Agricultural development has reduced the abundance of tallgrass prairies and oak savannas to less than 1% of Midwestern vegetation. These reductions have led to a critical decline in the abundance and diversity of plants and their insect pollinators. Understanding the key resources for butterfly pollinators is vital for preserving and restoring native habitats and their biodiversity. As the primary adult food source for many butterflies, the composition of nectar can have important consequences on butterfly longevity and reproduction. Literature suggests flowers pollinated by butterflies are likely to have high sucrose and/or high amino acid concentrations. The aim of this study was to survey the sugar and amino acid concentrations of nectar in 19 native tallgrass prairie and oak savanna forb species of Northwest Ohio. We used a refractometer to measure sugar (sucorse) concentration and ninhydrin tests coupled with image analysis to determine amino acids. Because many flowers preferred by butterflies are too small to be directly sampled with microcapillary tubes, we also developed a modified centrifugation method to extract nectar from flower clusters while excluding potentially contaminating pollen.
We found sugar concentrations ranging from 31.5 to 66% Brix, while total amino acid concentration varied from 14 to 256 ng/µL. Interestingly, the two species with the highest sugar concentrations, Asclepias sullivantii and Monarda punctata, were also the species with the lowest total amounts of amino acids. To examine the generality of this pattern, a post-hoc ANCOVA revealed a significant inverse relationship between amino acids and sugar concentrations across genera. These data reveal significant differences in the specific nutritional quality of different nectars that are also likely to impact pollinator populations. Considering nectar resources for butterfly conservation are commonly assessed only in terms of nectar species composition or floral abundance, habitat assessments that also include evaluation of nutritional quality of floral resources are likely to improve efforts to restore declining pollinator populations and, in turn, assist reproduction of the plants they service.