Synergy in the adulticidal efficacy of essential oils for the improvement of permethrin toxicity against Aedes aegypti L. (Diptera: Culicidae)

Background: In a previous screening prograrn for mosquitocides from local edible plants in Thailand, essential oils (EOs) of Cyperus rotundus, Alpinia galanga and Cinnamomum verum, were found to possess promising adulticidal activity against Aedes aegypti. With the aim of leducing usage of conventional insecticides and improving the management of resistant mosquito populations, this study was designed to determine the potential synergism in the adulticidal efficacy of EOs on permethrin toxicity against Ae. aegypti, both pyrethroid-resistant and -susceptible stiains. Methods: EOs extracted from rhizomes of C rotundus and A galanga as well as C verum barks were evaluated for chemical compositions and adulticidal activity against Muang Chiang Mai susceptible (MCM S) and Pang Mai Dang resistant (PMD R) stiains of Ae aegypti Adulticidal bioassays of EO permethrin mixtuies for synergistic activity were also performed on these Ae aegypti strains. Results: Chemical charactenzation by the GC-MS analytical technique demonstrated that 48 compounds were identified from the EOs of C. rotundus, A. galanga and C. verum, representing 80.22%, 86.75% and 97.24%, respectively, of all compositions. Cyperene (14.04%), beta-bisabolene (18.27%) and cinnamaldehyde (64.66%) were the main constituents of C. rotundus, A. galanga and C. verum oils, respectively in adulticidal bioassays, EOs of C rotundus, A. galanga and C. verum were effective in killing Ae. aegypti, both MCM-S and PMD-R strains, with LD50 values of 10.05 and 9.57 mu g/mg female, 7.97 and 7.94 mu g/mg female, and 3.30 and 3.22 mu g/mg female, respectively. The adulticidal efficacy against MCM-S and PMD-R Ae. aegypti of these EOs was close to that of pipeionyl butoxide (PBO, LD50 values = 6.30 and 4.79 mu g/mg female, respectively) but less pronounced than that of permethrin (LD50 values = 0.44 and 3.70 mu g/mg female, respectively). Nevertheless, combination-based bioassays discovered the accomplished synergism of EOs together with permethrin. Significant synergistic effects with permethrin against both the stiains of Ae. aegypti were recorded in the EOs of C. rotundus and A. galanga Addition of C. rotundus and A. galanga oils decreased the LD50 values of permethrin against MCM-S diamatically fiom 0.44 to 0.07 and 0.11 mu g/mg female, respectively, with syneigism ratio (SR) values of 6.28 and 4.00, respectively. Fuithermore, EOs of C. rotundus and A. galanga also reduced the LD50 values of permethrin against PMD-R drastically from 3.70 to 0.42 and 0.003 mu g/mg female, respectively, with SR values of 8.81 and 1233.33, respectively. Conclusions: The synergy of enhanced adulticidal toxicity lecorded from EO peimethrin combinations against both strains of A? aegypti piesents a promising roie of EOs as a synergist for improving mosquitocidai efficacy, particularly in situations where conventional compounds aie ineffective or inappropriate.