The effects of floral microclimate on pollen viability and visitor behavior

Flowers represent intricate microcosms shaped by their physical, chemical, and micrometeorological properties. Both thermogenic and non-thermogenic flowers (passively heated by insolation) that present a warm microclimate have been shown to attract pollinators, but emerging evidence suggests that floral humidity can also serve as an attractive cue. Here, we document a distinct microclimate within the flowers of both wild and domesticated squashes (Cucurbita spp.). Squash flower temperatures remain near ambient, but their humidity consistently exceeds ambient levels from bud to senescence. Experimental manipulations of the floral microclimate increased the occurrence of pollen tube rupture in a germination medium assay, highlighting a direct impact on plant fitness. To explore the role of the floral microclimate within a broader ecological context, we perforated squash flowers in farms to assess impacts on the behavior of their specialist squash bee pollinator (Xenoglossa pruinosa), generalist pollinators (bumblebees and honeybees), and specialist herbivores (cucurbit beetles). Perforated male flowers with reduced humidity received fewer visits from squash bees but not from generalist pollinators. Floral manipulations did not influence the foraging duration of any visitor but decreased the occupancy of male squash bees and cucurbit beetles in wilted flowers compared with controls. There was a positive correlation between ambient air dryness and the abundance of squash bees and cucurbit beetles residing within the humid wilted floral chambers. Our findings showcase squash flowers as humid microhabitats that influence reproductive success directly by affecting pollen viability and indirectly by altering interactions with the specialists, squash bees and cucurbit beetles.