Interactions Between Floral Resources and Bees (Hymenoptera: Megachilidae) in Commercial Alfalfa Seed Fields

Abstract

We monitored flower resources around 9 Megachile rotundata (F.) bee shelters in 7 commercial seed fields of alfalfa, Medicago sativa L. (Leguminosae) seed fields to understand better the interactions between floral resources and bee pollinators. A composite pattern of floral resource availability demonstrated an initial burst of bloom, followed by a linear decline in total flowers per raceme and an exponential decline in numbers of racemes with open flowers over the season. Open flowers per raceme declined exponentially at a more rapid rate than did total flowers per raceme, and standing crop of nectar sugar declined even more rapidly. Thus, floral resources available to bees declined to low levels within 3–4 wk of bee introduction. Pollinator activity was relatively constant within a week or 2 of release of bees into the fields. Numbers of open flowers and nectar availability declined more rapidly close to bee shelters than at a distance from them. We interpret the rapid decline in floral resources partly as a result of steady pollination over time. The decline in floral resources may contribute to poor bee yields in alfalfa seed fields.