Efficacy of 12 commercial essential oils as wheat protectants against stored-product beetles, and their acetylcholinesterase inhibitory activity

In this study, the insecticidal effect of 12 essential oils (EOs) from Coriandrum sativum, Tanacetum annuum, Boswellia carteri, Canarium luzonicum, Pogostemon cablin, Thymus vulgaris, Copaifera officinalis, Corymbia citriodora, Melaleuca cajuputi, Syzygium aromaticum, Santalum album, and Elettaria cardamomum as grain protectants was evalu- ated against Trogoderma granarium, Tribolium castaneum, Tribolium confusum and Tenebrio molitor. The EOs were applied on wheat at two concentrations (500 and 1000 ppm). Beetle mortality rates were evaluated after 4, 8, and 16 h, and 1, 2, 3, 4, 5, 6 and 7 days. The EO from S. aromaticum was the most effective against T. granarium adults, leading to 100% mortality after 16 h of exposure at 1000 ppm, and 83.3% at 500 ppm after 7 days. Complete mortality on this species was achieved testing the EO from B. carteri, after 6 days of exposure at 1000 ppm. EOs from S. album and M. cajuputi led to 96.7% mortality when tested at 1000 ppm. Mortality of T. granarium larvae was low for all tested EOs, except for S. aro- maticum and B. carteri, which exhibited moderate toxicity at 1000 ppm, reaching 56.7 and 51.1%, respectively. Mortality of T. castaneum adults was very low for all tested EOs, not exceeding 43.3%, as it was noted testing S. aromaticum EO. The EOs from E. cardamomum, T. annuum, and S. aromaticum caused ≥ 90% to T. castaneum larvae 7 days post-exposure. Concerning T. confusum adults, the tested EOs achieved low mortalities even at the higher concentration, which did not exceed 26.7%. The EOs from T. annuum, S. album, M. cajuputi, C. officinalis, S. aromaticum, C. citriodora and P. cablin exhibited high larvicidal activity 7 days post-exposure. The EOs from T. annuum and C. luzonicum were the most effective against T. molitor adults, at both tested concentrations, i.e., 92.2 and 83.3% at 500 ppm, 93.3 and 90.0%, respectively, after 7 days of exposure. Mortality of T. molitor larvae was low at both tested concentrations, reaching maximally 30.0%. The 12 EOs were also tested for inhibitory action against acetylcholinesterase (AChE). The most active samples were those from M. cajuputi, E. cardamomum and T. vulgaris, with IC50 values of 3.06 (±0.5), 4.05 (±0.8) and 4.13 (±1.7) mg mL-1, respectively. These results were not consistent with those from insecticidal tests, suggesting that other mechanisms of action are involved. Overall, the EOs from S. aromaticum, T. annuum and C. luzonicum exhibited high toxicity against T. granarium and T. molitor adults or T. castaneum and T. confusum larvae. They may be considered further for developing novel insecticides for managing beetle pests attacking stored products.