Evaluation of methyl salicylate, limonene, and thymol as potential biocidal against Trogoderma granarium everts larvae

The investigation of plant-based natural products, especially essential oils, as insecticide alternatives, responds to challenges as induced resistance and environmental drawbacks linked to prolonged synthetic insecticide use. Although essential oils demonstrate promising activity, their utilization faces hurdles related to standardization. In this study, contact/fumigation properties of three molecules (methyl salicylate, limonene and thymol) known for their activity against stored product beetles, were tested in laboratory bioassays toward young and old larval stages of Trogoderma granarium. The experiment revealed significant differences in their effects based on larval age and chemical concentration. Methyl salicylate exhibited concentration-dependent effects on both young and old larvae, with higher concentrations leading to accelerated mortality rates. For instance, young larvae exposed to 10 mg displayed an LT50 of 33.75 h, while old larvae showed an LT50 of 39.07 h. Similarly, at a concentration of 5 mg, young and old larvae exhibited LT50 values of 72.79 and 79.68 h, respectively, after treatment. Limonene also exhibited concentration-dependent effects, although with lower potency compared to methyl salicylate. Thymol showed efficacy only against young larvae of T. granarium. Statistical analyses confirmed the significant impact of each chemical on larval survival time, with tested chemicals displaying significant concentration-dependent variations in mortality rates. These findings suggest the potential of methyl salicylate, and to a lesser extent limonene and thymol, as effective biocidal agents against T. granarium larvae.