The chemical ecology of defense in arthropods

<i>Oreina cacaliae</i> (Coleoptera, Chrysomelidae) produces in its elytral and pronotal defensive secretion seneciphylline N-oxide together with small amounts of another pyrrolizidine alkaloid tentatively identified as senecionine N-oxide. This is a strong departure from the chemical composition of the defensive secretions in related species, characterized by complex mixtures of cardenolides, synthesized by the beetles from cholesterol. It is suggested that <i>O. cacaliae</i> sequesters the alkaloids from its host-plant, <i>Adenostyles leucophylla</i>. Other specimens of <i>O. cacaliae</i> from far distant populations feeding on <i>Senecio nemorensis</i>, <i>Petasites paradoxus</i> or <i>P. album</i> also produced pyrrolizidine alkaloids, but not <i>O. speciosissima</i> feeding on the same food plants and producing cardenolides. In addition to pyrrolizidine alkaloids, <i>O. cacaliae</i> secretes ethanolamine, which is also found in all the cardenolide-producing species.