Isolating the effects of floral temperature on visitation and behavior of wild bee and fly pollinators

Flower temperature impacts pollinator choice in laboratory settings and the foraging behavior of bees in natural populations. However, the direct effects of floral temperature on animal pollination in field conditions are difficult to isolate from other pollinator-perceived traits that may affect flower temperature, such as shape and color. We experimentally manipulated the temperature of model flowers while keeping insect-perceived color and floral shape constant by using infrared (IR) reflecting paint. We tested the efficacy of this method across an elevation gradient and evaluated the effect of flower temperature on pollinator visitation and behavior in populations of Argentina anserina. The cooling effect of the IR-reflecting treatment was effective in alpine populations (>3000m) but not at lower elevations (<2500m). A manipulative glasshouse experiment showed that the cooling effect of floral IR reflection was stronger under higher IR irradiance. This suggests that elevational change in solar intensity contributes to elevation-specific effects of IR reflectance on flower temperature. The impact of flower temperature on bee and fly visitation, basking, and the duration of forgaing depended strongly on air temperature. When air temperature was high (>30 degrees C), cooler flowers received more visits than warmer flowers. At high elevations, flies increased the time spent in warmer flowers when air temperatures were cool. However, flies foraged longer in cooler flowers when air temperatures were warm. Flower temperature impacted the likelihood of bees basking, but not their visitation rate or foraging duration. By experimentally isolating floral temperature from other floral traits, we found direct effects of flower temperature on the visitation and behavior of wild pollinators, especially during periods of more extreme air temperatures. Thus, pollinators have the potential to exert selection on floral temperature and the traits that shape it. Our study also highlights that floral IR reflectance impacts floral temperature in a context-dependent manner, calling for more investigations into this little-studied axis of floral variation.