Synergized potential, insecticidal and repellent activity of essential oils from two Rhododendron species against three stored product insects

ABSTRACT In this work, the essential oils (EOs) from leaves and flowers of Rhododendron spinuliferum and Rhododendron spiciferum were extracted by hydrodistillation, chemical compositions of EOs were analyzed by GC-MS and GC-FID. For R. spinuliferum and R. spiciferum, α- and β-pinene were common major compounds. It provided an opportunity to test the potential synergistic effect of α- and β-pinene among EOs. Firstly, four EOs and two major compounds (α- and β-pinene) exhibited fumigant, contact and repellent activities to three stored-product insects. To the red flour beetle, leaves of both species exhibited stronger contact toxicity than flowers did. This trend was diverse to the booklouse. Furthermore, binary mixture bioassays of α- and β-pinene were performed following natural proportions. Results of multiple linear regression showed that α-pinene, trans-β-farnesene and bornyl acetate were responsible for the contact toxicity to the red flour beetle and booklouse with diverse contribution.