Select Plant Essential Oils Enhance the Toxicity of Permethrin Against a Pyrethroid-susceptible and Pyrethroid-resistant Strain of <i>Aedes aegypti</i>

As resistance to current insecticidal treatments continues to develop throughout the world, new insecticides and synergists are critical to protect public health from mosquito-borne diseases. Accordingly, we screened 21 plant essential oils topically at 2 µg insect−1 and 10 µg insect−1 to determine their baseline toxicity and ability to synergize permethrin on a pyrethroid-susceptible strain of Aedes aegypti. We assessed knockdown at 1 h and mortality at 24 h post-application. At 10 µg insect−1, 6 of the 21 tested oils produced knockdown of &amp;gt;50% and 4 of the 21 oils produced mortality of &amp;gt;50%. Plant essential oils were then applied in combination with an LD25 of permethrin (0.4 μg insect−1). A co-toxicity factor as defined by Mansour et al. (J. Econ. Entomol. 1966, 59:307–311) was used as a benchmark to define additivity, synergism, or antagonism of permethrin activity. Synergism of knockdown was observed for nine of 21 essential oils, while synergism of toxicity was observed for 11 of 21 essential oils. The oils with the highest co-toxicities were juniper oil and cascarilla oil in synergizing knockdown and mortality, respectively. Juniper oil, however, caused the largest antagonistic effect on mortality. Select synergistic oils were also screened in combination with permethrin on a pyrethroid-resistant strain of Aedes aegypti, and almost all of those screened on both strains performed similarly. Overall, this study confirms the potential for plant essential oils to augment the efficacy of current, readily available insecticidal products on the market.