Spring nectar sources for solitary bees and flies in a landscape of deciduous forest and agricultural fields: production, variability, and consumption1

Abstract

Although solitary bees and flies are important pollinators, little information is available on the nectar rewards of flowers they visit or their consumption of them, because small quantities of nectar are difficult to extract and measure. We studied five such plants in deciduous forests and agricultural fields in west-central Indiana, USA: three native forest herbs [Claytonia virginica L., Dentaria laciniata Muhlenb. ex Willd., Erigenia bulbosa (Michaux) Nutt.] and two exotic agricultural weeds [Barbarea vulgaris R. Br., Stellaria media L. (Vill.)], which often grow in close proximity. Using spectrophotometry, we measured daily sugar accumulation (in caged flowers) and standing crops (in uncaged flowers) at two sites on four dates per species. Dentaria laciniata was the most rewarding species (mean daily accumulations: 185–404 µg sugar/flower), followed by moderately rewarding B. vulgaris and C. virginica (24–113 µg sugar/flower) and low-rewarding E. bulbosa and S. media (7–38 µg sugar/flower). The presence of species with similar rewards in both habitats suggests that generalist bee and fly species can be expected to forage in both. Each flower species showed much variation in mean sugar accumulation per flower among population samples and among individual plants. Solitary bees were the most common visitors to all species (47 to 81% of visitation), followed by flies and honey bees. Insects consumed much, but not all (45 to 90%) of daily nectar accumulation by the five species. The more sugar a flower population secreted, on average, the higher its standing crop; visitation did not reduce rewards to a common level.