The Hidden Nectaries on a Cherry Tree’s Leafstalk: Calling Ants from the Leaves, Not the Flowers

When cherry blossoms drift down like snow, the cherry tree’s story for the year seems to be over. But on the very branches the petals have left, a quieter scene begins as the pale green young leaves unfold. Look closely at a newly opened leaf and you will usually find two tiny red bumps, each about the size of a millet grain, sitting side by side where the leaf blade meets the leaf stalk. They look like a blemish or a random swelling, yet these little bumps are glands that secrete a sugary liquid. And the guests that come to drink it are not bees but ants. The cherry tree calls ants with its leaves rather than its flowers, and instead of carrying pollen, the ants carry off the caterpillars that chew the tender young leaves. This article follows the research to see when, how, and how far those two small dots protect the tree.

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Two Red Dots on the Leaf Stalk

These glands are called extrafloral nectaries, meaning nectar glands that sit outside the flower. In the cherry group (the genus Prunus) they usually sit on the upper part of the petiole or on the edge of the leaf blade near its base. Their appearance varies a good deal from species to species. In some they are no more than small red or green bumps or dots, while in others they are clear doughnut shapes colored pink to deep red that catch the eye at once. (OSU BYGL)

These glands have long served as a way to recognize cherry trees. Students in dendrology classes learned to pick out cherries by the glands on the leaf stalk well before research revealed what the glands actually do for the tree. (OSU BYGL)

The number and position of the glands also change with the season. Researchers observing sweet cherry noted that early in the season, when leaves are just emerging, a small nectary sits at the tip of each tooth along the leaf margin, and one to five large, brightly colored nectaries sit on the leaf base and petiole. Later in the season the marginal glands are no longer visible, and only those on the leaf base and petiole remain. (Pulice & Packer 2008)

Actually seeing nectar flow from the glands, however, is not easy. Indoors, where there are no ants, nectar can pool until it runs, but outdoors it is usually licked up by ants and other insects as soon as it is secreted, so the glands look only slightly moist. If you see no nectar, the gland may not be idle; its guests may simply have already been there. (Okayama University of Science Botanical Garden)

Aimed at a Different Partner Than Floral Nectar

Nectaries inside a flower draw pollinators such as bees and butterflies and help with pollination. Extrafloral nectaries have nothing to do with pollination. Their nectar is a carbohydrate-rich food that attracts ants and other arthropods, and many of them in return protect the plant from herbivores. This kind of relationship is called a protective mutualism. (Marazzi et al. 2013)

Ants that have fed on the nectar patrol back and forth across the leaves and carry off moth eggs and newly hatched caterpillars as food. Instead of fighting back directly with spines or poison, the tree uses nectar to bring in a third party and hands its defense over to it. This is called indirect defense, and the ants are often compared to bodyguards hired by the tree. (Okayama University of Science Botanical Garden)

Three Weeks of Nectar, Three Weeks to Hunt

The classic study of this alliance is a paper published in the journal Ecology in 1978 by ecologist David Tilman. His subject was the North American black cherry (Prunus serotina). Tilman found that this tree’s extrafloral nectaries are most active during the first three weeks after budbreak. The nectaries drew large numbers of Formica obscuripes, a species of the genus Formica known in English as the western thatching ant, especially to trees standing within about 20 meters of an ant colony. The number of ant visits per bud was highest just after budbreak and fell as the number of active nectaries declined. (Tilman 1978)

The ants’ target was the caterpillar of the eastern tent caterpillar moth (Malacosoma americanum), the leading defoliator of black cherry. This moth spends the winter as shiny, blackish-brown egg masses wrapped around twigs, and as soon as the caterpillars hatch in spring, they spin silk tents in the forks of branches. They favor trees in the rose family, especially those in the genus Prunus such as cherry. (OSU BYGL)

Here timing is decisive. Formica obscuripes could prey on these caterpillars only until about three weeks after budbreak. After that, the caterpillars had grown large enough that most of them escaped the ants. In other words, the three weeks when the nectaries are most active overlap with the three weeks when the caterpillars are most vulnerable to ants. Indeed, the farther a tree stood from an ant colony, the higher the tent caterpillars’ survival. (Tilman 1978; OSU BYGL)

Tilman worded his conclusion carefully. The relationship between the ants and the cherry is not an obligate symbiosis in which neither can live without the other, but a facultative mutualism formed as circumstances allow, and the timing of nectar production appears to be set so as to maximize the chance that ants will succeed in preying on tent caterpillar colonies. (Tilman 1978)

Damage a Leaf and the Nectaries Multiply

The number of nectaries is not simply fixed in advance. In 2008, Pulice and Packer experimented with sweet cherry seedlings grown in a greenhouse. On seedlings about four weeks after emergence, they removed half the area of each leaf with either a 3-millimeter hole punch or scissors, and kept applying the same damage to newly emerging leaves for three weeks. The result: compared with undamaged seedlings, the hole-punched seedlings had 39% more extrafloral nectaries per leaf and the scissor-cut seedlings 33% more. Nectaries increased even on leaves that were already fully formed when damaged, and some seedlings produced petioles with no leaf blade at all, carrying only several large nectaries that oozed nectar. (Pulice & Packer 2008)

