The spatial distribution of nonrewarding artificial flowers affects pollinator attraction.

Many species of orchids that do not offer food rewards to pollinators bloom in clusters, early in the season, and are polymorphic for corolla colour. Previous studies suggest that the foraging behaviour of insect pollinators may select for early blooming and colour polymorphism. I tested whether pollinator behaviour can also favour aggregated flowering in these species, in a two-stage laboratory experiment on naïve bumblebees, Bombus terrestris (L.). In the first stage, the bees were allowed to forage on three colours of artificial flowers that contained sucrose rewards. In the second stage, I added nonrewarding flowers of a fourth colour and recorded the bees' visits to them. The four types of artificial flowers were either arranged in spatially distinct clusters, or were randomly intermingled. I used two reward schedules for each spatial arrangement: constant refilling of reward-containing flowers and probabilistic refilling. Bees that foraged on clustered flowers flew more often to the nonrewarding patch, and made more visits to nonrewarding flowers, than bees that foraged on intermingled flowers. This tendency was obtained both in the constant reward and in the probabilistic reward schedules. The results support the hypothesis that pollinator attraction may select for clustered, synchronized blooming in flowers that do not contain nectar and pollen rewards. Copyright 2000 The Association for the Study of Animal Behaviour.