The attraction of bees to flowers is a sophisticated, multi-sensory process driven by co-evolution. Plants offer rewards (nectar and pollen) in exchange for pollination, and bees use specialized senses to locate these resources efficiently. This guidance relies on a combination of visual, chemical, and nutritional cues that direct the insect from its hive to the individual flower.
The Visual Spectrum: Color and Pattern
A bee’s perception of color differs significantly from human vision because it is shifted into the ultraviolet spectrum. Bees possess trichromatic vision, but their receptors are tuned to ultraviolet, blue, and green wavelengths. Consequently, the color red is generally invisible to bees, often appearing as a dark patch.
The colors most attractive to foraging bees fall within their sensitive range, including blue, violet, purple, blue-green, and yellow. Many flowers also display nectar guides, which are patterns only visible under ultraviolet light. These guides absorb or reflect UV light to create high-contrast patterns, directing the bee toward the center of the bloom where the nectar and pollen are located.
A specific color combination known as “bee’s purple” is created when a flower reflects a mixture of yellow or green wavelengths along with ultraviolet light. This distinct color is highly attractive to bees but is outside the realm of human vision.
The Chemistry of Attraction: Scent Signals
While visual cues guide bees at short range, scent provides the initial, wide-ranging signal of a flower’s presence. Flowers release a complex mixture of gaseous volatile organic compounds (VOCs) that form a scent plume detectable by a bee’s antennae. VOCs are the primary means of attraction over greater distances, drawing the insect into the immediate vicinity of the bloom.
The specific composition of VOCs helps the bee distinguish between plant species and assess a flower’s readiness to offer a reward. Common attractant VOCs include monoterpenes like (E)- and (Z)-\(\beta\)-ocimene, which attract generalist bees. This unique olfactory signature allows the bee to quickly identify and reinforce foraging behavior toward successful food sources.
Bees also use pheromones for communication within their colonies, distinct from floral VOCs. Some recruitment pheromones used by foraging bees mimic floral scents, signaling a newly discovered food source to nestmates. This internal chemical communication allows the colony to rapidly exploit a rich patch of flowers.
The Primary Reward: Nectar and Pollen
The nutritional reward reinforces a bee’s visit, consisting of nectar and pollen. Nectar is primarily a source of carbohydrates, providing energy for flight and metabolic functions. It is an aqueous solution containing varying concentrations of sugars, mainly sucrose, glucose, and fructose.
Nectar also contains trace amounts of amino acids, which influence a bee’s preference and contribute to the nectar’s overall nutritional value. Plants carefully regulate the sugar concentration in nectar to maximize bee visitation.
Pollen, conversely, is the bee’s main source of protein, lipids, and micronutrients, essential for larval development and colony health. Pollen protein content varies widely (10% to over 40% of dry weight) and contains all essential amino acids. The lipid fraction, including polyunsaturated fatty acids like linoleic and \(\alpha\)-linolenic acid, is also indispensable for bee nutrition.
Essential Non-Floral Attractants
A bee’s survival depends on non-floral environmental factors that attract them to a suitable habitat. Water is a fundamental resource, necessary for adult hydration, regulating hive temperature through evaporative cooling, and moistening crystallized honey. Bees prefer shallow water sources, such as wet moss or dishes with stones, which provide a safe place to perch while drinking.
Many bee species require specific sites for nesting and reproduction. The majority of native bees are ground-nesting and are attracted to patches of bare, undisturbed soil to excavate burrow systems. Other species, such as mason bees, are cavity-nesters, relying on hollow plant stems or wooden tunnels. They use materials like mud, resin, or leaf pieces to construct and protect their brood cells.
The attraction to salt and minerals drives a behavior known as puddling, where bees seek out moist soil or mud rich in dissolved inorganic compounds. This consumption of salts and minerals from non-floral sources supplements their diet and is important for reproductive success.