Extraction and chemical characterisation of agro-waste from turmeric leaves as a source of bioactive essential oils with insecticidal and antioxidant activities

Turmeric (Curcuma longa L.) is a significant crop that has historically been used worldwide as a medicinal plant, spice, food colouring agent, and a significant ingredient in cosmetic industries. After harvesting rhizomes, leaves are considered waste material. This research study aims to extract and chemically characterise the essential oil from the leaves waste of turmeric with an evaluation of different insecticidal, antioxidant, and phytotoxic activities. Subsequently, the contact toxicity, fumigant toxicity, and repellent activity were evaluated against two key stored grain insect species. The gas chromatography-mass spectrometry (GC-MS) characterisation revealed that α-phellandrene (28.95%), 2-carene (16.51%), eucalyptol (10.54%) and terpinolene (10.24%) were the major chemical constituents. The study's findings on the insecticidal effects of essential oils extracted from turmeric leaves revealed noteworthy repellent, contact (at 24 h, LC50 = 6.51 mg/cm2 for Tribolium castaneum and LC50 = 4.74 mg/cm2 for Rhyzopertha dominica) and fumigant toxicities (at 24 h, LC50 = 2.57 mg/L air for T. castaneum and LC50 = 2.83 mg/L air for R. dominica), against two key stored grain insects. In addition, turmeric leaf essential oil showed notable antioxidant activity (IC50 = 10.04 ± 0.03 µg/mL for DPPH assay; IC50 = 14.12 ± 0.21 µg/mL for ABTS assay. Furthermore, a phytotoxicity study was carried out on stored paddy seeds and no toxic effects were found on germination rate and seedling growth. So, it might be expected that the essential oils extracted from the turmeric leaf waste could be valorised and demonstrate their potential as safe botanical insecticides against stored-product insects, with noble antioxidant properties.