Abstract

Both temperature and flower colour have major effects on the foraging of pollinators. Despite both factors being important, no studies have examined the interactive effects of temperature and colour and how this may impact the maintenance of natural floral colour polymorphisms. We tested how temperature influenced bumblebees (Bombus spp.) visitation rates to Geranium sylvaticum (Linnaeus 1753) plants both in a greenhouse and field settings. G. sylvaticum shows colour polymorphism among populations, and individuals can be classified into two discrete morphs, dark morphs or pale morphs based on their visible light. We found that bumblebee visitation rates to plants and flowers increased linearly with temperature in the greenhouse and in the field, before decreasing at the highest temperatures in the greenhouse. In the greenhouse, there was an increasing preference for dark morphs at 23-26oC, whereas dark morphs received more visits than pale morphs at all temperatures. Temperature-related differences in reward quality along with differences between colour morphs are likely to drive this pattern, though other mechanisms could be important.