Abstract
Many plant essential oils and their terpenoid constituents possess bioactivities including insecticidal activity, and they sometimes act synergistically when mixed. Although several hypotheses for this have been proposed, the underlying mechanism has not been fully elucidated thus far. In the present study, we report that in larvae of the cabbage looper, Trichoplusia ni, most synergistic or antagonistic insecticidal activities among mixtures of plant essential oil constituents are pharmacokinetic effects, owing to changes in solubility as well as spreadability on a wax layer. Among the major constituents of rosemary (Rosmarinus officinalis) oil, in vitro analysis revealed up to a 19-fold increase in penetration of camphor in a binary mixture with 1,8-cineole through the larval integument, suggesting increased penetration as the major mechanism for synergy. A total of 138 synergistic or antagonistic interactions among 39 compounds were identified in binary mixtures via topical application, and these were highly correlated to changes in surface tension as measured by contact angle of the mixtures on a beeswax layer. Among compounds tested, trans-anethole alone showed evidence of internal synergy, whereas most of remaining synergistic or antagonistic combinations among the three most active compounds were identified as penetration-related interactions, confirmed via a divided-application bioassay.
Highlights
For over three decades, interest in botanical insecticides has been growing continuously
Since the discovery of synergistic activity of sesame oil to natural and synthetic insecticides by Eaglenson in 194014, many essential oils and their constituents have been reported to inhibit activity of detoxifying enzymes, which can lead to synergistic toxicity with synthetic insecticides[15,16,17]
We examined [1] in vitro penetration of the synergistic combinations of major constituents of rosemary oil, [2] insecticidal activity of 39 compounds generally found in plant essential oils as well as potential structure-activity relationships, [3] synergistic interactions among selected compounds and their mechanism of synergy, and [4] diagnostic methods to determine the involvement of penetration in synergistic combinations
Summary
Interest in botanical insecticides has been growing continuously. We previously suggested a penetration-enhancing effect through the insect cuticle as a mechanism of synergy between 1,8-cineole and camphor, the two major constituents of rosemary (Rosmarinus officinalis) oil, and in vivo analysis showed increased internal concentrations of the synergistic combination[21]. We examined [1] in vitro penetration of the synergistic combinations of major constituents of rosemary oil, [2] insecticidal activity of 39 compounds generally found in plant essential oils as well as potential structure-activity relationships, [3] synergistic interactions among selected compounds and their mechanism of synergy, and [4] diagnostic methods to determine the involvement of penetration in synergistic combinations
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