Synergistic terpene combinations as larvicides against the dengue vector Aedes aegypti Linn.

Most of the essential oils (EO) known for mosquitocidal activity contain terpenes as part of their chemical constituents. In this study, four monoterpenes namely γ-terpinene (T), R-(+)-limonene (L), carvacrol (C), and trans-anethole (A) were screened against late third instar Aedes aegypti Linn. larvae singly and in combination to find out the synergistic interaction. The monoterpenes γ-terpinene, R-(+)-limonene, trans-anethole, and carvacrol showed larvicidal activity with LC50 values of 9.76, 11.88, 50.19, 48.57 ppm and LC90 values of 16.99, 17.78, 65.21, 75.02 ppm, respectively for Ae. aegypti when tested individually. Prepared binary combinations at LC20 values and studied the synergistic interactions based on cotoxicity factor (CTF) and found that all the binary combinations showed synergistic interactions (CTF > 20). Further, the binary (1:1), ternary (1:1:1), and quaternary (1:1:1:1) terpene combinations on equal weight basis were prepared, evaluated against Ae. aegypti larvae and synergistic interactions were studied by Wadley's method. Among the eight combinations two combinations viz. LC (LC50 10.42 ppm) and LA (LC50 11.86 ppm) were showing synergistic interactions (R > 1.5) and others were exhibiting additive effect (1.5 ≥ R > 0.5) and none of the combinations showed antagonism (R < 0.5). Among the two methods used the Wadley's method found suitable when combinations were made on an equal weight basis. The two binary combinations found in this study are promising synergistic mixtures which has scope in designing mosquito larvicidal EOs by enriching the EOs with terpenes or by producing EO combinations with these terpenes as chemical constituents in the correct proportions.

Larvicidal activity of selected essential oil in synergized combinations against Aedes aegypti.

This study was conducted to attain an alternate plant essential oil (EO) based mosquito larvicide against the dengue vector Aedes aegypti. Here three plants were selected based on their local availability, safety and insecticidal properties. EOs were extracted from Syzygium aromaticum (Myrtaceae) flower buds, fruits of Illicium verum (Schisandraceae) and Trachyspermum ammi (Apiaceae) by hydro-distillation and tested against Ae. aegypti larvae individually and in combinations to find synergistic interactions. Chemical constituent analysis of EOs was done by GC/MS/MS analysis and the main constituents in T. ammi were thymol (93.58%) and terpinen-4-ol (2.16%), in S. aromaticum eugenol 56.32% and caryophyllene 17.91% and in I. verum trans-anethole 53.05% and p-anisaldehyde 12.47%. The EOs from T. ammi, I. verum and S. aromaticum and their top components thymol, trans-anethole and eugenol exhibited larvicidal activity with LC50 values 39.48, 41.30, 66.90, 59.76, 50.19 and 60.89 mgL-1 respectively against Ae. aegypti larvae. The values for the co-toxicity factors for the binary combinations of the EOs were > 20 showing synergistic interactions among the binary mixtures. The respective LC50 values for the 1:1 binary combinations (S. aromaticum + I. verum), (S. aromaticum + T. ammi) and (I. verum + T. ammi) were 49.07, 48.54 and 27.67 mgL-1. ß-cyclodextrin inclusion complex made with I. verum + T. ammi combination showed an LC50 value of 23.93 mgL-1. On the whole the outcome of this study draw attention to the capability of synergistic EO combinations to emerge as a safe and environment friendly effective larvicide to control Aedes mosquitoes.