A response like this, in which defense is stepped up after an attack, is called an induced defense. Note, however, that this experiment counted nectaries; it did not measure whether the extra nectaries actually attracted more ants or reduced damage. The authors themselves wrote that more research is needed under field conditions with ants and herbivores present. (Pulice & Packer 2008)

Keep the Ants Out and the Leaves Get Eaten

Whether nectaries really protect the tree was tested more directly on the peach, another member of the genus Prunus. Mathews and colleagues planted trees of the same cultivar (‘Lovell’) that either had or lacked extrafloral nectaries on their leaves and compared them. In the first year after planting, ants dominated the young trees with nectaries, those trees hosted fewer herbivorous insects, and their leaves suffered half as much feeding damage as trees without nectaries. Their trunks also grew faster, and in later years they produced three times as many buds. (Mathews et al. 2009)

The decisive evidence came from keeping ants away. When ants were prevented from climbing into the canopies of trees with nectaries, leaf damage in May, when new leaves were coming out, rose sharply. This shows that the ants really were guarding the trees while new leaves emerged. For more on the peach blossom and peach fuzz, see Peach Fuzz Begins Inside the Flower. (Mathews et al. 2009)

Not a Secret Only Cherry Trees Keep

Extrafloral nectaries are not a special organ found only in cherry trees. According to Weber and Keeler, who compiled 135 years of records in the journal Annals of Botany in 2013, extrafloral nectaries have been reported in 3,941 species in 745 genera and 108 families of plants. That is about 1 to 2 percent of vascular plant species and about 21 percent of families, and most species with these nectaries are concentrated in the rosids, the large group that includes the rose family to which cherries belong. (Weber & Keeler 2013)

The same study’s phylogenetic analysis suggests that extrafloral nectaries occur in at least 457 independent lineages. It also estimates that there may be as many unreported cases as reported ones. The same defensive arrangement appears again and again across plant groups with different roots. The cherry is simply one well-studied example among them. (Weber & Keeler 2013)

The Yoshino Cherry in the Park, and the Terms of the Alliance

The Yoshino cherry (Prunus × yedoensis) we gaze up at every spring has these glands too. NC State University’s Extension gardening resource explains that the tip of the Yoshino cherry’s petiole bears two or more large, wart-like glands, and that one or more of them commonly touch the base of the leaf blade. In the Yoshino cherry, 70 to 80 percent of glands touch the blade, whereas in the double-flowered ‘Kwanzan’ cherry 80 to 90 percent sit apart from it, a difference that helps tell the two trees apart by their leaves alone. (NC State Extension) A resource from the Okayama University of Science Botanical Garden in Japan shows photographs of the leaf nectaries of the Somei-yoshino cherry and explains that leaf nectar flows mainly from the time new shoots appear until the leaves harden. Once leaves have grown and hardened, caterpillar damage also declines. (Okayama University of Science Botanical Garden)

There is a point to be careful about, though. The effect of ants protecting the tree by catching caterpillars has been studied in black cherry, sweet cherry seedlings, and peach trees; it has not been separately confirmed by experiment in the Yoshino cherry. It is more accurate to understand it as a principle established across several Prunus species. Nor are ants always on the tree’s side. In experiments with broad beans, more ants visited the plants as aphid numbers rose, but the ants shifted toward harvesting the sugary honeydew excreted by aphids rather than the nectar from the extrafloral nectaries. (Katayama & Suzuki 2003) This is a conditional alliance that changes with timing, partners, and the other food nearby.

So the two red dots on a cherry leaf stalk are a mechanism placed where they are easy to overlook, matched even in their timing. Nectar flows when the young leaves are at their most tender, and that nectar summons guests strong enough to guard them at exactly that moment. When leaves are damaged, more glands appear; when leaves harden, the nectar stops. This is not a decision the tree calculates, but an arrangement precisely designed to fit together from the start. Even in this one small nectary that God made, there is a finely woven design that protects life in places we rarely notice. Next spring, after the blossoms fall, take a look at the tip of a young cherry leaf’s stalk. If you are lucky, you may meet an ant going back and forth between the two dots.

References

• Tilman, D. (1978) Cherries, Ants and Tent Caterpillars: Timing of Nectar Production in Relation to Susceptibility of Caterpillars to Ant Predation — Ecology
• Eastern Tent Caterpillars, Extrafloral Nectaries, and Ants — OSU BYGL (2019)
• Extrafloral Nectaries, Myrmecophiles and Other Trivial Pursuits — OSU BYGL (2021)
• Pulice & Packer (2008) Simulated herbivory induces extrafloral nectary production in Prunus avium — Functional Ecology
• Mathews, Bottrell & Brown (2009) Extrafloral nectaries alter arthropod community structure and mediate peach (Prunus persica) plant defense — Ecological Applications
• Weber & Keeler (2013) The phylogenetic distribution of extrafloral nectaries in plants — Annals of Botany
• Marazzi, Bronstein & Koptur (2013) The diversity, ecology and evolution of extrafloral nectaries — Annals of Botany
• Prunus × yedoensis (Yoshino cherry) — NC State Extension Gardener Plant Toolbox
• Leaf Nectaries — Okayama University of Science Botanical Garden (in Japanese)
• Katayama & Suzuki (2003) Changes in the use of extrafloral nectaries of Vicia faba and honeydew of aphids by ants with increasing aphid density — Annals of the Entomological Society of America

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