SEQUESTRATION OF INGESTED [C-14] SENECIONINE N-OXIDE IN THE EXOCRINE DEFENSIVE SECRETIONS OF CHRYSOMELID BEETLES

Oreina cacaliae (Chrysomelidae) sequesters in its elytral and pronotal defensive secretion the N-oxides of pyrrolizidine alkaloids (PA N-oxides) from its food plant Adenostyles alliariae (Asteraceae). [C-14]Senecionine N-oxide was applied for detailed studies of PA N-oxide sequestration. An average of 11.4 % of total radioactivity is taken up by individual beetles which had received [C-14]senecionine N-oxide with their food leaves 8 days before. An average of 28.9% of the ingested radioactivity could be recovered from the defensive secretions collected twice, i.e., 5 and 8 days after tracer feeding. The tracer transfer into the secretion seems to be a slow but progressive process as indicated by the high percentage of tracer still recovered from the secretion sampled after 8 days. Chromatographic analysis revealed that [C-14]senecionine N-oxide is the only labeled compound in the defensive secretion. Beetles that fed on tertiary [C-14]senecionine sequestered only trace amounts of radioactivity (exclusively present as labeled N-oxide) in their secretions. O. speciosissima, a species also adapted to PA containing food plants, was shown to sequester [C-14]senecionine N-oxide with the same efficiency as a cacaliae. a bifrons, a specialist feeding on Chaerophyllum hirsutum (Apiaceae), rejected PA treated leaf samples already at very low PA concentrations (10 nmol/leaf piece). In both O. cacaliae and O. speciosissimia, [C-14]senecionine N-oxide applied by injection into the hemolymph is rapidly transferred into the glands. O. bifrons, not adapted to pyrrolizidine alkaloid containing plants was unable to sequester [C-14]-senecionine N-oxide in the secretion but rapidly eliminated the tracer with the frass. Again, only traces of labeled [C-14]senecionine N-oxide were found in the defensive secretions of the two PA adapted species if labeled senecionine was injected. It is suggested that the beetles are adapted to the N-oxide form of PAs, similarly as their food plants, and that they lack the ability to efficiently N-oxidize tertiary PAs. No indication for de novo PA synthesis by the beetles was found in tracer feeding experiments with the biogenetic PA precursor putrescine.