Effects of rosemary, thyme and lemongrass oils and their major constituents on detoxifying enzyme activity and insecticidal activity in Trichoplusia ni

•Synergy mechanisms for rosemary, thyme, and lemongrass oils were explored.•No apparent relationship between detoxifying enzyme activity and toxicity was found.•Enzyme inhibitors can sometimes decrease toxicity by impeding penetration.•Care should be given when using enzyme inhibitors to measure degrees of resistance.

Although there have been many reports on the synergistic interactions among the major constituents of plant essential oils regarding insecticidal activity, their underlying mechanism of synergy is poorly understood. In our previous studies, we found each of the two most abundant constituents of rosemary (Rosmarinus officinalis L.), thyme (Thymus vulgaris L.) and lemongrass (Cymbopogon citratus Stapf.) essential oils can be synergistic against the larvae of the cabbage looper, Trichoplusia ni at their natural proportion or equivalent blending ratios. In the present study, we investigated whether the enhanced toxicity between the major constituents could be the result of inhibited enzyme activity of cytochrome P450s, general esterases or glutathione S-transferases which are highly related to the development of insecticide resistance. Overall, although some combinations showed mild inhibitory activity, at least for these essential oils and their major constituents, inhibition of detoxication enzyme activity is unlikely to be a direct cause of increased toxicity in the cabbage looper. The results point to other factors, such as multiple modes-of-action or enhanced penetration through the cuticular layer, playing important roles in the elevated insecticidal activity. Moreover, application of enzyme inhibitors sometimes resulted in decreased activity when mixed with the target compounds, but these antagonistic interactions disappeared when they were applied separately, suggesting that the enzyme inhibitors can sometimes influence the penetrations of toxicants.

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