Chemical composition and biological efficacy of Acorus calamus (L.) rhizome essential oil on Sitophilus oryzae (L.), Rhyzopertha dominica (F.), and Oryzaephilus surinamensis (L.) as stored-grain protectants

Key depredating agents affecting grains are insects which lead to considerable losses across post-harvest food supply chain. As their control strategies, plant-derived essential oils have become optimal competitors to synthetic insecticides. In view of this background, chemical composition, insecticidal activity in comparison with ultrastructural impact on cuticular topography, repellency and feeding deterrence of Acorus calamus (L.) rhizome essential oil were investigated towards Sitophilus oryzae (L.), Rhyzopertha dominica (F.), and Oryzaephilus surinamensis (L.). Chemical profile of oil was examined by gas chromatography-mass spectrometry (GC-MS). Morphological alterations on insects' cuticles triggered by oil's direct exposure were analyzed by scanning electron microscopy (SEM). alpha-Asarone (89.05%) was the major constituent of oil. Insecticidal bioassays showed that O. surinamensis (LC50 = 1.92 mu l/g) was more susceptible than R. dominica (LC50 = 4.78 mu l/g) and S. oryzae (LC50 = 9.61 mu l/g) upon 24 h of exposure. Degree of insects' repellence was arranged in the declining order of O. surinamensis (RC50 = 0.96 mu l/cm(2)) > S. oryzae (RC50 = 1.02 mu l/cm(2)) > R. dominica (RC50 = 1.56 mu l/cm(2)) after 1 h, wherein estimated values increased after 5 h of exposure. According to flour-disc bioassays, oil had the weakest feeding deterrence on O. surinamensis (AC(50) = 4.37 mg/disc) leading to post-ingestive lethality induced by phagostimulation, than against S. oryzae (AC(50) = 0.88 mg/disc) and R. dominica (AC(50) = 1.80 mg/disc). SEM observations reflected severe impact on insects' cuticular topography suggesting that oil molecules might be entered into their bodies via cuticular penetration and oral ingestion influencing contact/post-ingestive toxicity. Hence, results demonstrate the potentiality of A. calamus essential oil for sustainable insect pest management in stored-grain ecosystems.