A Plant Pathogen Influences Pollinator Behavior and May Influence Reproduction of Nonhosts

Some plant pathogens attract pollinating insects and thus have the potential to influence the pollination of flowers, just as different flower species can influence one another's pollination. Showy, flower—like pseudoflowers caused by the rust fungus Puccinia monoica on Arabis holboellii (Brassicaceae) commonly co—occur with flowers of Anemone patens (Ranunculaceae). I evaluated the effects of pseudoflowers and anemone on each other's visitation, and the effect of pseudoflowers on the seed set of anemone. I expected that at low overall “flower” densities visitation would be facilitated in patches containing a mixture of these species. At low densities, pseudoflowers or flowers alone may be too rare to attract sufficient visitors, but the addition of the other species could make mixtures more attractive and thus increase visitation rates. At high densities I expected that “flower” in mixtures would compete for pollinators because it is more energy efficient for pollinators to concentrate visits on the most rewarding species. I used artificial arrays to separate the effects of “flower” density and relative frequency on visitation. Local density was not a good predictor of the influence of one species on the other. Instead, visitation patterns depended most strongly on the kind of visitor (flies vs. bees). Visitation to A. patens by flies was facilitated by the presence of pseudoflowers at both high and low densities, whereas visitation to pseudoflowers was not significantly influenced by A. patens at either density. Local density affected visitation by bees, but relative frequency did not. In addition to observing visitation, I also quantified the effects of interspecific movement of both pollen and spores on the reproductive success of A. patens. Although the presence of pseudoflowers sometimes facilitates visitation to A. patens, this effect could be counterbalanced by competition through interspecific insect movement: sticky pseudoflowers remove pollen from visiting insects, and fungal spermatia deposited on flower stigmas reduce seed set.