Toxicity of the Essential Oil of Illicium difengpi Stem Bark and Its Constituent Compounds Towards Two Grain Storage Insects

During our screening program for new agrochemicals from Chinese medicinal herbs, the essential oil of Illicium difengpi stem bark was found to possess strong insecticidal activities against the maize weevil, Sitophilus zeamais (Motschulsky) (Coleoptera: Curculionidae) and red flour beetle, Tribolium castaneum Herbst (Coleoptera: Tenebrionidae). A total of 37 components of the essential oil of I. difengpi were identified. The main components of the essential oil were safrole (23.61%), linalool (12.93%), and germacrene D (5.35%). Bioactivities-directed chromatographic separation on repeated silica gel columns led to the isolation of two compounds: safrole and linalool. Safrole showed pronounced contact toxicity against both insect species and (LD₅₀ = 8.54 for S. zeamais; 4.67 µg/adult for T. castaneum) and was more toxic than linalool (LD₅₀ = 24.88 for S. zeamais; 8.12 µg/adult for T. castaneum). The essential oil acting against the two species of insects showed LD₅₀ values of 13.83 and 6.33 µg/adult, respectively. Linalool also possessed strong fumigant toxicity against both insect species (LC₅₀ = 10.02 for S. zeamais; 9.34 mg/L for T. castaneum) and was more toxic than safrole (LD₅₀ = 32.96 and 38.25 mg/L), while the crude essential oil acting against the two species of insects showed LC₅₀ values of 14.62 and 16.22 mg/L, respectively. These results suggest that the essential oil of I. difengpi stem bark and the two compounds may be used in grain storage to combat insect pests.