Insecticidal activity of the essential oil and polar extracts from Ocimum gratissimum grown in Ivory Coast: Efficacy on insect pests and vectors and impact on non-target species

Ocimum gratissimum L. (white wild basil) is an aromatic herb cultivated in Western Africa for culinary and pest control purposes. The current research evaluated the insecticidal activity of white wild basil essential oil, ethanolic and water extracts against insect pests and vectors, i.e., the tobacco cutworm Spodoptera littoralis, the housefly Musca domestica, and the filariasis vector Culex quinquefasciatus. Furthermore, the toxicity of the essential oil and polar extracts against the non-target earthworm Eisenia fetida was assessed. The chemical profiles of the essential oil and polar extracts were obtained by GC–MS and HPLC-DAD analyses. Acute toxicity experiments were conducted on larvae of C. quinquefasciatus and S. littoralis and adults of M. domestica and E. fetida, to determine the LC50 and LC90 values of the oil and polar extracts. Chronic toxicity was evaluated on S. littoralis feeding on tomato discs treated with essential oil and polar extracts. The essential oil was dominated by thymol (50.0%) and p-cymene (16.8%), whereas ethanolic and aqueous extracts were characterized by carvacrol (13%) and thymol (11%), and shikimic acid (3%) and rosmarinic acid (2%), respectively. The essential oil was significantly more active on target insects than extracts, showing LC50/LD50 of 39.6 mg L−1 on C. quinquefasciatus, 72.2 μg adult−1 on M. domestica and 30.2 μg larva−1 on S. littoralis. Furthermore, the essential oil and ethanolic extract at sublethal doses (10 and 70 μg cm-2, respectively) affected the survival of S. littoralis larvae from the third day on. White wild basil oil LD50,90 at day 5 were 2.8 and 12.3 μg cm-2. Finally, the essential oil and polar extracts were not toxic to E. fetida over the positive control α-cypermethrin. Overall, our study showed that the essential oil of white wild basil is a potential candidate as a functional ingredient in insecticidal formulations to manage agricultural moth pests and insect vectors of public importance.