Efficacy of Two Monoterpenoids, Carvacrol and Thymol, and Their Combinations against Eggs and Larvae of the West Nile Vector <i>Culex pipiens</i>

• Main Authors: Mohammad Reza Youssefi, Mohaddeseh Abouhosseini Tabari, Aryan Esfandiari, Sohrab Kazemi, Ali Akbar Moghadamnia, Stefania Sut, Stefano Dall’Acqua, Giovanni Benelli, Filippo Maggi
• Format: Article
• Language: English
• Published: MDPI AG 2019-05-01
• Series: Molecules
• Subjects: biopesticide; essential oil; ovicidal activity; larvicidal activity; mosquito vector
• Online Access: https://www.mdpi.com/1420-3049/24/10/1867

<i>Background</i>: Insect vector control is facing the challenges of resistance development and environmental hazards caused by synthetic pesticides. This has led to a considerable market opportunity for botanical insecticides. In this scenario, our study investigated the potential of selected bioactive monoterpenoids, carvacrol and thymol, as safe and effective tools to control the West Nile vector <i>Culex pipiens</i>. Furthermore, the combined effect of thymol-carvacrol mixtures and their possible interactions were assessed. <i>Methods</i>: For determining larvicidal and ovicidal 50% lethal concentration (LC₅₀), each monoterpenoid was tested at different concentrations (5&#8211;500 mg/L). Then, the fixed ratio method was used for evaluating their combinational efficacy. <i>Results</i>: Carvacrol was more toxic against larvae of <i>Cx. pipiens,</i> with a LC₅₀ value of 14 mg/L, whereas thymol exhibited a LC₅₀ value of 49 mg/L. Comparable trends of efficacy were observed when toxicity on <i>Cx. pipiens</i> eggs was investigated, with LC₅₀ values of 7 and 13 mg/L for carvacrol and thymol, respectively. In combinational toxicity assays, the mixture thymol-carvacrol at 1:4 ratio achieved a synergistic effect against larvae of <i>Cx. pipiens,</i> whereas an additive effect was observed on eggs. Other ratios showed antagonistic effects. <i>Conclusions</i>: Overall, our findings pointed out that the 1:4 ratio of thymol-carvacrol blend can enhance the insecticidal efficacy on <i>Cx. pipiens</i> young instars and can be considered further as active ingredient for developing botanical insecticides to be used in mosquito control operations.