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Toxicities of thymol, citronellal, eugenol and rosemary oil to control Agriotes obscurus (Coleoptera: Elateridae) in laboratory and greehouse bioassays Waliwitiya, Ranil


Toxicities were determined for four naturally occurring monoterpenoid essential oils on late instar larvae of Agriotes obscurus in laboratory and greenhouse bioassays. Both contact and fumigant toxicities were determined for thymol, citronellal, eugenol and rosemary oil. Thymol had the greatest contact toxicity (LD₅₀=196.0 μg/larva) while citronellal and eugenol were significantly less toxic (LD₅=404.9 μg/larva and 516.5 μg/larva, respectively). Rosemary oil did not show any significant contact toxicity, at the highest dose tested, 1600 μg/larva. In fumigant bioassays, citronellal was the most toxic to wireworm larvae (LD₅=6.3 μg/cm ) followed by rosemary oil (LD₅=15.7 μg/cm ), thymol (LD₅=16.9 μg/cm³), and eugenol (LD₅=20.8 μg/cm³). Phytotoxicities were also evaluated based on corn seed germination and seedling development. In laboratory bioassays, thymol and citronellal significantly inhibited seed germination and development while rosemary oil had only minimal phytotoxic effects. Based on these laboratory results, four treatments were selected for repellency trials under greenhouse conditions. Citronellal (800 pg/seed), thymol (400 μg/seed), rosemary oil (1600 μg/seed) and eugenol (1600 pg/seed) were evaluated for their ability to protect corn seeds from Agriotes obscurus feeding damage. In the bioassay, wireworm health, seed germination, seed damage, and seedling height were evaluated. Rosemary oil treated seeds produced the highest number of distressed wireworms (56%, Odds Ratio = 2.64) compared to all other treatments while not creating any phytotoxic effects. Seeds treated with thymol were the least damaged (90.7% undamaged, Odds Ratio = 0.153) compared to the control while thymol and citronellal retarded seedling height compared to the control. Eugenol produced the lowest seed germination (25.3% germination) while citronellal, rosemary oil and thymol had no negative impact on seed germination.

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