Specific compounds extracted from plants can control insect pests. The objective of this study was to evaluate the toxicity of deodar oil (phytopesticide) to adult mealworms Tenebrio molitor (Coleoptera: Tenebrionidae) compared with carbosulfan (synthetic insecticide), which exibits cholinesterase (ChE), glutamic pyruvic transaminase (GPT), and glutamic oxaloacetic transaminase (GOT) activities. The insecticides were applied through feeding, and the LC 50 (lethal concentration) was calculated using the Finney method. The LC 50 of deodar oil was higher than that of carbosulfan. The doses of both deodar oil and carbosulfan inhibited the ChE activity (p > 0.05) and enhanced the GPT and GOT activities (p < 0.05) in mealworm adults. Alterations in the activity of these biomarkers indicated that deodar oil could effectively control adult mealworms, being an environmentally low-impact method that can replace the use of chemical products.