That there are sweet rewards in plant flowers that entice insects to visit most everyone knows. The fragrance of flowers alone often gives this fact away.
From the plant’s point of view, the gift of sweets is self-serving. The insect leaves happily nourished and inadvertantly covered with some amount of pollen that it will perhaps drop off at the very next flower in the garden or meadow, boy meets girl. Win-win, so common in nature where competition gets far too much of our attention. The small world can be amazingly cooperative.
But when I first saw our heirloom purple-hull beans occupied by black ants, my first impulse was “them or us” assuming wrongly that the insects were there to cause trouble. Running for the Sevin dust would have been a common mistake too many gardeners make thinking the only good bug is a dead one. These, it seems, we should let live.
Just what purpose they serve–from the bean’s point of view–when they come to the “extrafloral nectaries” (found somewhere else on the plant than the flowers) and the sweet secretions they offer, botanist and entomologists are not entirely sure. The sugary offering seems to come in this case from the vertical structures at the junctions where the fruits arise. Can you see them especially in the lower right inset? (The red color, too, may have a purpose as a visual sign of sweets.)
Most likely, the ants defend their sweet treats against the threat of aphids and scale insects and perhaps others that would do the plant harm. So it is in the interest of bean plant and ant to live together in this way.
It is our best interest as gardeners to leave these ants alone to do what they have done generation after generation, providing an environmental service to the bean, and thus, to the bean eaters in our household.
Interested? Read Many Plants Have Extrafloral Nectaries Helpful to Beneficials from the University of Florida and even MORE on plant secretory structures in general.