Evaluating the Sublethal Effects of Origanum vulgare Essential Oil and Carvacrol on the Biological Characteristics of Culex pipiens biotype molestus (Diptera: Culicidae)

SIMPLE SUMMARY: In recent decades, essential oils from various plant species have been extensively tested as low-risk larvicides showing acute toxicity and/or insect growth regulatory properties through various mechanisms of action on mosquitoes. In the laboratory, we evaluated the chronic effects of larvicidal sublethal (LC(50)) concentrations of essential oil from Origanum vulgare and its major component carvacrol on biological parameters of the principal West Nile virus vector Cx. pipiens biotype molestus. The short-term (24 h) treatment of mosquito larvae with LC(50) concentrations produced significantly delayed mortality and morphological abnormalities to surviving larvae and pupae and resulted in failed adult emergence, indicating a potential growth inhibition mode of action for the tested materials. The results reported herein promote carvacrol and carvacrol-rich oregano oil as effective larvicides against Cx. pipiens biotype molestus at doses lower than the acute toxic ones, suggesting a use for these botanical insecticides that is safer for the environment and less costly. ABSTRACT: Culex pipiens is a mosquito species complex spread worldwide that poses a serious threat to human health as the primary vector of West Nile virus. Its control is mainly based on larvicidal applications with synthetic insecticides on mosquito breeding sites. However, the excessive use of synthetic larvicides may provoke mosquito resistance issues and negative side effects to the aquatic environment and human health. Plant-derived essential oils, including those from the Lamiaceae family, can be eco-friendly alternative larvicidal agents causing acute larval toxicity and/or growth inhibitory effects on the developmental stages of mosquitoes through different modes of action. In the current laboratory study, we evaluated the sublethal effects of carvacrol-rich oregano essential oil and pure carvacrol on Cx. pipiens biotype molestus, the autogenous member of the Cx. pipiens species complex, after the exposure of 3rd–4th instar larvae to LC(50) concentrations. The short-term (24 h) larvicidal treatment with the sublethal concentrations of both tested materials exhibited an acute lethal effect on the exposed larvae as well as significant delayed mortality for surviving larvae and pupae. Larvicidal treatment with carvacrol reduced the longevity of the emerged males. In addition, the morphological abnormalities that were observed at the larval and pupal stage along with failed adult emergence indicate the potential growth inhibitory properties of the tested bioinsecticides. Our findings suggest that carvacrol and carvacrol-rich oregano oil are effective plant-based larvicides at doses lower than the acute lethal ones, thus promoting an environmentally friendly and more affordable perspective for their use against the WNV vector Cx. pipiens biotype molestus.