Penetration-enhancing effect of rosemary essential oil components as a possible mechanism of synergy

The penetration-enhancing effect of a binary mixture of rosemary essential oil constituents, and its correlation to increased toxicity were investigated. GC-MS analyses revealed that the major constituents of rosemary essential oil were 1,8-cineole (37.6%) and camphor (20.2%), and a binary mixture of them following their relative proportions in the essential oil showed synergistic insecticidal activity via topical application in third instar larvae of the cabbage looper, Trichoplusia ni. Groups of thirty larvae were topically treated individually with 1,8-cineole, camphor, or as a mixture of them at doses based on the LD₅₀ concentration of the essential oil, and compounds penetrating through the cuticle were extracted with hexane and analyzed by GC-MS. The penetration of 1,8-cineole did not change in the presence of camphor, whereas that of camphor was significantly enhanced when mixed with 1,8-cineole. Injection into the body cavity of fifth instar larvae indicated that camphor was more toxic than 1,8-cineole, although it was less toxic when applied topically. A possible mechanism for enhanced penetration of camphor by 1,8-cineole is reduced surface tension of the mixture. Contact angles of 50% acetonic solutions on a layer of beeswax showed that the surface tension of 1,8-cineole was lower than that of camphor. Another probable reason is a change of camphor from solid to liquid status, when mixed with 1,8-cineole. This is the first report attempting to explain the synergistic insecticidal interaction of essential oil components, from a physiochemical perspective.