Ant–aphid interactions increase ant floral visitation and reduce plant reproduction via decreased pollinator visitation

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, visit flowers and can 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. Interactions 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. We manipulated the abundance of cotton aphids ( Aphis gossypii ) on cotton ( Gossypium hirsutum ) over three years to test how the interaction between honeydew‐producing aphids and the Argentine ant ( Linepithema humile ) affects floral visitation by ants, floral visitation by bees, and seed production. Increasing aphid abundance increased ant abundance on cotton plants, and floral visitation by ants was positively related to ant abundance on leaves. The duration of visits by honey bees, the most common floral visitor aside from ants, declined with both increasing aphid abundance and increasing ant floral visitation. Two measures of seed production declined with increasing aphid abundance and reductions in the duration of bee visits. 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 interactions 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.