Floral colour signal increases short-range detectability of a sexually deceptive orchid to its bee pollinator

Abstract

Orchids of the genus Ophrys are pollinated by males of solitary bees and wasps through sexual deception. The flowers mimic the behaviourally active compounds of the sex pheromone of receptive females and thus attract males that seek to copulate. Odour is the main attractant while visual stimuli have been assumed so far to play only a minor role. In contrast to most species of the genus, Heldreich's orchid Ophrys heldreichii, which is pollinated by males of the long-horned bee Tetralonia berlandi, possesses a bright pink perianth that appears conspicuous to a human observer. We investigated the role of this floral colour signal in pollinator attraction. We filmed approach flights of male bees to flowers in which we removed the original perianth and in which we substituted the perianth with an artificial one of a particular selected colour. At distances >30 cm, male search time correlated only with wind speed but not with the spectral parameters of the perianth, i.e. chromatic and green receptor-specific contrast. By contrast, in the close range (<30 cm), where the perianth subtends a visual angle of at least 5 deg. to the bee's eye, search time decreased with increasing green receptor contrast between perianth and background; however, no correlation with chromatic contrast or wind speed was found. Our results indicate that pollinators are first attracted by olfactory signals from a distance. Once in the vicinity of the flower where spatial vision of the males is sufficient, they are guided exclusively by vision. However, it can be expected that possession of a ;non-private' colour signal would increase the risk of pollen loss in sexually deceptive orchids by accidentally attracting non-specific flower visitors. We therefore discuss the occurrence of colour signals in the genus Ophrys in respect to the species-specific visual system of the pollinators.