Phenology and nectar chemistry contribute to sex-specific foraging patterns in the rare declining butterfly Speyeria idalia idalia

Background/Question/Methods

Variation of foraging behavior in plant-pollinator mutualisms influences the structure of pollination networks, pollination effectiveness, and can shape community diversity. While foraging differences between pollinator species have been widely studied, less is known about the role of intraspecific variation in pollinator behavior. Sex-specific differences in floral visitation related to differing reproductive investment, sensory bias, sexual dimorphism, and/or differences in phenology are particularly likely to generate intraspecific variation in the behavior of pollinator species. We examined 21 years of floral visitation by the at-risk butterfly, the eastern regal fritillary (Speyeria idalia idalia). This species is protandrous, with males emerging earlier than females to patrol for emerging females, which then persist longer into the season than males. We first evaluated visitation over the entire period in order to determine whether males and females differ in their patterns of floral use. We then assessed the role that phenology plays by comparing these patterns of visitation with patterns of visitation when males and females overlap in their phenology. Finally, we quantified amino acid and carbohydrate concentrations of male- and female-associated flower species in order to understand the role that different nutrient requirements may play in driving observed differences.

Results/Conclusions

While males and females did not differ in the richness of plant species visited (p >0.05), females associated with a greater effective number of species (3.35 ESN for females vs. 2.59 ESN for males p< 0.05), with dissimilarity of visitations higher than expected by chance in 20 of 21 years (Morisita-Horn dissimilarity index of 0.38 ± 0.22 SD). While diminished, differences persisted when we exclusively examined periods of phenological overlap between males and females (ESN of 3.23 for females, 2.46 for males, MH index of 0.17). Males preferred Asclepias syriaca and A. tuberosa¸ whereas females preferred Cirsium discolor, Cirsium pumilum, Monarda fistulosa, and Centaurea spp. in both the full (χ2 = 759.52, df =19, p < 0.01) and phenological overlap χ2 = 269.9 df =18, p < 0.01) dataset. Flower species visited by females were more likely to contain higher concentrations of overall carbohydrates, glucose, threonate, and galactinol. Total amino acid concentrations did not differ, though female-associated plants contained significantly greater concentrations of proline, glycine, and leucine. Sex-specific foraging persists across decadal timescales in our system, demonstrating that sex-specific foraging is a promising avenue of research into intraspecific variation in plant-pollinator networks.