Ants helping plants: extrafloral nectaries

In the last couple of weeks I have posted about ants affecting plants negatively one way or another: taking nectar from flowers but failing to fertilise the plant; farming scale insects and defending them from their predators. Ants, however, also have mutualistic relationship with plants. Take cherry trees. As their green cherries are growing, their young, tender leaves are at their juiciest. Many caterpillars and other herbivorous insects would love to have a go at them. But cherries have a sweet way to call for bodyguards: they secrete nectar in red floral glands at the base of each leaf - also called extrafloral nectaries. These are irresistible to black garden ants (Lasius niger) (above) and they secrete nectar in small quantities so they encourage ants to scout around the tree to find the next leaf. Should they encounter a caterpillar trying to get to the leave, or an insect land on it, the ants act aggressively like they would do to defend aphids or scale insects, and keep the insects at bay. Nectaries are produced by a large diversity of flowering plants and experiments have demonstrated that if ants are excluded from a nectary bearing plant, the plant is rapidly damaged by herbivorous insects. In the American black cherry, nectar production in nectaries is seasonal and it is precisely timed to the period when tent caterpillars are small enough to be able to be predated by an ant. When ants are disturbed by an insect, they immediately attacked the insect and if small, they carried it back to their nest. This seasonal match between nectar production and caterpillar attack suggests that the main function of nectaries is ant attraction, as a deterrent from herbivorous caterpillars.

An ant inspecting the nectary, a glistening drop of nectar in the middle of it. Smaller nectaries are also present at the end of each serration in young leaves.

Two ant on nectaries.

More information
Tilman, D. (1978). Cherries, Ants and Tent Caterpillars: Timing of Nectar Production in Relation in Relation to Susceptibility of Catepillars to Ant Predation Ecology, 59 (4) DOI: 10.2307/1938771