Pollination biology of Heracleum sphondylium L. (Apiaceae): the advantages of being white and compact

Abstract

Two questions were addressed in the present study: (1) What are the main pollinators of the two subspecies of H. sphondylium?, and (2) Do the studied plants share the pollinators' set or are they attractive for different groups of insects? The survey showed that among 40 insect taxa visiting both subspecies of H. sphondylium approx. only 53% carried significant pollen loads. However, the Pollinator Importance Coefficient (IC) calculated for each insect group, and based on observation of insects' abundance, within-umbel activity and pollen load revealed that only two taxa in case of H. s. ssp. sibiricum (Thricops nigrifrons, Eriozona syrphoides) and four in case of H. s. ssp. sphondylium (T. nigrifrons,E. syrphoides, Meliscaeva cinctella and Arge ustulata) were truly important pollinators. Although both subspecies were visited by similar insects, H. s. ssp. sphondylium, with its characteristic compact and white umbels, was visited more frequently by Diptera and Hymenoptera, while yellow-greenish loose umbels of H. s. ssp. sibiricum were preferred by Coleoptera. This paper indicates that the concept of faithful pollinators may also apply to a broader spectrum of Apiaceae, usually considered primitive in terms of pollination strategies, and suggests possible ways of differentiation in two closely related taxa.