The responses of the cabbage seed weevil (Ceutorhynchus assimilis ) to the odour of oilseed rape (Brassica napus ) and to some volatile isothiocyanates

The responses of the cabbage seed weevil, Ceutorhynchus assimilis Payk. (Coleoptera: Curculionidae) to volatiles from oilseed rape (Brassica napus L.) were tested using a linear track olfactometer. Weevils were attracted towards the odour of rape during a short period before diapause and for most of their post-diapause life. Odours from both the flowering and green parts of the plant were attractive, but the odour of a non-host plant (Bellis perennis L.) was not. An entrainment extract of flowering rape volatiles in pentane was attractive, but significantly less so than the odour of flowering rape itself. Attraction was found to 3-butenyl and 4-pentenyl isothiocyanate (NCS), but not to 2-phenylethyl isothiocyanate. A mixture of the three NCSs was more attractive than the individual NCSs. However, the concentration required to produce a response was still relatively high. Isothiocyanates, along with other volatiles from rape, probably play a role in host plant recognition by the seed weevil.