Bioefficacy and biochemical responses to Acorus calamus essential oil-loaded nanoemulsions in Sitophilus oryzae

Post-harvest stored grain losses caused by insect pests are a growing concern, emphasizing the need for eco-friendly alternatives like plant-derived essential oils (EOs). However, due to their low solubility and persistence, EOs are formulated into nanoemulsions (NEs), which enhance their stability and effectiveness. Therefore, NEs based on 6% (v/v) Acorus calamus rhizome essential oil (ACEO) were evaluated for their contact insecticidal, fumigant, and repellent activity, alongside their impact on antioxidant systems and acetylcholinesterase activity against S. oryzae. GC/MS analysis revealed that the chemical profile of ACEO was predominantly composed of α-asarone (80.46%). ACEO was encapsulated into four NEs at varying EO:Tween80 ratios: ACNE1 (1:1), ACNE2 (1:3), ACNE3 (1:4), and ACNE4 (1:6). The particle sizes decreased with increasing Tween80 concentration, ranging from 329.62 nm (ACNE1) to 270.20 nm (ACNE2), 162.5 nm (ACNE3), and 30.49 nm (ACNE4). The Polydispersity Index (PDI) values were observed as 0.231, 0.124, 0.515, and 0.339 for ACNE1, ACNE2, ACNE3, and ACNE4. The zeta potential, which is the electrical potential difference between a particle's surface and the surrounding liquid that reflects the stability of colloidal dispersions, resulted in -12.60 mV, -11.58 mV, -7.07 mV, and -1.18 mV for ACNE1, ACNE2, ACNE3, and ACNE4, respectively. The ACNE4 demonstrated significantly enhanced insecticidal properties, including a six-fold increase in contact and fumigant toxicity as well as a five-fold repellency compared to ACEO after 24 hours. ACNEs also altered enzyme activities in S. oryzae, increasing oxidized glutathione levels while reducing glutathione, catalase, peroxidase, and acetylcholinesterase activities. The current study provides a first-time account of introducing ACNEs leading to the efficient, cost-effective use of ACEO in stored-grain insect pest management.