Resistance to insecticides and synergistic and antagonistic effects of essential oils on dimefluthrin toxicity in a field population of Culex quinquefasciatus Say.

In this work, we firstly tested five spatial repellent pyrethroids, meperfluthrin, dimefluthrin, heptafluthrin, metofluthrin and transfluthrin, to determine the susceptibility of pyrethroids to field strains of Culex quinquefasciatus using adult topical bioassay. The results showed that though field strains exhibited the highest resistance to dimefluthrin among the selected five pyrethroids, it still can be considered low resistance in the scale of Cui et al. (2006; 2007). Then, the aim of this study was to optimise the synergistic efficacy of essential oils combined with dimefluthrin and explore the major contribution composition of eucalyptus oil, basil oil and cinnamon oil as natural synergist of dimefluthrin against the field populations of C. quinquefasciatus. GC-MS analysis showed 1,8-cineole, eugenol and trans-cinnamaldehyde were the main chemical components of eucalyptus oil, basil oil and cinnamon oil, respectively. The results of bioactivity showed that eucalyptus oil and 1,8-cineole have highly fumigant knock-down activity to the adults, showing KT50 (the median knockdown time) of 5.76 and 4.27 min at the concentration of 24.2 µL/L; basil oil and eugenol, cinnamon oil and trans-cinnamaldehyde have highly fumigant toxicity to the adults, showing LD50 of 1.00 and 0.79, 1.26 and 1.03 µL/L, respectively. Three effective main essential oil components were selected to prepare binary mixtures, which combined with dimefluthrin against the field population of Culex quinquefasciatus. 1,8-cineole+eugenol (9:1, w/w), 1,8-cineole+trans-cinnamaldehyde (1:1, w/w) and trans-cinnamaldehyde+eugenol (9:1, w/w) combined with dimefluthrin (10:1, w/w) were the most synergistic interaction, showed SR (synergistic ratio) values of 1.2471, 1.5709 and 1.1969; KT50 of 11.68, 9.51 and 10.67 min respectively, by quadrate box method. In addition, to validate the stable synergistic interaction of 1,8-cineole+trans-cinnamaldehyde (1:1, w/w) combined with dimefluthrin (10:1, w/w), the SR values were about 1.3, and KT50 values were 38.72–50.26 min by simulated house method. Overall, our results pointed out the promising potential of these essential oils to increase the efficacy of dimefluthrin. It might be expected that these essential oils could be developed to a useful botanical synergist of dimefluthrin for the control of the field populations of C. quinquefasciatus.