Many wild bees prefer flowers in the violet-blue range—in part because these blossoms tend to produce high volumes of nectar. But it’s not easy for plants to produce blue flowers. Instead, a new study shows that many have evolved “blue halos” to allure bees, nanoscale structures on their petals that produce a blue glow when light hits them. The blue halo is created by tiny, irregular striations—usually lined up in parallel fashion—and is found in all major groups of flowering plants pollinated by insects, the scientists report today in Nature. They made their find by using scanning electron microscopy to examine every type of angiosperm—or flowering plant—including grasses, herbaceous plants, shrubs, and trees. The size and spacing of the nanoscale structures vary greatly, yet they all generate a blue or ultraviolet (UV) scattering effect particularly noticeable to bees, which have enhanced photoreceptor activity in the blue-UV parts of the spectrum. The scientists tested this attraction by exposing bumble bees to artificial flowers with three surfaces: smooth, iridescent, and striated to produce the blue halo. Despite the color of the flower, the bees preferred those with the blue halo. For us humans, the blue halo effect is most visible on flowers with dark pigments (like the South African Ursinia speciosa above), but not on lighter colored blooms.