Acute toxicities of three naturally occurring monoterpenoid essential oil constituents and the essential oil of rosemary were tested against late instars of Agriotes obscurus (L.) (Coleoptera: Elateridae). Both contact and volatile toxicities of thymol, citronellal, eugenol, and rosemary oil were determined. Also, phytotoxicity of these compounds was evaluated on corn germination and seedling development. Thymol had the greatest contact toxicity (LD₅₀ = 196.0 μg/larva), whereas citronellal and eugenol were less toxic (LD₅₀ = 404.9 and 516.5 μg/larva, respectively). Rosemary oil did not show any significant contact toxicity, even at 1,600 μg/larva. In terms of volatile toxicity, citronellal was the most toxic to wireworm larvae (LC₅₀ = 6.3 μg/cm³) followed by rosemary oil (LC₅₀ = 15.9 μg/cm³), thymol (LC₅₀ = 17.1 μg/cm³), and eugenol (LC₅₀ = 20.9 μg/cm³). Thymol, eugenol, and citronellal significantly inhibited corn seed germination and development, whereas rosemary oil had only minimal phytotoxic effects.