Sensory basis of host-plant selection:: In search of the "fingerprints" related to oviposition of the cabbage root fly

The oviposition preference of the cabbage root fly, Delia radicum (Diptera, Anthomyiidae), was studied using leaf surface extracts of 24 different plant species that covered the whole span of preference rankings. The oviposition data were related to the content of the extract fractions containing either the glucosinolates or CIF ("cabbage identification factor", 1,2-dihydro-3-thia-4,10,10b-triaza-cyclopenta[:a.]fluorene-l-carboxylic acid). We observed a significant correlation between oviposition preference and the leaf surface content of benzyl and indolyl glucosinolates, substances that belong to the most active stimulants in oviposition assays, and in electrophysiological recordings from the tarsal D(4,3)-sensilla. However, there was not a significant correlation between the extract fraction containing CIF and the recorded neural activity in the tarsal C(5)-sensillum containing the CIF sensitive neuron. When this lack of correlation was investigated it was revealed that the leaf surfaces of two unacceptable host plants, Capsella bursa-pastoris and Tropaeolum majus, contain inhibitory compounds. Our data strongly support the hypothesis put forward by T. JERMY that "fingerprints" (specific mixtures of stimulatory and inhibitory plant compounds) mediate host-plant selection.