FIELD DISCRIMINATION OF OILSEED RAPE, BRASSICA-NAPUS VOLATILES BY CABBAGE SEED WEEVIL, CEUTORHYNCHUS-ASSIMILIS

The response of the cabbage seed weevil, Ceutorhynchus assimilis to ye llow water traps baited with some components of oilseed rape (Brassica napus) odor was studied in a series of field experiments. Four isothi ocyanates (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 N CS were attractive when released individually during the period when w eevils were immigrating to the host crop, but allyl NCS was not. A mix ture of these four isothiocyanates was attractive during the periods o f weevil immigration and dispersal, However, during the host colonizat ion phase, 2-phenylethyl NCS, like the mixture of the four isothiocyan ates, elicited no response or was repellent. Phenylacetonitrile and be nzyl alcohol were attractive during the colonization phase. The fatty acid derivative (Z)-3-hexen-1-ol did not affect weevil catch in an exp eriment conducted when the insects were already established in the cro p, but it depressed the catch in another conducted when weevils were d ispersing from the crop. These results suggest that yellow traps baite d 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.