Sequestration of ingested [¹⁴C]senecionine <i>N</i>-oxide in the exocrine defensive secretions of chrysomelid beetles

<i>Oreina cacaliae</i> (Chrysomelidae) sequesters in its elytral and pronotal defensive secretion the <i>N</i>-oxides of pyrrolizidine alkaloids (PA <i>N</i>-oxides) from its food plant <i>Adenostyles alliariae</i> (Asteraceae). [¹⁴C]Senecionine <i>N</i>-oxide was applied for detailed studies of PA <i>N</i>-oxide sequestration. An average of 11.4% of total radioactivity is taken up by individual beetles which had received [¹⁴C]senecionine <i>N</i>-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 [¹⁴C]senecionine <i>N</i>-oxide is the only labeled compound in the defensive secretion. Beetles that fed on tertiary [¹⁴C]senecionine sequestered only trace amounts of radioactivity (exclusively present as labeled IV-oxide) in their secretions. <i>O. speciosissima</i>, a species also adapted to PA containing food plants, was shown to sequester [¹⁴C]senecionine <i>N</i>-oxide with the same efficiency as <i>O. cacaliae</i>. <i>O. bifrons</i>, a specialist feeding on <i>Chaerophyllum hirsutum</i> (Apiaceae), rejected PA treated leaf samples already at very low PA concentrations (10 nmol/leaf piece). In both <i>O. cacaliae</i> and <i>O. speciosissima</i>, [¹⁴C]senecionine <i>N</i>-oxide applied by injection into the hemolymph is rapidly transferred into the glands. <i>O. bifrons</i>, not adapted to pyrrolizidine alkaloid containing plants was unable to sequester [¹⁴C]-senecionineN- oxide in the secretion but rapidly eliminated the tracer with the frass. Again, only traces of labeled [¹⁴C]senecionine <i>N</i>-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 <i>N</i>-oxide form of PAs, similarly as their food plants, and that they lack the ability to efficiently <i>N</i>-oxidize tertiary PAs. No indication forde <i>novo</i> PA synthesis by the beetles was found in tracer feeding experiments with the biogenetic PA precursor putrescine.