For all of the amazing symbioses ants and plants share, there is one thing ants seem to get in the way of... plant sex. That's right, plants have found a use for ants in pretty much every way except for when it comes to reproduction (with some exceptions of course). Ants being what they are, they can easily become a force to be reckoned with. For this reason, many plant species have co-opted ants as defense agents, luring them in with nectar-releasing glands, a resource that ants guard quite heavily.
When it comes to flowering, however, ants can become a bit overbearing. Research done at the University of Toronto shows that the invasive European fire ant has a tendency to guard floral nectar so heavily that they chase away pollinators. By observing fire ants and bumblebees, they found that ants change bumblebee foraging behaviors. The fire ants often harassed and attacked bumblebees as they visited flowers, causing them to spend significantly less time at each flower, a fact that could very well result in reduced pollination for the plant in question.
This reduction in pollination is made even more apparent for dioecious plants. Since ants are after nectar and not pollen, male flowers received more bumblebee visits than nectar-producing female flowers. This could become quite damaging in regions with heavy fire ant infestations.
As it turns out, the ants don't even need to be present to ward off bumblebees. The mere scent of ants was enough to cause bumblebees to avoid flowers. They apparently associated the ant smell with being harassed and are more likely to not chance a visit. Of course, this study was performed on using an invasive ant species. Because so many plant species recruit ants for things like protection and seed dispersal, it is likely that under natural conditions, the benefit of associating with ants far outweighs any costs to reproductive fitness. More work is needed to see if other ant specie exhibit such aggressive behavior towards pollinators.
Photo Credit: Lalithamba (https://www.flickr.com/people/45835639@N04)