Toxicity and metabolic mechanisms underlying the insecticidal activity of parsley essential oil on bean weevil, Callosobruchus maculatus

Control of the bean weevil, Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae), in stored cowpea beans, Vigna unguiculata (L) Walp., when accomplished, is mainly achieved by applications of phosphine. However, the long-term use of this insect control practice has contributed to the selection of resistant populations, enhancing the need to develop alternative control tools. Due to their diverse biologically active compounds, the essential oils of aromatic plants such as parsley, Petroselinum sativum L., have been suggested as suitable alternatives for controlling insect pests. Here, we evaluated the toxicity (including the metabolic mechanisms underlying the insecticidal activity) of parsley essential oil against C. maculatus. Parsley essential oil controlled C. maculatus in a concentration-dependent manner (concentration increases resulted in lower weight losses and better germination levels of beans) with a fumigant toxicity (LC50: 489.5 mu L L-1 air) significantly lower than that observed for phosphine (LC50: 35.7 mu L L-1 air). However, when applied at their LC10 values (Parsley essential oil: 399.3 mu L L-1 air. Phosphine: 18.6 mu L L-1 air), the parsley essential oil caused significantly higher reductions in the emergence of C. maculatus compared with phosphine. Additionally, application of the synergists piperonyl butoxide (PBO), triphenyl phosphate (TPP) and diethyl maleate (DEM) demonstrated that glutathione S-transferases enhanced the parsley essential oil toxicity, while cytochrome P450-dependent monooxygenases formed part of the resistance mechanisms used by C. maculatus to mitigate the toxicity of the essential oil. Our findings suggest that parsley essential oil may be integrated into the control of C. maculatus only when synergized with PBO.