Impact of Flower Standing Crop and Pollinator Movement on Alfalfa Seed Yield

For agricultural crops that depend on pollination, the impact of flower abundance and pollinator movement on seed or fruit yield is of economic importance, and may have implications for crop and pollinator management. Using alfalfa, Medicago sativa L. (Leguminosae), I tested the hypothesis that seed set is lower when standing crop of open flowers is high than when standing crop is low. This could occur if pollinators move more among flowers on the same plant, causing self-pollination, when flowers are abundant. In a greenhouse experiment I compared seed set for 3 simulated pollinator movement patterns between flowers on the same versus different plants, under the 2 standing crop regimes generated by varying the number of flowers hand-pollinated each day. Treatments maximized, minimized, and were intermediate in movement between flowers on different plants. Significantly more seeds were produced per pod when flower standing crop was low than when standing crop was high. I hypothesized that movement pattern would have a greater effect when standing crop was high rather than low. Despite a trend in the expected direction, this interaction was not significant. Field observations of within versus between plant movement of the pollinator, Megachile rotundata (F.) (Megachilidae), indicated that the bees visit more flowers per raceme when standing crop is high than when standing crop is low. In a series of hand pollinations among flowers on the same plant, seeds per pollination decreased between the 1st flowers pollinated and later flowers pollinated in succession. Alfalfa may yield less seed when flowers are abundant, suggesting that early introduction of bees into alfalfa seed fields is desirable to maintain a low flower standing crop.