Alfalfa Flower Color Associated with Differential Seed Set by Leaf‐cutter Bees1

Abstract

AbstractMany studies have been carried out to demonstrate that alfalfa (Medicago sativa L.) plants vary in their attractiveness to bees. Flower color has often been implicated as one of the factors in attractiveness, but the reports are conflicting and inconclusive. Field observations by the authors indicated that yellow flowered alfalfa plants were tripped less frequently and set less seed than the common purple flowered plants.To investigate this phenomenon, an initial study of 18 pairs of plants pollinated by leaf‐cutter bees, Megachile pacifica (Panzer), revealed that the yellow flowered plants produced only one‐half as much seed (P = 0.05) as plants with purple flowers. A subsequent more detailed study was then carried out in an attempt to determine the relative importance of flower color per se as a factor in alfalfa pollination and seed set.Twenty pairs of yellow and purple flowered plants were studied in a spaced‐plant plot of ‘Ladak’ alfalfa which contained yellow flowered plants in a predominantly purple‐blue flowered population. Leaf‐cutters were used for pollination. Percent tripping was determined by observations of 100 florets on racemes taken at random from each plant. Other observations made were: number of florets (20 racemes), percent pods, seeds/pod, seed weight (g/200 seeds), and seed yield (g/plant). On the average, it was found that yellow flowered plants were tripped much less frequently (16% vs. 43%, P = 0.01), produced fewer seeds/pod (P = 0.01), lower percent pods (P = 0.01), and yielded half as much seed as purple flowered plants (P = 0.10).It was concluded that if flower color per se is not the predominant factor responsible for the lower tripping and seed set on the yellow flowered plants, nevertheless it is associated with this phenomenon. This study indicates the need for more comprehensive studies to determine the cause(s) for the observed differential tripping and seed set of yellow flowered plants.