, contact time- plant and insect-dependency (p<0.05). In space fumigation studies, C. lusitanica oil at 20 μl/L caused 67 and 100% mortality in T. castaneum and A. obtectus, respectively, 168 hr post-fumigation whereas E. saligna oil at 20 μl/L air, caused 80 and 100% mortality in T. castaneum and A. obtectus, respectively. Leaf essential oils of E. saligna and C. lusitanica at a low dose of 10μl/L evoked 100 and 85% kill, respectively, against adult C. chinesis after 168 h exposure. However, the two plant oils induced a relatively low fumigant toxicity of 65% and 67%, respectively against adult S. zeamais even at a higher dose of 20μl/L. In Grain fumigation, C. lusitanica was toxic to A. obtectus causing 93 - 100% mortalities and 12-17 % in T. castaneum at at100 μl/L after 3-10 days of exposure. Results confirm that C. lusitanica and E. saligna essential oils are promising grain fumigants with a possibility of incorporation in stored product insect pest management in small-holder agriculture.