The response of the cabbage seed weevil, Ceutorhynchus assimilis to yellow water traps baited with some components of oilseed rape (Brassica napus) odor was studied in a series of field experiments. Four isothiocyanates (NCS), five other amino acid derivatives (aromatic compounds), and two fatty acid derivatives are detected by peripheral olfactory receptors of C. assimilis. 3-Butenyl, 4-pentenyl, and 2-phenylethyl NCS were attractive when released individually during the period when weevils were immigrating to the host crop, but allyl NCS was not. A mixture of these four isothiocyanates was attractive during the periods of weevil immigration and dispersal. However, during the host colonization phase, 2-phenylethyl NCS, like the mixture of the four isothiocyanates, elicited no response or was repellent. Phenylacetonitrile and benzyl alcohol were attractive during the colonization phase. The fatty acid derivative (Z)-3-hexen-1-ol did not affect weevil catch in an experiment conducted when the insects were already established in the crop, but it depressed the catch in another conducted when weevils were dispersing from the crop. These results suggest that yellow traps baited with 2-phenylethyl NCS can be used to monitor immigration into crops in the spring and that phenylacetonitrile may be useful for assessing numbers of weevils colonizing the crop throughout the summer.