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 and camphor, and a binary mixture of them showed synergistic insecticidal activity via topical application in third instar larvae of the cabbage looper, Trichoplusia ni. In the mixture, penetration of camphor was significantly enhanced both in vivo and in vitro, whereas that of 1,8-cineole did not change much compared to individual application. Injection into hemocoel of fifth instar larvae indicated that camphor was more toxic than 1,8-cineole, although it was less toxic than 1,8-cineole 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.

• Jun-Hyung Tak(Faculty of Land and Food Systems, University of British Columbia)
• Murray B. Isman(Faculty of Land and Food Systems, University of British Columbia)