Synergistic interactions among the major constituents of lemongrass essential oil against larvae and an ovarian cell line of the cabbage looper, Trichoplusia ni
2017
Tak, Jun-Hyung | Jovel, Eduardo | Isman, Murray B.
Since plant essential oils are composed with vast numbers of constituents, they often show complex interactions among the components. Although the research interest on the insecticidal activity of plant essential oils has been gaining more attention recently, most of the studies still focus on simple screening of active plant source or identification of active compounds. In the present study, insecticidal activity and synergistic interactions among the four major constituents of lemongrass (Cymbopogon citratus) essential oil were examined via topical application against the third instar larvae of the cabbage looper, Trichoplusia ni. Two synergistic binary combinations—citral + limonene and citral + geranyl acetate—were identified at the equivalent mixing ratios. The former was especially synergistic in larvae and additionally with respect to cytotoxicity in the ovarian cell line of the cabbage looper. Morphological observations indicated different cytotoxic modes of action of citral and limonene. GC–MS analysis of larval extracts in vivo revealed several metabolites of citral and limonene, with geranic acid and neric acid (from citral), and limonene-1,2-diol (from limonene) as the major ones. The insecticidal activity of geranic acid was very similar to that of the parent compound, citral, but limonene-1,2-diol failed to show any toxicity, indicating that inhibition of the metabolism could be a good strategy to enhance toxicity. Further, larval extracts following topical administration of a binary mixture of the two compounds revealed higher internal concentrations of both compared to their individual application, suggesting the possibility of enhanced cuticular penetration as the mechanism of synergy.
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