Antifeedant effects of common terpenes from Mediterranean aromatic plants on Leptinotarsa decemlineata

Essential oil terpenes are secondary metabolites produced in different biological pathways as pollinator attraction signals, but also as defense mechanisms against insect pests, herbivores, microorganisms, competing plants, etc. In this context, we have evaluated a total of 24 terpenes commonly found in Mediterranean aromatic plants, including 8 monoterpene hydrocarbons, 9 oxygenated monoterpenes, 2 esterified monoterpenes, 3 sesquiterpene hydrocarbons and 2 oxygenated sesquiterpenes in order to determine their antifeedant effects on the Colorado potato beetle (Leptinotarsa decemlineata Say), a major pest of potato crops. Terpene hydrocarbons showed low antifeedant activity, whereas the oxygenated sesquiterpene (-)-alpha-bisabolol with an inhibition activity of 96.3% was the most active, followed by carvacrol (90.9%), (+)-Terpinen-4-ol (87.1%) and thymol (81.5%). Other terpenes like (1S)-(-)-verbenone (72.9%), (+/-)-camphor (63.4%) and linalyl acetate (60.7%) showed moderate activity. Subsequently, terpenes showing antifeedant activity against this insect were also tested in allelopathic assays to determine potential damage to the crops. Results showed phytotoxic effects for (1S)-(-)-verbenone on Lactuca sativa germination, and for carvacrol on both the seed germination and the leaf and root growth of Lolium perenne. In conclusion, (-)-alpha-bisabolol is the best option to develop natural antifeedant formulations against L. decemlineata on the basis of its high antifeedant and low phytotoxic activities.