Evaluation of inflorescence visitors as pollinators of Echinacea angustifolia (Asteraceae): comparison of techniques

Inflorescences (heads or capitula) of the putative self-incompatible species, purple coneflower (Echinacea angustifolia (DC) Cronq. (Asteraceae)), were visited by insects representing the Coleoptera, Diptera, Hymenoptera, and Lepidoptera, in accordance with a generalist pollination syndrome. Measurement of the effectiveness of insect species as pollinators was accomplished by permitting solitary visits to receptive, central disc florets of virgin (previously bagged) heads. Four parameters were quantified: total stigmatic pollen load and proportion of pollen grains germinated, numbers of pollen tubes at style bases, and percentages of total receptive florets that had retracted (shrivelled) styles. Quantifying total and germinated pollen grains proved ineffective, partly owing to the tendency of self-pollen to initiate pollen tubes. The most effective pollinators were Apidae, especially bumble bees (Bombus spp.) and the European honey bee (Apis mellifera L.) (mean: 39 - 61% of styles retracted). Other noteworthy pollinators were cloudless sulfur butterflies (Phoebis sennae L.--Pieridae; mean 47% of style bases with pollen tubes), golden blister beetles (Epicauta ferruginea Say--Meloidae; 44%), and grasshopper bee flies (Systoechus vulgaris Loew--Bombyliidae; 22%). Sunflower leafcutter bees (Megachile pugnata Say) were less effective (4% of styles retracted). Promisingly, analysis of the proportion of retracted styles provided similar results to the established technique of pollen-tube quantification, but had the significant advantages of being completed more rapidly, without a microscope, and in the field. The quantitative technique of retracted-style analysis appears well suited for prompt measurement of inflorescence-visiting insects as pollinators of many asteraceans.