Investigation of insecticidal activity of two Rhododendron species on stored-product insects

This study was designed to investigate the insecticidal activity of the essential oils (EOs) and extracts from Rhododendron rufum and Rhododendron przewalskii. The EOs were extracted from the leaves of R. Rufum and R. przewalskii by hydro-distillation and their chemical components were analyzed by gas chromatography-mass spectrometry (GC-MS). The repellency, contact toxicity and antifeedant activity of the EOs and extracts were evaluated against Sitophilus oryzae and Tribolium castaneum along with those of their main components. A total of nine compounds were identified from the EO of R. Rufum, and the most abundant component was myristicin (79.72%). The EO of R. Rufum exhibited repellent activities at different levels and its main compound myristicin showed contact toxicity and repellent effects against S. oryzae and T. castaneum. Meanwhile, by bioassay-guided fractionation, four compounds with strong antifeedant activities against T. castaneum, 24-methylenecycloartanyl-2'E, 4'Z-tetradecadienoate (1), methyl thyrsiflorin B acetate (2), friedelin (3) and Excoecarin R1 methyl ester (4) were separated and identified from the ethanol extract of R. przewalskii for the first time. Considering the significant anti-insect activities, the EOs and extracts of R. Rufum and R. przewalskii might be used in integrated pest strategies, establishing a good perspective for the comprehensive use of natural plant resources of Rhododendron genus.