Not all toxic butterflies are toxic: Patterns of chemical sequestration in a subtropical community of Troidini swallowtails

Background/Question/Methods

Swallowtail butterflies in the tribe Troidini (Papilionidae), a group that sequesters chemical defenses from its host plant, have been a model group for development of theory on host plant chemical sequestration. Troidini butterflies specialize on plants in the genus Aristolochia (Aristolochiaceae), which possess toxic alkaloids called aristolochic acids (AAs), and sequester AAs as larvae, rendering both larvae and adults chemically defended against most predators. Although Troidini butterflies are predominantly tropical, with many co-occurring species concentrated in the lowland forests of Central and South America, most work on this group has focused on a single species in North America at the northern most range of the distribution of this tribe. The goal of this study is to analyze the chemical composition (i.e., aristolochic acid content, using HPLC chemical analyses) of both swallowtail butterflies and their Aristolochia host plants in areas where greater diversity of Troidines is found. We aim also at examining whether variation exists in host plant preference and performance among co-occurring butterfly species reared on different Aristolochia host plants. We conducted this study in Iguazú National Park, where at least five Troidini butterflies species and two species of Aristolochia co-occur.

Results/Conclusions

We find that not all host plant species used by Troidines possess the alkaloids that these butterflies sequester as larvae. Aristolochia triangularis, a very abundant species commonly used by Troidini butterflies, did not possess AAs. Also, preference experiments showed that A. triangularis was the most preferred species by Troidines. Further, contrary to the currently accepted paradigm, in our study system most Troidines do not possess chemical defenses, showing that toxicity itself can be polymorphic within a population. This suggests that in addition to these butterflies being involved in Batesian mimicry complexes, they might also be involved in automimicry, with butterflies without AAs benefiting from the presence of toxic co-specifics. We also found that if the opportunity to feed on a more tender host plant is given, these swallowtails butterflies choose to feed on this host plant indiscriminately of its AA content. Therefore, the amount of AAs, which can serve as defense against predators, seems not to play a role in their larval food preference. By studying Troidini species in regions previously understudied (i.e., outside of North America), and in areas where they reach higher diversity, we are obtaining a more complete picture of the chemical ecology of this model group.