Larvicidal and adulticidal activity of irradiated Thuja orientalis essential oil by gamma radiation for controlling Culex pipiens

Culex pipiens mosquitoes are the primary vector of the West Nile virus worldwide and have a wide distribution. To avoid the development of resistance in insect pests to conventional synthetic pesticides, there have been many attempts to study botanical pesticides. The increasing resistance of insect pests to synthetic pesticides is a threat to the diversity of ecosystems, which makes the study of botanical pesticides all the more important. The study aimed to determine the efficacy of Thuja orientalis essential oil irradiated with 1, 3 and 5 KGy compared to unirradiated T. orientalis as larvicidal agents against the third instar larvae of C. pipiens using six different concentrations (15, 20, 25, 30, 35 and 40 ppm) and as an adulticidal agent against approximately three-day-old female adults 0.2, 0.4, 0.6, 1 and 2%. The results generally showed that the irradiated essential oil with 5 KGy had the highest efficacy by LC₅₀ (20.16 ppm) followed by essential oil irradiated with 3 and 1 KGy (LC₅₀, 23.16 and 26.00 ppm, respectively) compared to unirradiated ones (LC₅₀, 27.28 ppm) after 24 h from the exposure for the third instar larvae. The knockdown effect for adults increased significantly with higher concentrations of the oil, across different exposure times ranging from 5 to 60 min. The time to knock down 100% of mosquito population to Thuja orientalis essential oils at the highest concentration 2% were 20 min at 0, 1, and 3 KGy and 10 min at 5 KGy. Our results indicate that gamma irradiation of T. orientalis fruit improves the efficacy of their essential oil against Culex pipiens and the irradiated oil could be successfully used for management of these insects. The head capsule, antenna, and the last abdominal segments of the 3rd instar larvae exposed to the essential oil of unirradiated or irradiated Thuja orientalis unripe were examined by the scanning electron microscope (SEM).