Sequestration, Maintenance, and Tissue Distribution of Pyrrolizidine Alkaloid <i>N</i>-Oxides in Larvae of Two <i>Oreina</i> Species

<i>Oreina cacaliae</i> and <i>O. speciosissima</i> are leaf beetles that, as larvae and adults, sequester pyrrolizidine alkaloid <i>N</i>-oxides (PAs) as defensive compounds from their host plants <i>Adenostyles alliariae</i> and <i>Senecio nemorensis</i>. As in most <i>Oreina species</i>, <i>O. speciosissima</i> is also defended by autogenously produced cardenolides (mixed defensive strategy), whereas <i>O. cacaliae</i> does not synthesize cardenolides and is exclusively dependent on host-plant-acquired PAs (host-derived defense). Adults of the two <i>Oreina species</i> were found to have the same PA storage capacity. The larvae, however, differ; larvae of <i>O. speciosissima</i> possess a significantly lower capability to store PAs than <i>O. cacaliae</i>. The ability of <i>Oreina larvae</i> to sequester PAs was studied by using tracer techniques with ¹⁴C-labeled senecionine <i>N</i>-oxide. Larvae of the two species efficiently take up [¹⁴C]senecionine <i>N</i>-oxide from their food plants and store the alkaloid as <i>N</i>-oxide. In <i>O. cacaliae</i>, there is a slow but continuous loss of labeled senecionine <i>N</i>-oxide. This effect may reflect the equilibrium between continuous PA uptake and excretion, resulting in a time-dependent tracer dilution. No noticeable loss of labeled alkaloid is associated with molting. Senecionine <i>N</i>-oxide is detectable in all tissues. The hemolymph is, with ca. 50–60% of total PAs, the major storage compartment, followed by the integument, with ca 30%. The alkaloid concentration in the hemolymph is approximately sixfold higher than in the solid tissues. The selectivity of PA sequestration in larvae is comparable to PA sequestration in the bodies of adult beetles.