The sunflower, belonging to the genus Helianthus, is a visually striking flower frequently covered with buzzing insects, confirming a strong attraction for bees. However, this attraction is not uniform, as the plant employs sophisticated biological mechanisms that appeal to some bees more than others. The relationship between the sunflower and its visitors is a complex interaction of visual cues, physical structure, and variable nutritional rewards. Understanding this partnership requires a deeper look into the flower’s design and the different preferences of the bee species that depend on it.
Why Sunflowers Are Magnetic to Pollinators
The sunflower head is not a single flower but an inflorescence known as a capitulum, a dense cluster of hundreds of tiny individual flowers called florets. The outermost yellow ray florets are sterile and serve primarily as a visual advertisement and landing platform for foraging insects. The true reproductive units are the disc florets packed into the center, which open sequentially to present pollen and nectar over several days.
A major draw for bees is the sunflower’s unique pattern of light reflection, which is invisible to the human eye. Bees can see ultraviolet (UV) light, and sunflowers display a UV-absorbing “bullseye” pattern at the center of the flower head. This pattern acts like a runway, visually guiding the bee directly toward the nectar and pollen resources concentrated in the disc florets. Genetic research has shown that the size of this UV bullseye can vary and is controlled by a single gene, with larger patterns generally attracting more bees.
The compounds responsible for UV absorption are flavonol glycosides, which serve a dual purpose for the plant. These molecules create the visual target for pollinators and also help the sunflower manage water loss. Sunflowers growing in drier climates often have larger UV patterns, correlating with a greater ability to retain moisture. This trait efficiently addresses both the plant’s reproductive success and its survival. Each floret also produces nectar that is a reliable source of sugar, the bee’s primary energy fuel.
The Pollinator Spectrum Which Bees Visit Sunflowers
Sunflowers support a wide array of bee species, categorized into generalists and specialists based on their diet. European honey bees (Apis mellifera) and common bumblebees (Bombus species) are generalists that visit almost any flower providing sufficient resources. They are frequent visitors to sunflower fields, which often provide a mass-flowering resource during the late summer when other floral sources may be scarce.
In contrast, native insects known as long-horned bees (Melissodes species) are specialists, or oligolectic, meaning their diet is highly focused on the sunflower family (Asteraceae). Species like Melissodes trinodis and Melissodes agilis are colloquially known as “sunflower bees” because they rely almost exclusively on Helianthus pollen to provision their underground nests. Female bees actively seek out pollen-producing plants to feed their developing young.
The quality of the sunflower’s pollen for bee nutrition presents a nuanced picture. While the pollen is sometimes labeled as having a lower crude protein content compared to other floral sources, it offers unique health advantages. Studies have shown that consuming sunflower pollen can reduce the infection intensity of the gut parasite Crithidia bombi in bumblebees. This medicinal quality suggests that sunflower pollen provides a significant benefit to bee health, especially when included as part of a varied diet.
Sunflower Cultivation and Bee Nutrition
Modern agriculture and ornamental gardening have introduced considerable variation in the sunflower’s value to bees. Most commercial and garden sunflowers are hybrids bred for specific traits, and these breeding efforts often impact the floral rewards available to pollinators. A major distinction exists between traditional oilseed varieties and many ornamental cultivars.
Oilseed varieties, grown for seed and oil production, are typically male-fertile and produce copious amounts of pollen and nectar. These types are highly sought after by wild bees and specialists like Melissodes, who require the pollen to reproduce. In contrast, many ornamental and cut-flower varieties are bred to be male-sterile, or “pollen-less,” a trait desirable for florists and consumers who want to avoid falling pollen.
Pollen-less hybrids still produce nectar, which attracts generalist nectar-foragers like honey bees seeking a sugar boost. However, the lack of pollen means they are nutritionally incomplete for bees that require it for larval development and colony protein stores. Wild bees show a strong preference for visiting male-fertile plants because they forage for both nectar and pollen to meet their colony’s diverse dietary needs.
The difference in floral resources between male-fertile and male-sterile lines can influence how bees behave on the plants. When both types are grown together, differences in nectar volume or other traits can lead to bees exhibiting high constancy, meaning they are less likely to move between the two types. This reduced crossover can negatively affect the pollination required for producing hybrid sunflower seeds. For gardeners aiming to support a diverse and healthy bee population, choosing pollen-rich, traditional varieties ensures the fullest range of nutritional rewards for all bee visitors.