Comparative study on the repellency and chemical profiles of different chilli peppers formulations against Sitophilus oryzae (L.) (Coleoptera: Curculionidae) in stored wheat

Abstract

The weevil Sitophilus oryzae (L.) (Coleoptera: Curculionidae) is the major insect pest of stored grains, including wheat. This study tested the repellency effectiveness of three formulations, including chilli dry powder (CDP), chilli ethanol extract (CEE), and chilli crude extract (CCE), from different chilli peppers of Capsicum annuum L. and Capsicum chinense Jacquin varieties (Solanaceae), against S. oryzae using a modified cup bioassay arena. Repellency responses varied significantly (P ≤ 0.001) within the three different formulations, where the CEE formulation at the concentration of 5 g (w/w) demonstrated the highest effectiveness, effectively deterring the highest number of insects (mean number of repelled insects ± SE) after 24 h of treatment across tested chilli varieties such as Guntur Sannam chilli (GSC) (15.0 ± 0.6), Kashmiri chilli (KC) (16.0 ± 0.0), and Bhut Jolokia chilli (BJC) (18.0 ± 0.6). CDP formulation closely followed, displaying notable repellency activity against the same varieties. Conversely, the CCE formulation exhibited the least effectiveness in repelling the adult weevils. Thus, the repellency activity observed was time-, concentration-, and formulation-dependent. Further, the percentage repellency (PR) assay conducted by following the area preference method demonstrated that the Bhut Jolokia chilli exhibited the highest repellency percentage (61.2%) at a concentration of 0.251 μg/cm², by categorizing it as a class IV repellent. Gas chromatography-mass spectrometry (GC-MS) analysis detected the presence of capsaicinoids like capsaicin and dihydrocapsaicin, alongside various non-capsaicinoids such as n-hexadecanoic acid and n-pentadecylacetamide, etc., in the ethanolic fraction of chilli pepper varieties, suggesting that the observed differences in the repellency activity might be attributed to varying chemical compositions, warranting further detailed exploration in this domain. These findings provide valuable primary insights into the prospective application of chilli pepper formulations from different varieties as a repellent against S. oryzae.