Insecticidal Effects of Monoterpenes on Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae)

Twenty eight monoterpenes including monoterpene hydrocarbons and oxygenated monoterpenes (borneol, borynl acetate, camphene, camphor, 3-carene, carvone, 1,8-cineole, citronellal, β-citronellene, β-citronellol, dihydrocarvone, fenchol, fenchone, geranyl acetate, isomenthol, limonene, limonene oxide, linalool, linalyl acetate, menthol, menthone, myrcene, nerol, neryl acetate, α-pinene, β-pinene, terpinen-4-ol, α-terpineol), the active compounds of essential oils obtained from different plant species were tested against adults of Sitophilus zeamais Motschulsky under laboratory conditions. The monoterpenes were applied at contents of 10, 20 and 30 μl for liquid compounds and 10, 20 and 30 μg for solid compounds. The results show that most of the monoterpenes have significantly insecticidal effect on the tested insects. Insecticidal effects of monoterpene hydrocarbons were found to be lower than those of oxygenated monoterpenes. The ketone and aldehyde and epoxide derivatives of oxygenated monoterpenes were also found to be more toxic as compared with their other derivatives. Mortality percentage of S. zeamais adults, after 96th h of exposure at the maximum dose (30 μl/μg) of oxygenated monoterpenes including borneol, fenchol, linalool, menthol, terpinen-4-ol, α-terpineol (alcohols group); 1,8-cineole, limonene oxide (epoxides group); camphor, carvone, citronellal, dihydrocarvone, fenchone, menthone (ketones and aldehydes group) and neryl acetate (esters group) attained 100%. Concurrently, 3-carene from monoterpene hydrocarbons showed 100% mortality after 96th h of exposure at the maximum dose (30 μl). Carvone, dihydrocarvone, fenchone, limonene oxide, menthone and terpinen-4-ol from these compounds showed 100% insecticidal effect after 48th h of exposure. Among the monoterpenes tested, carvone, dihydrocarvone, menthone and terpinen-4-ol showed the strongest insecticidal activities with 100% of mortality at all doses (96 h after treatment) and then 1,8-cineole, fenchone, linalool and limonene oxide showed stronger insecticidal activities in comparison with other monoterpenes with lethal doses (LD50) values of 1.989, 2.445, 2.445 and 3.235 μl (96 h after treatment) against the test insects, respectively. Mortality rate of S. zeamais adults increased significantly (p < 0.01), as the dosage level and/or exposure time increased. Based on the present results, it can be concluded that the oxygenated monoterpenes may have a potential action for control of S. zeamais a