Pyrrolizidine derivatives in the Lepidoptera

The Monarch butterfly Danaus plexippus) was found to sequester and store pyrrolizidine alkaloids when fed on homogenized leaves of Senecio vulgaris. It is thought that the presence of these alkaloids in the butterfly may contribute towards its defence mechanism, by making it unpalatable to potential predators. Unlike many other danaids, the Monarch is not dependent on pyrrolizidine alkaloids as precursors of its sex pheromones. 

Moths of the families Ctenuchidae and Arctiidae are also attracted to plants containing pyrrolizidine alkaloids. In particular, larvae of Nyctemera annulata BoisJuval (Arctiidae) (the Magpie moth) ingest pyrrolizidine alkaloids from a variety of Senecio species. Benn et al. have shown that larvae feeding on S. spathulatus A. Rich are able to store pyrrolizidine alkaloids. These alkaloids subsequently appeared in the adult moths and their eggs, and also in a parasite of N. annulata larvae. 

Male Danaus butterflies feed on plants containing pyrrolizidine alkaloids and convert the alkaloids into dihydropyrrolizines such as da-naidone (86), danaidal (87), and hydroxydanaidal (88). These derivatives are used as pheromones 48). Danaid butterflies are also able to store pyrrolizidine alkaloids. This may act as a defence mechanism by rendering 

A one-step synthesis of the pheromone danaidal (87) from N-formylproline and propargylic aldehyde has been reported by Pizzorno and Albonico (229). 

---

The occurrence of pyrrolizidine derivatives in the Lepidoptera is not confined to butterflies, but has also been observed in the tiger moth, where their function may be similar to that established in one species of butterfly. Male tiger moths (fam. Arctiidae) possess scent organs in the form of inflatable coremata which have a pheromone-distributing function. Some of these species are known to use plants containing pyrrolizidine alkaloids, e.g. Heliotropium europaeum and Echium... 

---