Pyrrolizidine alkaloids: different acquisition and use patterns in Apocynaceae and Solanaceae feeding ithomiine butterflies (Lepidoptera: Nymphalidae)

Abstract

Pyrrolizidine alkaloids (PAs) often serve as chemical mediators of plant-herbivore-predator interactions. Butterflies (Danainae and Ithomiinae) and moths (Arctiidae) usually acquire PAs from plant sources (larval host plants, flowers or withered leaves visited by adults — pharmacophagy) and thereby become chemically protected against predators; they also use PAs as pheromone precursors. Study by GC-MS of PAs in three species of Ithomiinae butterflies, their larval host plants and adult alkaloid sources showed three different acquisition patterns: (1) larvae of the primitiveTithorea harmoniasequester PAs from their food plantPrestonia acutifolia(Apocynaceae: Echitoideae), and adults may also acquire these alkaloids from plant sources; (2) larvae of the more derivedAeria olenafeed onPrestonia coalita, in whose leaves no PAs were detected, but freshly emerged adults sometimes contain PAs and males intensively seek and sequester these alkaloids in plant sources; and (3) larvae of the still more advancedMechanitis polymniafeed on several PA-freeSolanumspecies, and adult males sequester the alkaloids from various plant sources. Males and females of all three species contain mostly two PAs, the diastereoisomeric retronecine monoesters lycopsamine and intermedine, stored in the N-oxide form. Larval host plants and adult plant sources showed a large array of PA structures, the most abundant and frequent being lycopsamine and its diastereoisomers intermedine, echinatine, rinderine and indicine, and the deoxy-analogues supinine and amabiline. Bioassays with wild caught and freshly emerged adults suggest that protection against predation by the orb weaving spiderNephila clavipesmay be dependent on PA concentration and maybe some spider idiosyncrasies, but freshly emergedAeria olenawithout PAs are also liberated byNephila, suggesting other protective compounds. The role of this spider as a selective pressure for PA acquisition by ithomiines is not clear.