Major monoterpenoids from Dracocephalum moldavica essential oil act as insecticides against Culex quinquefasciatus with synergistic and antagonistic effects

Essential oils (EOs) are considered effective, safe, and eco-friendly alternatives to synthetic products for the management of insect pests and vectors. Dragonhead (Dracocephalum moldavica L., Lamiaceae) is an aromatic crop widely cultivated in Iran for its valuable citrus-like EO. This work aimed to investigate the dragonhead EO efficacy on Culex quinquefasciatus Say, the major lymphatic filariasis vector. Moreover, the main EO components (geranyl acetate, geraniol, citral, and linalool) and their binary mixtures were tested to assess the possible antagonistic or synergistic interactions aiming to propose artificial mixtures to be exploited in the agrochemical field. The assays demonstrated the EO larvicidal efficacy (LC50 of 33.8 ppm) as well as its tarsal contact toxicity and fumigant effect on adults (LC50 of 37.5 and 5.6 ppm, respectively). The time needed to achieve larval mortality after application of the EO at its LC90 was significantly different among the three larval stages. Specifically, the 90 % mortality in 2nd instar larvae was observed after 182.7 min, a significantly shorter period of time compared with that of the 3rd and 4th instar (LT90 estimated at 355.3 and 302.8 min, respectively). Regarding the EO components, citral and geranyl acetate showed the best larvicidal action (LC50 of 73.3 and 80.9 ppm, respectively), even if all the tested binary mixtures demonstrated a synergistic effect. Regarding tarsal contact toxicity and fumigant effects, these were more significant for citral (LC50 of 11.6 μg cm−2 and 0.23 ppm, respectively) and geraniol (LC50 15.4 μg cm−2 and 1.13 ppm, respectively). Moreover, the fumigation test revealed a synergism only for the mixture linalool/citral, while the binary mixtures geranyl acetate/geraniol, linalool/geraniol, and geraniol/citral demonstrated an antagonistic effect. This study reported for the first time the insecticidal potential of D. moldavica EO and its components on C. quinquefasciatus. The results obtained, together with the already well-established cultivation of the plant, support the possible industrial application of this EO and artificial mixtures made up of its main constituents as effective insecticidal agents.