Nectar in oak savannas: implications for butterfly conservation

Restoring critically imperiled midwestern oak savanna habitat is frequently guided by the requirements of the federally endangered Karner blue butterfly (Plebejus melissa samuelis). Although studies often correlate butterfly population size or density with nectar species abundance, nectar characteristics are seldom considered. We compared floral nectar resources across 15 sites categorized by Karner blue butterfly occupancy by quantifying the floral abundance, nectar volume, and sugar concentration for 22 species, calculating their mean nectar availability per stem, measuring environmental variables, and estimating a species' nectar sugar contribution to the landscape across seasons. Species identity predicted nectar volume and sugar concentrations. Mean nectar volume ranged between 0.02 and 2.20 μl and sugar ranged between 3.06% and 61.26% Brix per flower. Rubus flagellaris and Ceanothus americanus contributed the most nectar sugar per stem in the spring and summer, respectively. Sites with a history of occupation had 11 times greater nectar sugar available in the spring than previous release locations or restorations without occupation. Improved understanding of seasonal distribution, quality, and abundance of oak savanna nectar resources is likely to aid habitat restoration planning and conservation efforts for pollinators of this critically imperiled habitat.