Essential oils and phenylpropanoids from Piper: Bioactivity and enzyme inhibition in Sitophilus zeamais and Tribolium castaneum

This study aimed to evaluate the insecticidal potential of essential oils (EOs) from four Piper species and selected phenylpropanoids against Triboliu m castaneum and Sitophilus zeamais, focusing on fumigant, topical toxicity, repellent effects, and enzyme inhibition related to detoxification and motor function. EOs from Piper aduncum, P. asperiusculum, P. auritum, and P. holtonii were obtained by steam distillation and analyzed by GC-MS. Apiol, dillapiole, and myristicin were isolated from the EOs using flash chromatography, while safrole was acquired commercially. Fumigant activity was tested using the vial-in-vial method, topical toxicity via direct application, and repellency through the area preference method. Enzymatic assays for acetylcholinesterase (AChE), catalase (CAT), and glutathione S-transferase (GST) were performed spectrophotometrically. Major constituents included dillapiole and apiol in P. aduncum, myristicin and dillapiole in P. asperiusculum, safrole in P. auritum, and apiol and dillapiole in P. holtonii. P. auritum EO showed potent fumigant and topical toxicity against T. castaneum (LC₅₀: 4.7–7.2 μL/L air; LD₅₀: 84.4–120.0 μg/insect), while P. asperiusculum EO was effective topically against S. zeamais (LD₅₀: 108.2–123.2 μg/insect). P. holtonii EO was highly repellent (90 % and 80 % repellency at 0.1 μL/cm²). Safrole exhibited the highest toxicity among constituents (LD₅₀: 34.4 and 12.6 μg/insect for S. zeamais and T. castaneum, respectively). Structure–activity analysis indicated decreased toxicity with more methoxy groups. Enzyme inhibition ranged from low to moderate (0–50 %), suggesting secondary involvement in insecticidal action. These results support the potential of Piper EOs and phenylpropanoids as eco-friendly pest control agents.