From chemotype to control: insecticidal efficacy of piperitone and 2-cyclohexen-1-one analogues against dual-stage stored-product pests

Eucalyptus dives leaf oil, representing the piperitone chemotype, was characterized by GC/MS and evaluated for insecticidal efficacy against larval and adult stages of Tribolium castaneum and Plodia interpunctella. GC/MS analysis revealed that piperitone comprised 47.09% of the oil, followed by α-phellandrene (16.96%) and p-cymene (9.01%). The piperitone-rich E. dives oil exhibited fumigant- and contact-dependent insecticidal activity against P. interpunctella and T. castaneum, with toxicity varying by species, developmental stage, and exposure mode. Adult P. interpunctella showed high susceptibility to fumigant exposure, whereas T. castaneum displayed comparatively lower sensitivity across both life stages. Adult P. interpunctella exhibited an order-of-magnitude greater susceptibility to fumigant exposure than larvae, highlighting vapor-phase uptake as a dominant driver of toxicity in piperitone-rich chemotypes. Piperitone, corresponding to the dominant constituent identified in the oil, was confirmed as a contact-active compound, showing stronger larvicidal activity against T. castaneum than P. interpunctella. Comparative evaluation of selected 2-cyclohexen-1-one analogues, used as simplified structural models of the piperitone cyclohexenone core, revealed consistent structure-activity relationships (SARs), with 3-methyl-substituted analogues exhibiting higher toxicity than other structural variants. By integrating chemotype-resolved composition analysis, component-level bioassays, and parallel fumigant and contact assays across two pests and life stages, this study delineated how ketone-dominant Eucalyptus chemotypes translate structural features into exposure-dependent insecticidal outcomes. Across the cyclohexenone analogues, empirical SARs indicated higher toxicity for analogues bearing smaller substituents at the 3-position than at the 4-position. These findings underscore the potential of piperitone-rich E. dives oil and cyclohexenone analogues as environmental grain protectants.