Toxic and behavioural effects of carvacrol and thymol on Frankliniella occidentalis larvae

Bioassays using bean leaf discs were conducted to investigate toxic and behaviour-modifying effects of carvacrol and thymol, two monoterpenoid phenols, on second instar larvae of Frankliniella occidentalis (Thysanoptera: Thripidae) at concentrations of 0.1% and 1%. We tested the contact toxicity of carvacrol and thymol to F. occidentalis larvae and studied the settlement preference of thrips larvae for treated or untreated bean leaf discs and the effects of the compounds on their feeding activity. Mortality rates after 24 h were 24.1% for carvacrol at 1% concentration, 16.5% for carvacrol at 0.1%, 7.7% for thymol at 1% concentration and 3.6% for thymol at 0.1%. Thrips larvae showed a significant preference for untreated leaf discs in choice tests with carvacrol 1% or thymol 1%. Like-wise, in the no-choice tests feeding damage after 24 h was significantly lower on leaf discs treated with 1% carvacrol or 1% thymol compared to the control, but not on discs treated with 0.1% concentration of either compound. We conclude that an application of carvacrol or thymol to bean leaves does not result in a high mortality of F. occidentalis larvae but affects their behaviour.