Carvone and less volatile analogues as repellent and deterrent antifeedants against the pine weevil, Hylobius abietis

  The monoterpenoid carvone ( 1 ) has been shown to have strong antifeedant effects on Hylobius spp. However, because of the high volatility of carvone, long‐time protection of conifer seedlings in the field using this compound has not been possible. We demonstrate, in several bioassay steps, that less‐volatile, heavier analogues retain their pre‐ingestive feeding inhibition activity in the large pine weevil, Hylobius abietis (L.) for a longer time. The first step in the evaluation of the biological activity of 12 carvone analogues was a micro‐assay, a choice test lasting 4 h. Compounds active at 100 nmol/cm 2 were further dose–response tested to give the effective dose needed to inhibit feeding by 50% (ED50). Of the 14 compounds tested, including both carvone enantiomers, seven heavier analogues were active at low doses (had low ED50 values). As expected from their lower vapour pressure compared with carvone, the heavier analogues proved more resistant to evaporation before testing. Thus, whereas the effect of 8‐hydroxy‐ p ‐menth‐en‐6‐one 4 declined after 2 days, some of the compounds with high molar masses, such as the alkylhydroxymenthenones 6 and 8 , retained a stable activity for 4 days. The retained activity at 4 days was strongly correlated to molecular mass and boiling point. When tested on natural material (host Pinus sylvestris L. twig sections for 48 h), the heavier analogues showed a rather low activity. Probably, the activity of the more volatile compound carvone ( 1 ) is due to a repellent effect (olfactory mode) rather than the deterrent effects (gustatory mode) of the heavier compounds. In agreement with the relatively low activity on twigs in the laboratory, the hydroxymenthenone 4 was not active in the field when tested for 2 months as a 1 : 9 mixture with a polar wax.