EFFECTS OF BUMBLEBEE VISITS ON THE SEED SET OF PEDICULARIS, RHINANTHUS AND MELAMPYRUM (SCROPHULARIACEAE) IN THE NETHERLANDS

Abstract

The pollination ecology of five species of Rhinanthoideae (the annual species Rhinanthus serotinus, R. minor and Melampyrum pratense, the biennial Pedicularis sylvatica and the annual and biennial P. palustris) has been investigated. All species contain pollen and nectar and are frequently visited by sternotribically and nototribically pollinating bumblebees. The effectiveness of pollen transfer has been measured by the use of fluorescent powder. In R. serotinus and M. pratense no differences exist in percentages of fluorescent stigmas of flowers nototribically or sternotribically visited by bumblebees. R. minor flowers, visited sternotribically, have very low percentages of fluorescent stigmas. This indicates that the pollen-covered venter cannot touch stigmas enclosed by the galea; the movements of the bumblebees probably caused self-pollination. P. palustris, R. serotinus and M. pratense flowers are very frequently perforated by nectar-collecting short-tongued bumblebees. P. sylvatica and R. minor flowers are very rarely perforated. On these species nectar is mainly collected by nototribically pollinating bumblebees. Seed production and dependence upon pollination by bumblebees (Bombus Latr. spp.) are considered. A range from high dependence upon bumblebee visits for seed production in P. palustris, to medium dependence in P. sylvatica and R. serotinus and virtual independence in R. minor and M. pratense is established. No species is completely self-sterile. Seed set in caged plants is due to favourable morphology and position of flowers. Close proximity of thecae and stigma or a downward curving of the pistil under pollen chamber in Melampyrum and Rhinanthus insure seed set in caged plants. In Pedicularis these characteristics for self-pollination are absent. The importance of bumblebees for the five Rhinanthoideae and the reciprocal importance of these pollen and nectar providing plants for bumblebees is discussed. The importance of alternative pollination by honeybees, thrips and wind is evaluated.