Toxicity of monoterpenes against larvae and adults of Colorado potato beetle, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae)

In the present study, 30 pure monoterpenes were tested for their toxicity against first, second and third instars larvae and adults of Colorado potato beetle (Leptinotarsa decemlineata Say). The majority of tested compounds were found to be toxic to the larvae and the adults, however variable degree of toxicity with a range of 20–100% mortality. In general, monoterpene hydrocarbons exhibited high toxicity as compared with oxygenated monoterpenes. Among the tested monoterpene hydrocarbons, β-pinene, γ-terpinene, 3-carene as well as myrcene showed strong toxicity against all instars larvae. On the other hand, 1,8-cineole, fenchone, linalool and terpinen-4-ol were more toxic against the larvae as compared with the toxicities of other oxygenated monoterpenes. Borneol, fenchol, geranyl acetate, menthol, nerol acetate and α-terpineol showed weak toxicity against the larvae, whereas no significant toxicity was observed for isomenthol against the larvae. Some tested compounds showed different toxicities against both adults and larvae. For instance, menthone was more toxic against the adults than the larvae. Likewise, monoterpene hydrocarbons showed the higher toxicity against the adults than oxygenated monoterpenes. Limonene, α-pinene and β-pinene caused 100% mortality against the adults at two doses after 24 h of exposure. Among the tested oxygenated monoterpenes, 1,8-cineole, fenchone and menthone were much more effective against the adults. The present results indicated that 1,8-cineole, fenchone, β-pinene and γ-terpinene can be used as potential control agents against both the larvae and adults of Colorado potato beetle.