Bumblebee visits to different colour morphs of the Washington lupine, Lupinus polyphyllus

Abstract

We examined the flower visits of bumblebees (Hymenoptera, Apoidea, Bomb us spp.) to Lupinus polyphyllus Lindley (Fabaceae) in southern Finland. The racemes of the plant bear nectarless flowers which appear in three main colour morphs, blue, red and white, of which blue is the most common. Blue inflorescences had the greatest number of flowers and their proportion increased towards the end of the flowering period, whereas white flowers seemed to fade away more quickly and drop easier than those of the other morphs. Bumblebees seemed to prefer blue inflorescences over red and white in the beginning of the flowering period, but in the end of the period red and white inflorescences were visited more often than expected on the basis of their proportion in the field. Bumblebee species differed slightly in their colour preferences, for instance, B. lapidarius seldom visited white flowers. When the temperature was low and the wind was high, the visits to blue flowers were more numerous than expected, whereas in cloudy conditions red flowers were often visited. Bumblebees moved between the same coloured inflorescences more often than expected, because the colour selected in the beginning was favoured during the whole foraging trip and the nearest inflorescence having another colour was skipped. The pollinator behaviour favoured the existing proportions of colour morphs in the lupine populations.