Ants and cherry laurel nectaries

 On passing by a hedge of Cherry Laurel, Prunus laurocerasus, we notice ants. Lots of Black Garden Ants, Lasius niger, walking all over the leaves. We creep closer and see little round marks on the base of the leaves that the ants are paying special attention to. They are nectaries, nectar producing organs in plants, which are most commonly found at the base of flowers. In about 1% of plants (including several species of the genus Prunus, including cherries) these organs are also found on leaves and are called 'extrafloral nectaries'. Extrafloral nectaries are part of a mutualistic relationship between plants and ants, in which both parts benefit. The plants give away the sweet nectar which feeds the ant, and in turn the ant defends the plant. By visiting the plant to obtain nectar ants come across herbivorous insects, particularly caterpillars, that might feed on the leaves, and prey upon them.
 Other nectar-loving insects take advantage from this easy to reach nectar production, and often, bees and bumblebees are found feeding in nectaries. Bees and bumblebees behave in this case as 'nectar robbers' as they do not benefit the plant at all (unlike when they feed on the floral nectaries). In this case, the ant is the one benefiting the plant. If you like the bright green, glossy leaves of the cherry laurel to stay that way, you might want to leave these ants well alone.

Ant feeding on a nectary. There is another pale, round nectary just above the ant.

Male Bombus pratorum visiting the cherry laurel

Nectar from cherry leaves

I was watching male bumblebees around the cherry tree a few days ago when I saw some honeybees around. On close inspection, I noticed they were visiting extrafloral nectaries, little red swellings in the petiole of cherry leaves, which secrete nectar, usually taken by ants. The bee would visit several leaves in succession, but being high in the tree, it was tricky to get good shots. I noticed an early bumblebee, Bombus pratorum, doing the same. So, even when not in bloom, cherry trees are producing nectar used by bees and bumblebees. Many other plants have these nectar glands, including willows, cherry laurel and plums.

honeybee sucking nectar from the nectary

Male Bombus pratorum doing the same

A close up of a cherry extra floral nectary

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