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.