Abstract
Synergistic interactions between constituents of essential oils have been reported for several areas of research. In the present study, mechanisms that could explain the synergistic action of the two major insecticidal constituents of rosemary oil, 1,8-cineole and camphor against the cabbage looper, Trichoplusia ni were investigated. 1,8-Cineole was more toxic than camphor when applied topically to larvae, and when coadministered in their ratio naturally occurring in rosemary oil, the binary mixture was synergistic. However, when injected directly into larvae, camphor was more toxic than 1,8-cineole. GC-MS analyses showed that penetration of topically-applied camphor was significantly enhanced when it was mixed with 1,8-cineole in the natural ratio. A bioassay combining injection and topical application methods confirmed the increased penetration of both compounds when mixed, showing the same bioactivity as seen for higher amounts applied individually. Lowered surface tension as well as increased solubility of camphor by 1,8-cineole, along with the interaction between 1,8-cineole and the lipid layer of the insect’s cuticle may explain the enhanced penetration of camphor. Because of the similarities in biological function of animal and microbial membranes, our finding has potential for application in other fields of study.
Highlights
Synergistic interactions between constituents of essential oils have been reported for several areas of research
The individual contributions of the two most abundant compounds, 1,8-cineole and camphor in rosemary oil and their combined effect to the toxicity of the oil were examined (Fig. 1). In both topical and fumigant bioassays, an artificial mixture consisting of ten major constituents of rosemary oil (> 1% in concentration) reproduced the bioactivity of natural rosemary oil
Plant essential oils have been a subject of research by many disciplines because of their wide range of bioactivities including antimicrobial, insecticidal, therapeutic and medicinal effects
Summary
Synergistic interactions between constituents of essential oils have been reported for several areas of research. Mechanisms that could explain the synergistic action of the two major insecticidal constituents of rosemary oil, 1,8-cineole and camphor against the cabbage looper, Trichoplusia ni were investigated. Not unlike the pharmaceutical activity of drugs in animals and humans, insecticidal activity is the result of a series of complex actions and counteractions between a toxicant and an insect’s tissues These complex dynamics of toxicity can be simplified into three categories - penetration, activation (= target site interaction) and detoxification[1]. Our main question was whether there is increased penetration of a specific combination of rosemary essential oil constituents, and if this could account for their observed synergistic action.
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