ABSTRACT
Ligusticum sinense Oliv. cv. is a species of Umbelliferae (Apiaceae), a large plant family in the order Apiales. In this study, L. sinense hexane extract nanoemulsion gel (LHE-NEG) was investigated for mosquito repellency and compared to the standard chemical, N,N-diethyl-3-methylbenzamide (DEET), with the goal of developing a natural alternative to synthetic repellents in protecting against mosquito vectors. The results demonstrated that LHE-NEG afforded remarkable repellency against Aedes aegypti, Anopheles minimus, and Culex quinquefasciatus, with median protection times (MPTs) of 5.5 (4.5-6.0), 11.5 (8.5-12.5), and 11.25 (8.5-12.5) h, respectively, which was comparable to those of DEET-nanoemulsion gel (DEET-NEG: 8.5 (7.0-9.0), 12.0 (10.0-12.5), and 12.5 (10.0-13.5) h, respectively). Evaluation of skin irritation in 30 human volunteers revealed no potential irritant from LHE-NEG. The physical and biological stability of LHE-NEG were determined after being kept under heating/cooling cycle conditions. The stored samples of LHE-NEG exhibited some changes in appearance and differing degrees of repellency between those kept for 3 and 6 heating/cooling cycles, thus providing slightly shorter MPTs of 4.25 (4.0-4.5) and 3.25 (2.5-3.5) h, respectively, when compared to those of 5.0 (4.5-6.0) h in fresh preparation. These findings encourage commercially developed LHE-based products as an alternative to conventional synthetic repellents in preventing mosquito bites and helping to interrupt mosquito-borne disease transmission.
ABSTRACT
BACKGROUND: In a previous screening program 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 reducing 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 strains. 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) strains of Ae. aegypti. Adulticidal bioassays of EO-permethrin mixtures for synergistic activity were also performed on these Ae. aegypti strains. RESULTS: Chemical characterization 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%), ß-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 µg/mg female, 7.97 and 7.94 µg/mg female, and 3.30 and 3.22 µg/mg female, respectively. The adulticidal efficacy against MCM-S and PMD-R Ae. aegypti of these EOs was close to that of piperonyl butoxide (PBO, LD50 values = 6.30 and 4.79 µg/mg female, respectively) but less pronounced than that of permethrin (LD50 values = 0.44 and 3.70 ng/mg female, respectively). Nevertheless, combination-based bioassays discovered the accomplished synergism of EOs together with permethrin. Significant synergistic effects with permethrin against both the strains 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 dramatically from 0.44 to 0.07 and 0.11 ng/mg female, respectively, with synergism ratio (SR) values of 6.28 and 4.00, respectively. Furthermore, 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 ng/mg female, respectively, with SR values of 8.81 and 1233.33, respectively. CONCLUSIONS: The synergy of enhanced adulticidal toxicity recorded from EO-permethrin combinations against both strains of Ae. aegypti presents a promising role of EOs as a synergist for improving mosquitocidal efficacy, particularly in situations where conventional compounds are ineffective or inappropriate.
