Abundance and Spatial Distribution of Bees Visiting Male-sterile and Male-fertile Sunflower Cultivars in California

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Abstract
Observations on the density and distribution of bees visiting two male-sterile (MS) and one male-fertile (MF) cultivar of sunflowers over a 3-yr period in commercial seed production fields in California revealed the honey bee, Apis mellifera L., to be the most abundant pollinator, making 98.9% of all visits. Honey bees were uniformly distributed across the male-sterile rows. Comparisons of bee density (mean number of bees per head) by cultivar indicated a preferential pattern of visitation in the order MS206 (1.25, 1.46, and 0.62, in three plots) > MS207 (0.18, 0.19, and 0.22) > MF207 (0.08, 0.07, and 0.07). Difference in nectar production may be a possible cause of discrimination, in addition to differences in inflorescence type, size, and color. Discrimination by honey bees may be an important limiting factor in pollination because bees confining their visits to one parent would reduce pollen flow.