POLLINATORS' STRATEGIES IN FINDING FLOWERS

ABSTRACT

Two phases of foraging flights of hymenopteran pollinators are discussed: localization of food sources over far distances (hundreds of meters to several kilometers); and spotting of flowers within their visual catchment area. In the first part, evidence from navigational tasks with honeybees is presented which favors the interpretation that bees possess a rich and unique spatial memory of qualified and localized objects. Depending on the motivation, the bee is rather free to navigate with reference to this memory. In particular, bees are guided towards feeding places with specific expectations of their signal and reward properties. In the second part, the processes guiding the bee during its final approach to the flower are analyzed. When arriving in the close vicinity of a rewarding flower, bees first detect and recognize the achromatic green signal and then the chromatic color signal. The dependence on the optical signals of the flowers and the habitat features is studied in a comparison between plants growing in the Israeli Mediterranean and desert habitats. We find that the green contrasts of flowers in desert plants are less prominent than in Mediterranean plants because the green signal of the desert background is more similar to that of flowers, not because the green signals of desert and Mediterranean plants are different. These results are interpreted on the assumption that the green signal of flowers used in further distance detection is an adaptive property of plant species only in the context of all features supporting navigation of insects. The low density growth of desert plants may allow for the possibility that the plants are located by insect pollinators as specified places relative to landmarks. Therefore, further distance visual signals emanating from the flower may be less important in a desert habitat, and reduced green contrast does not become an unfavorable property in desert plants. We conclude that both habitat features and flower signals contribute to the navigational system of insect pollinators, and that the evolutionary development of flower signals needs to be evaluated in the context of the plant species' habitat.