Chemical constituents of the essential oil extracted from Rhododendron thymifolium and their insecticidal activities against Liposcelis bostrychophila or Tribolium castaneum

• Five constituents from Rhododendron thymifolium essential oil were isolated for the first time.
• Insecticidal and repellent activities of the essential oil were studied for the first time against Liposcelisa bostrychophila and Tribolium castaneum.
• Insecticidal and repellent activities of germacrone, α-oxobisabolene, γ-elemene, selina-3,7(11)-diene, β-sesquiphellandrene were studied for the first time against L. bostrychophila and T. castaneum.
• The crude essential oil of R. thymifolium possess similar contact toxicity to pyrethrins against L. bostrychophila.
• Germacrone showed comparable contact toxicity with the positive control, pyrethrins against T. castaneum.

In this work, we investigated the chemical constituents of the essential oil extracted from the leaves of Rhododendron thymifolium and their insecticidal and repellent activities against Liposcelis bostrychophila and Tribolium castaneum. 14 chemical components were identified from the essential oil and included germacrone (20.83%), γ-elemene (11.10%), selina-3,7(11)-diene (6.18%), α-oxobisabolene (5.39%), β-sesquiphellandrene (2.44%), β-elemenone (35.17%), Z,Z,Z-1,5,9,9-tetramethyl-1,4,7,-cycloundecatriene (3.64%) and 1-(4-ethylphenyl)-3-methyl-pyrazol-(4H)-one (2.91%). Among them, the first five components were isolated and identified. The results showed that the essential oil and five pure compounds possessed the obvious repellent activity and various levels of contact activities against the two species of stored insects. The essential oil possessed the contact toxicity (LD50 = 19.63 μg/cm2) which was quite close to the toxicity of positive control pyrethrins (LD50 = 18.72 μg/cm2) against L. bostrychophila. The five pure compounds might individually or together effect the contact toxicity of the essential oil against L. bostrychophila. The essential oil possessed the contact toxicity (LD50 = 29.82 μg/adult) against T. castaneum, but only germacrone revealed strong contact toxicity (LD50 = 0.97 μg/adult), which was comparable with the toxicity of pyrethrins (LD50 = 0.24 μg/adult) among five compounds against T. castaneum. The existence of germacrone might be one of the key elements which affected the contact toxicity of R. thymifolium essential oil against T. castaneum. The insecticidal and repellent activities of essential oil and five compounds were evaluated, the results provide a basis of the development and utilization in the future for control of insects.

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