COS 31-4
Ant-aphid mutualisms increase ant floral visitation and reduce plant reproduction via decreased pollinator visitation

Tuesday, August 12, 2014: 9:00 AM
Carmel AB, Hyatt Regency Hotel
Katherine E. LeVan, Ecology, Behavior, Evolution, University of California, San Diego, La Jolla, CA
David A. Holway, Division of Biological Sciences, University of California, San Diego, La Jolla, CA
Background/Question/Methods

For plant species that depend on animal-mediated pollination, reproduction hinges on adequate access to pollinators. Even in the presence of intact pollinator assemblages, negative interactions among floral visitors can compromise pollination services. Ants, for example, often visit flowers and discourage visitation by other insects, but usually do not perform pollination themselves. Effects on plant reproduction that result from this type of interaction may be compounded by factors that increase the likelihood or extent of floral visitation by ants. Mutualisms between ants and honeydew-producing insects alter the activity and local abundance of ants on plants, but the degree to which these interactions increase floral visitation by ants and in turn disrupt pollination services remains incompletely understood. Here, we manipulated the abundance of cotton aphids (Aphis gossypii) on cotton (Gossypium hirsutum) over a three-year period to test how the mutualism between honeydew-producing aphids and the Argentine ant (Linepithema humile) affects floral visitation by ants, floral visitation by bees, and cotton seed production.

Results/Conclusions

In each year of the study increasing aphid abundance increased ant abundance on cotton leaves, and floral visitation by ants was positively related to per leaf ant abundance. The duration of visits to cotton flowers by honey bees, the most common floral visitor aside from ants, declined with both increasing aphid abundance and increasing ant floral visitation. Seed set and seed mass both declined with increasing aphid abundance and decreasing bee visitation. Aphid herbivory alone, however, did not affect seed production, which was independent of aphid abundance on plants that were manually cross-pollinated under greenhouse conditions. Our results illustrate that mutualisms between ants and honeydew-producing insects can enhance levels of floral visitation by ants and in turn disrupt pollination services enough to have measurable effects on plant reproduction. Although numerous studies demonstrate that such mutualisms enhance host plant growth and reproduction, our results indicate that the manifestation of these positive indirect effects can depend on the foraging behavior of the focal ant species. Given the ubiquity of interactions between ants and honeydew-producing insects, the implications of this study extend to a wide variety of plant species that rely on animal-mediated pollination.