Extrafloral nectary composition within and across selected Passiflora species : do patterns support the hypothesis that differential herbivore tactics promote alternative EFN traits?
This study determined extrafloral nectar (EFN) amino acid composition of 31 species of the salicoid genus Passiflora known for diverse EFN morphology to characterize quantitative and qualitative variation in EFN amino acid profiles within and between species. The work was motivated by the question of whether such diversity is driven by distinct tactics used by specialists herbivores i.e. whether EFN traits attract mutualistic defenders appropriate to tactics used by specialists herbivores. The coevolutionary relationship between Passiflora and Heliconius butterflies presents an ideal system to understand how herbivore pressure can drive indirect defenses such as EFNs. EFN nectar composition might differentially favor mutualistic defenders ranging from tiny egg parasitoids to large ants and wasps. While most Passiflora species analyzed are host to several heliconiine species, our study focused on the oviposition behavior of Heliconius that impacts vulnerable new shoots. Past observations indicate that Heliconius species that deposit single eggs on host new shoots experience high mortality from eggparasitoids. By contrast, Heliconius that lay groups of eggs on new shoots, are thought to satiate local egg parasitoids. Thus, Passiflora species vulnerable to group attack may rely on attracting predators like social insects with potential of a functional response to richer resources. This study sought evidence that signals EFN traits reflect these extreme modes of Heliconius oviposition tactics. Though Passiflora subgenus classifications were a significant contributor to EFN amino acid composition (r2=0.299) pointing to a taxonomic origin of specific amino acid compositions, species classifications were a larger contributor (r2=0.526), highlighting that composition could be driven by species specific herbivore pressure. The final part of the study examined known ant defenders that patrol two species, P. auriculata and P. vitifolia, hosts to heliconiine/Heliconius species with mass egg laying strategies. Neither Passiflora species showed significant difference in EFN amino acid composition between field populations and were used to model synthetic nectars for lab-testing preferences of the ants Crematogaster laeviscula and Pseudomyrmex gracilis. Synthetic nectar trials revealed ant preferences reflective of relationships witnessed in the field and preference for higher diversity of EFN amino acid composition