THE RAISON-DETRE OF PYRROLIZIDINE ALKALOIDS IN CYNOGLOSSUM-OFFICINALE - DETERRENT EFFECTS AGAINST GENERALIST HERBIVORES

In this study we tested whether pyrrolizidine alkaloids (PAs) of Cynoglossum officinale serve as antifeedants against herbivores. Total PA N-oxide extracts of the leaves significantly deterred feeding by generalist herbivores. Specialist herbivores did not discriminate between food with high and low PA levels. Three PAs from C. officinale, heliosupine, echinatine, and 3'-acetylechinatine, equally deterred feeding by the polyphagous larvae of Spodoptera exigua. Although the plants mainly contain PAs in their N-oxide form, reduced PAs deterred feeding by S. exigua more efficiently than PA N-oxides. On rosette plants, the monophagous weevil Mogulones cruciger significantly consumed more of the youngest leaves, which had the highest PA level and the highest nitrogen percentage. Larvae of Ethmia bipunctella, which are oligophagous within the Boraginaceae, did not discriminate between leaves. All generalist herbivores tested significantly avoided the youngest leaves with the highest PA levels. In the field, the oldest leaves also were relatively more damaged by herbivores than the youngest leaves. It is hypothesized that the skewed distribution of PAs over the leaves of rosette plants reflects optimal defense distribution within the plant.