A study of insecticidal synergy of plant essential oil constituents against Trichoplusia ni (2015)
Chemical compositions of plant essential oils can be affected by many environmental and biological factors. Understanding the role of individual constituents and their interactions to overall insecticidal bioactivity is prerequisite to the use of essential oils as an alternative to conventional insecticides. In the present study, the chemical compositions of Rosmarinus officinalis (rosemary), Thymus vulgaris (thyme) and Cymbopogon citratus (lemongrass) essential oils were analyzed by gas chromatography-mass spectrophotometry (GC-MS), and relationships between chemical composition and toxicity of the constituents, and synergistic interactions of the major constituents of the oils were evaluated against third instar larvae and an ovarian cell line of the cabbage looper, Trichoplusia ni, via different application methods. To explore underlying mechanisms of synergy, penetration through the insect cuticle and inhibitory activities on three detoxifying enzyme systems were investigated. The most abundant constituents of rosemary, thyme and lemongrass oils were 1,8-cineole, thymol and citral, respectively, and their overall contributions to in vivo and in vitro toxicity varied according to the application methods, as did their designation as major active principles. A weak correlation between insecticidal activity and cytotoxicity was observed, indicating limitation of insect cell cultures as a screening tool for novel insecticides. Several synergistic interactions were found among the major constituents of each oil, including 1,8-cineole+camphor, thymol+p-cymene and citral+limonene. Analysis by GC-MS showed a significant penetration-enhancing effect of topically applied camphor by 1,8-cineole in their synergistic binary mixture. 1,8-Cineole was more toxic than camphor when applied topically to larvae, but a bioassay via injection revealed greater toxicity of camphor than 1,8-cineole. A bioassay combining injection and topical application confirmed the increased penetration of both compounds when mixed, showing the same bioactivity with higher amounts applied individually. A similar pattern of enhanced penetration of insecticides through the cuticle of T. ni in other synergistic combinations was observed as well. Lowered surface tension and increased solubility along with the interaction between essential oil constituents and the lipid layer of the insect’s cuticle may explain their enhanced penetration. Although some mild enzyme inhibitory activities were observed in essential oil-treated larvae, no correlation was observed between detoxicative metabolism and synergistic toxicity.