Bioactivity of fenchone and Plectranthus glandulosus oil against Prostephanus truncates and two strains of Sitophilus zeamais

The toxic, repellent and reproduction inhibitory effects of Plectranthus glandulosus essential oil and one of its major compounds, fenchone, were evaluated against adult Prostephanus truncatus (Horn) and for two strains of Sitophilus zeamais Motschulsky from Cameroon and Germany. Maize grains were coated with the oil at 0, 10, 20, 40 and 80 ll/40 g grain and fenchone at 0, 20, 40, 80 and 120 ll/40 g grain. Mortality was assessed over a 7-day period, followed by the determination of F1 progeny production. The repellence of the two products was evaluated at five rates (0.5, 1, 3, 5, 10 ll/half disc) in a choice bioassay on filter paper. The oil and fenchone caused significant dose-dependent mortality to the three insects, with the oil achieving 100% and 90% mortality for the two S. zeamais strains and P. truncatus, respectively, within 1 day of exposure at the dosage of 80 ll/40 g grain. The insects were less susceptible to fenchone compared with the crude oil. The German strain of S. zeamais was susceptible to the oil than the Cameroonian strain. At the dosage of 20 ll/40 g grain, S. zeamais F1 progeny emergency was completely inhibited by the oil, while 80 ll/40 g grain treatment with fenchone prevented also insect emergence. Plectranthus glandulosus oil generally evoked a maximum per cent repellency of 100% for the three insects, but fenchone caused 55–60% to S. zeamais and 80% to P. truncatus. These results suggest that fenchone, though a major constituent of P. glandulosus oil, may only be a minor component of its bioactivity.