Insecticidal activities of the essential oil of Rhynchanthus beesianus rhizomes and its constituents against two species of grain storage insects

The aim of this research was to evaluate insecticidal activities of the essential oil of Rhynchanthus beesianus rhizomes against adults of Liposcelis entomophila and Tribolium castaneum. Gas chromatography-mass spectrometry analyses revealed the presence of 44 compounds with β-eudesmol (19.1%), elemol (8.1%), α-terpineol (8.0%), methyl eugenol (6.5%), and caryophyllene (4.8%) being the major constituents. Bioactivity-directed chromatographic separation of the oil led to the isolation of four constituents, elemol, β-eudesmol, methyl eugenol, and α-terpineol. The essential oil exhibited fumigant toxicity against the adults of L. entomophila and T. castaneum with LC50 values of 0.57 and 4.96 mg/L air while the two isolates, methyl eugenol and α-terpineol possessed fumigant toxicity against the booklice (LC50 = 0.15 and 0.48 mg/L air, respectively) and the beetles (LC50 = 1.81 and 4.96 mg/L air, respectively). The oil also possessed contact toxicity against the booklice and the beetles with LD50 values of 121.56 μg/cm2 and 54.93 μg/adult, respectively, while the two isolates β-eudesmol and elemol showed contact toxicity against L. entomophila (LD50 = 99.21 and 35.19 μg/cm2, respectively) and T. castaneum (LD50 = 35.26 and 8.89 μg/adult, respectively). The results indicate that the oil of R. beesianus rhizomes and its isolates have potential as a source for natural insecticides.