Abstract
Danaine butterflies sequester pyrrolizidine alkaloids (PAs) from several plant sources. The underlying mechanisms that mediate this interaction have not been explored. It is also underappreciated whether species that differ in PA demands forage differently. Decision-making in the southern monarch butterfly Danaus erippus (low PA-demand) was compared with the queen butterfly D. gilippus (high PA-demand) in relation to withered and freshly damaged leaves of Crotalaria spectabilis (monocrotaline-rich plant). The ultrastructure of the proboscis and mesothoracic legs of D. erippus, D. gilippus and D. plexippus was also analysed. Attraction of D. erippus and D. gilippus to withered or freshly damaged leaves of C. spectabilis was negligible. The two Danaus species did not scratch leaves and did not differ in feeding time on this PA plant. Butterflies did not visit vegetative PA sources in the field. The proboscis and mesothoracic legs of all Danaus species studied did not present hooks and/or spines for scratching leaves. Basiconic sensilla were found on the surface of the proboscis of all butterfly species analysed. In contrast to the current knowledge on Danaus, these results revealed that common species of the Americas are not attracted to vegetative PA sources. It is suggested that Danaus species may have diversified independently with respect to PA assimilation capacities and use.
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