The yellow fever mosquito, Aedes aegypti, is one of the most medically important mosquito species due to its ability to spread viruses of yellow fever, dengue fever and Zika in humans. In this study, the insecticidal activity of individual plant essential oil-derived monoterpene compounds including thymol, p-cymene, and trans-anethole as well as their binary mixtures were evaluated via a topical application against insecticide-susceptible Orlando strain of Ae. aegypti female adults. Among the compounds tested, thymol was the most active, and both p-cymene and trans-anethole displayed mild but synergistic interaction when blended with thymol in 1:1 ratios. To understand the possible synergy mechanisms in the mixtures, chemical analyses were conducted via GC-MS. Compared to the topical application of thymol alone, 1.48 times greater recovery of thymol was observed when mixed with p-cymene, indicating p-cymene enhanced the cuticular penetration of thymol in the mixture. On the other hand, trans-anethole failed to show any increase in the thymol recovery, suggesting a different synergistic mechanism from that of p-cymene. Interestingly, thymol and trans-anethole showed distinctive differences in their toxic response when they were individually administered. The mosquitoes exhibited paralysis with no movement when they were applied with the LD50 of thymol, whereas trans-anethole-treated mosquitoes displayed uncoordination and convulsions. Whereas thymol inhibited the nerve firing in Drosophila melanogaster larval CNS, lower concentration of trans-anethole showed excitation of nerve discharges, consistent with the observed behavioral response in the penetration studies, suggesting their different modes-of-action could be the underlying mechanism of their synergy.

Insecticidal activity of essential oil-derived compounds and their possible synergy mechanisms in the yellow fever mosquito, Aedes aegypti

2019 Fall National Meeting

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