Detoxification of pyrrolizidine alkaloids by the harvestman <i>Mitopus morio</i> (Phalangidae) a predator of alkaloid defended leaf beetles

The harvestman <i>Mitopus morio</i> (Phalangidae) is a generalist predator. It is known to prey on larvae of the chrysomelid leaf beetle <i>Oreina cacaliae</i> defended by plant acquired pyrrolizidine alkaloids (PAs). Tracer feeding experiments were performed to determine how harvestmen tolerate protoxic PAs. Minced meat containing either [¹⁴C]senecionine or [¹⁴C]senecionine <i>N</i>-oxide was fed to <i>M. morio</i> and subsequently feces and bodies were analyzed. Labeled alkaloid <i>N</i>-oxide remained stable and was eliminated almost unaltered with the feces; only 10% was recovered as tertiary PA. In contrast, approximately 80% of labeled tertiary alkaloid (senecionine) ingested with the diet was <i>N</i>-oxidized and eliminated; the remaining 20% consisted of unchanged senecionine and a polar metabolite of unknown structure. Harvestmen process their diet by excreting digestive juice, indicated by bleaching of the meat color. Analysis of the processed diet revealed some N-oxidation of [¹⁴C]senecionine, suggesting the gut as the site of <i>N</i>-oxidation. Analysis of the bodies of harvestmen 80 hours after the tracer feeding pulse revealed only trace amounts of the polar metabolite. Neither senecionine nor its <i>N</i>-oxide could be detected in the body extracts. The results are discussed in relation to the strategies of PA adapted insects to avoid accumulation of tertiary PAs in living tissues.