Plant essential oil terpenoids represent a valuable library of potential new insecticidal chemistries that could synergize or replace currently available insecticides on the market today. For this study, we evaluated the potential of 41 plant essential oils to enhance various commercially relevant insecticides, both applied topically and via vapor phase exposures. Among these combinations of plant essential oils and commercially available insecticides, numerous plant essential oils were capable of significantly synergizing the effects of insecticides on <i>Aedes aegypti</i>. In topical applications, 26 oils were capable of synergizing pyrethroid toxicity via co-toxicity factor analysis. If we extend this analysis to synergism of knockdown or mortality at 1 hr or 24 hr post-application, respectively, 34 oils were capable of significantly synergizing pyrethroids, which demonstrates the utility of these chemistries in future insecticidal formulations. One oil, fir needle, was particularly unique in that it significantly synergized the knockdown of various insecticides at 1 hr post-application but did not significantly synergize 24-hr mortality. Because of this finding, we assessed the ability of fir needle oil to synergize other insecticidal modes of action. Fir needle oil strongly synergized the knockdown effects of other neonicotinoids, on both a pyrethroid-susceptible and pyrethroid-resistant strain (12-24 fold), however, enhancement of 24-hr mortality was not appreciable. In order to better understand this effect, we utilized GC/MS to identify the primary constituents of this oil that were responsible for its bioactivity. Two primary constituents were then screened for their ability to produce synergism, and delta-3-carene was identified as the most bioactive constituent within this oil. In fact, this constituent produced significantly higher synergism of clothianidin than the whole oil itself (4.9-fold vs. 2.4-fold). Subsequent work also demonstrated that this synergism was not mediated by direct effects on the nervous system (as fir needle oil was not capable of synergizing pyrethroids directly on the mosquito nervous system at 100 ppm (a level that produced significant block by other plant essential oils). Further discussion will be provided regarding the ability of plant essential oils to synergize commercially available insecticides and hypotheses regarding their mechanism.

Synergistic plant essential oils: exploring their mechanisms of action

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ACS Fall 2022

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