Comparative and synergistic activity of <i>Rosmarinus officinalis</i> L. essential oil constituents against the larvae and an ovarian cell line of the cabbage looper, <i>Trichoplusia ni</i> (Lepidoptera: Noctuidae)

BACKGROUND: Plant essential oils are usually complex mixtures, and many factors can affect their chemical composition. To identify relationships between the composition and bioactivity of the constituents, comparative and synergistic interactions of the major constituents of rosemary essential oil were evaluated against third-instar larvae and an ovarian cell line of the cabbage looper, Trichoplusia ni, via different methods of application. RESULTS: The major constituents of the rosemary oil we used were 1,8-cineole, (±)-camphor, (+)-α-pinene and camphene. Via topical application to larvae, 1,8-cineole was identified as the major active compound, whereas via fumigation, 1,8-cineole and (±)-camphor, and in a cytotoxicity assay, (+)-α-pinene, were determined to be the major active principles. Several combinations of these constituents exhibited synergistic insecticidal activities when topically applied, particularly among combinations of three major constituents, (±)-camphor, (+)-α-pinene and camphene. A binary mixture of 1,8-cineole and (±)-camphor showed enhanced activity, with a synergy ratio of 1.72. CONCLUSION: Based on our results, the insecticidal activity of rosemary oil appears to be a consequence of the synergistic interaction between 1,8-cineole and (±)-camphor, and (±)-camphor should be considered a promising synergizing agent.