Flower color offers a warm welcome to bees

Pollinators often use floral color signals as an indication of nutritional reward. Because bees have to maintain a certain body temperature, floral heat could be an additional incentive for them to land on one flower instead of another. Scientists at the Universities of Cambridge and London in the UK speculated as to whether bees prefer warmer flowers and if they can use floral color as a cue to decide which plants to visit.

As described in the Aug. 3 issue of Nature, the team introduced bees to two identical feeders: one at 18.5 °C and the other at 18.5, 22.5, 27 or 29.5 °C. The bees chose the warmer feeder, especially when the temperature difference was greater than 4 °C. Although this confirmed the first part of the hypothesis, it only exhibited the bees’ ability to use spatial positioning to identify heat.

Next, they exposed the bees to four purple flowers at 28.8 °C and four pink flowers at 20.8 °C. The flowers were positioned randomly and offered solutions with equal amounts of sucrose. The bees landed on the purple flowers 58 percent of the time. Conversely, when the pink flowers were warmer, the bees chose the pink flowers 61.6 percent of the time. They demonstrated no color preference when the flowers were the same temperature, nor did they exhibit a temperature preference when the flowers were the same color.

This led the researchers to conclude that bees use color, rather than other perceptual mechanisms, to gauge which flowers are warmer. They speculate that their findings may have evolutionary implications, suggesting that flora within and between species may adapt their temperature to encourage pollination.