Abstract
Carlina acaulis root essential oil (EO) is one of the most potent mosquito larvicides (LC50 < 2 ppm). This EO is mainly composed of carlina oxide (> 90%). Poor water solubility and rapid degradation from UV light and oxygen in the environment limit the real-world use of this EO. Herein, we developed nanocarrier-based formulations, namely micro- and nanoemulsions (ME and NE, respectively) containing C. acaulis EO or carlina oxide (both at 0.5%) as active ingredients (a.i.). The larvicidal activity of ME and NE was evaluated against Culex quinquefasciatus. The highest larvicidal activity was achieved by the ME containing 0.5% of the EO (M1); its LC50(90) was 579.1 (791.3) µL L−1. Sublethal effects of this ME and its a.i. were assessed testing both at the LC16, LC30, LC50 and LC90 on mosquito larvae exposed to each product for 1–7 h, and then monitoring mortality for 18 days. At variance with the EO, ME application, even at LC16, led to 100% mortality at 18 days. The EO and its encapsulated form were scarcely toxic to human keratinocytes (HaCaT) and human fibroblast (NHF A12) cell lines. The acute toxicity of C. acaulis EO and its ME (M1) was also evaluated in Wistar rats through oral administration; EO LD50 was 1098 mg kg−1 bw, whereas its ME, even at 5000 mg kg−1 bw (considered the upper testing limit to establish safety to mammals), was not toxic. This study highlights the outstanding efficacy of C. acaulis EO ME for developing long-lasting and safe larvicides against Cx. quinquefasciatus.
Highlights
Developing highly effective insecticides, acaricides and arthropod repellents is a challenge of high economic importance nowadays (Isman 2015; Stevenson et al 2017; Benelli and Pavela 2018a, b; Chaieb et al 2018; Benelli et al 2019a; Petrović et al 2019)
Despite a huge number of research products on the topic (Isman and Grieneisen 2014; Chellappandian et al 2018), moving plant-based insecticides from laboratory to the field is still difficult, due to both formulation and regulatory issues (Pavela et al 2019a). Concerning the former, it should be noted that essential oil (EO) chemical composition is difficult to standardize and their efficacy in the field is often limited to a short period of time due to the high volatility, low stability and photo- and thermal degradation of the EO mixture once exposed into the environment (Pavela and Benelli 2016)
A prototype formulation titrated in 0.5% of C. acaulis EO with high efficacy on larvae of Cx. quinquefasciatus (LC50 = 579.1 μL L−1) was developed
Summary
Developing highly effective insecticides, acaricides and arthropod repellents is a challenge of high economic importance nowadays (Isman 2015; Stevenson et al 2017; Benelli and Pavela 2018a, b; Chaieb et al 2018; Benelli et al 2019a; Petrović et al 2019). Despite a huge number of research products on the topic (Isman and Grieneisen 2014; Chellappandian et al 2018), moving plant-based insecticides from laboratory to the field is still difficult, due to both formulation and regulatory issues (Pavela et al 2019a) Concerning the former, it should be noted that EOs chemical composition is difficult to standardize and their efficacy in the field is often limited to a short period of time due to the high volatility, low stability and photo- and thermal degradation of the EO mixture once exposed into the environment (Pavela and Benelli 2016). The use of C. acaulis EO and carlina oxide as new green insecticides is limited by their high lipophilicity and volatility as well as susceptibility to chemical degradation that reduce their persistence and diffusion in the environment In this framework, formulating effective and highly stable micro- and nanoemulsions can represent a valuable approach to allow the practical employ of C. acaulis EO or carlina oxide as active ingredients for developing commercial mosquito larvicides. The ME/NE showing the highest efficacy against Cx. quinquefasciatus larvae was selected—along with the C. acaulis EO—for acute oral toxicity experiments on Wistar rats and for dermal toxicity on human keratinocytes and fibroblasts, to shed light on the possible toxicity of both products on a vertebrate and cell models and to provide new insights to fulfil the requirements of regulatory agencies
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