The insecticidal toxicities of essential oils and other active components extracted from Eucalyptus dives leaves (as well as structural analogues) were studied against stored-product insects, Plodia interpunctella and Tribolium castaneum. 3-Carvomenthenone was purified from E. dives oil, and the structures were elucidated by electron ionization mass spectra, 1H-nuclear magnetic resonance (NMR), 13C-NMR, heteronuclear multiple quantum coherence, 1H-1H correlation spectroscopy, and distortionless enhancement by polarization transfer NMR. Using the fumigant method against P. interpunctella larvae and adults, cyclohexenone exhibited the strongest toxicity (50% lethal dose [LD50] against larvae and adults, 2.45 and 3.63 μg/cm3), followed by methylcyclohexenone (2.95 and 4.24 μg/cm3), seudenone (3.02 and 4.44 μg/cm3), and 3-carvomenthenone (52.4 and 68.7 μg/cm3). Using the contact method, cyclohexenone (LD50 against larvae and adults, 17.25 and 19.35 μg/cm2) exhibited the most potent toxicities against T. castaneum larvae and adults, followed by methylcyclohexenone, seudenone, and 3-carvomenthenone. No functional radical on the backbone (2-cyclohexen-1-one) was more toxic than other chemicals. Structure-activity relationships between 3-carvomenthenone analogues and toxicities indicated that the toxicity of 3-carvomenthenone, cyclohexenone, methylcyclohexenone, and seudenone might be conferred through the dermal organs of T. castaneum and P. interpunctella. This study indicates that E. dives oil, 3-carvomenthenone, cyclohexenone, methylcyclohexenone, and seudenone have potential as natural agents to control stored-product insects.
Keywords: Carvomenthenone; Eucalyptus dives; Plodia interpunctella; Structure-activity relationships; Tribolium castaneum.