The impact of flower size, height, color, density and uv markings on bee pollination of sweet peas, Lathyrus odoratus (Fabaceae).

Abstract

The obligate mutualism between flowering plants and their pollinators began over 100 million years ago; long enough that plants have evolved to attract pollinators (Buchmann and Nabhan 1996). Which traits are most attractive to pollinators is a subject of continual study. The purpose of this study is to determine which traits of Lathrys odoratus (sweet peas) result in longer and more honey bee and bumblebee visits. I observed that flower color, density, size, and height all differed among plants. Thus I predicted that characteristics that would make patches more visible to bees, such as bright color, high density, large size, and tall stems would result in more and longer visitations. Ultraviolet markings can also lead to increased bee visitation (Jones et al. 1986). Therefore, I predict that if UV markings were applied to pea plants, that plants with the markings would receive more and longer visitations. To test these hypotheses, ten 2m x 2m plots and seven 1m x 1m plots were randomly selected from 5 patches of pea plants at UMBS in Pellston, Michigan. For each of the 2m x 2m plots we recorded the proportion of stems and number of flowers of each color, the corolla length and banner height for 20 randomly selected flowers, the total number of stems, and the height of the four plants closest to the corners of the plot. We also recorded light measurements at the four corners of each plot before and after each observation period to test whether light was a confounding variable. Out of all these variables only stem density (p = 0.073, R2 = 0.347, df = 1) and flower density (p = 0.034, df = 8, R2 = 0.448) had a signficant effect on the length of visitations (p = 0.073, R2 = 0.347, F-ratio = 4.245, df = 1) and none had a significant effect on number of bee visits. For the uv portion of the experiment, the number and length of bee visits were recorded for stems that had their lateral petals sprayed with UV paint and stems that did not. There was no significant difference between stems that were sprayed and those that were not sprayed in terms of number and length of bee visits. Overall, it can be concluded from this study that patches with higher stem density have longer bee visitation times. Increased knowledge of the interrelationships between pollinators and their plant counterparts is becoming more important since the pollinator populations we depend on to pollinate a third of our food supply are declining all over the world from human interventions (Buchmann and Nabhan 1996).