Bacteria species known to induce the biosynthesis of crucifer-specific phytoalexins have ear-lier been shown to be associated with Delia flies. Eleven crucifer-specific phytoalexins and related synthetic com-pounds were applied on surrogate paper leaves and offered to cabbage root flies in oviposition assays. Since three of these compounds (methoxybrassinin, cyclo-brassinin, brassitin) proved to be significantly stimula-tory whereas the remaining metabolites had no effect, the reaction of the fly appears to be structure-specific. Inactive phytoalexins had no inhibitory effect on ovipo-sition. 100 mg of methoxybrassinin per surrogate leaf was as stimulatory as 0.05 gle (gram leaf equivalent) of a methanolic host-leaf (Brassica oleracea) extract. Thus the three active phytoalexins can explain only part of the activity of host-pant extracts but might induce a preference for infected plants.
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