Ant Plants… Without the Ants

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Some plants are designed to attract ants … and that’s a good thing! Source: The Cliparts

Did you know that some houseplants, maybe some you already grow, are actually “ant plants?” Myrmecophytes, to use the scientific name. The word myrmecophyte comes from the Greek myrmex (ant) and phuton (plant).

These plants live in symbiosis with ants, a form of mutualism where each species helps the other. There are more myrmecophytic plants than you might think: actually thousands of species in more than 100 families of plants. They are, in fact, very common in nature. And not all are tropical plants that make good houseplants. There are myrmecophytic plants in all climates, worldwide. Probably quite a few in your own garden. But some an plants are more “anty” than others!

Myrmecophytes function by providing either food or shelter to ants and, in return, ants offer them certain services, including:

  • protection against insects and other animals;
  • protection against invasive plants;
  • suppression of harmful fungi and bacteria;
  • nourishment (from their excretions as well as dead ants);
  • seed distribution (sometimes);
  • pollination of flowers (very rarely).

Ant Hotels

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Cutting open the tuber of Myrmecodia tuberosa reveals the convoluted interior where ants take up residence. Source: thephytophi8e.blogspot.ca.

Myrmecophytes that offer ants dwellings to live in are the ones we most readily recognize as ant plants and the ones that most strike our imagination. Generally, these ant plants bear some sort of conspicuously swollen organ that ants can inhabit, often already hollow or, if not, designed to be hollowed out. Likewise, there may already be an opening for ants to penetrate, but if not, there’s usually a spot with thinner than usual skin designed to be punctured. The host organ can be a stem, a tuber, a rhizome, a leaf or a spine. Such residences are called domatia or, to be more specific, myrmecodomatia. Or ant hotels!

Special Organs to Feed Ants

This is a much more subtle form of symbiosis. So subtle, indeed, that it often goes unobserved until someone carefully studies the behavior of ants visiting the plant. For that reason, there are almost certainly many more myrmecophytic plants than is presently known.

Here’s how it works.

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Extrafloral nectaries on the leaves of Prunus laurocerasus. This kind of nectary is actually quite common. Source:  Luis Fernández García, Wikimedia Commons

Most plants only produce nectar, a sugar-rich liquid, in the center of their flower and use it to attract pollinators to come and pick up their pollen and transfer it to other plants. Myrmecophytic plants, though, have nectaries rich in sugars located elsewhere on the plant, often on leaves or stems. They’re called “extrafloral nectaries” and are usually designed to attract ants for protection and cleaning.

You certainly already know at least one myrmecophytic garden plant: the peony (Paeonia lactiflora). It bears nectaries on the outside of the flower bud that attract ants and the ants in turn help protect the flower to come from predators.

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The yellowish growths on the tips of these bull’s horn acacia leaflets are food bodies, designed to attract and to feed ants. Source: botany.wisc.edu

Some myrmecophytes go a step further and produce special small growths rich in lipids, sugars and proteins on their leaves or elsewhere. Called food bodies (they also bear various other names, like Beltian bodies, Beccarian bodies, etc., depending on where they’re found on the plant, but or the purpose of this article, food bodies will suffice), these growths are designed so ants can harvest them and bring them to bring back to their nest to feed the colony.

Ant Plants Sans Ants

Sometimes myrmecophylia is optional: the plant and the ant can very well survive one without one another. In fact, most plants that only offer extrafloral nectaries (and they make up the majority of ant plants) aren’t seeking to attract a specific kind of ant: any passing ant will do. If the plant fails to attract ants, it will still probably get along just fine.

On the other hand, there are cases where mutualism is obligatory, at least in the wild.

If the right ant is not present, the plant won’t be able to reproduce or will be eaten by herbivores. And the ant species in question depends entirely on its host plant. It can no longer survive on its own.

In cultivation, however, it’s possible to cultivate at least the plant without the ants if not vice versa. Nor do you need to worry that outdoor ants will invade and move into your houseplants. Ant hotel type myrmecophytes will only accept one specific host species, not just any old ant that comes around and, unless you live where the plant species is native, it’s not going to find its favorite ants. It will be up to you to feed and care for the plant rather than ants.

Ant Plants as Houseplants

The text that follows presents some myrmecophytic plants that can be grown as houseplants. All these plants will need good light (bright light with some full sun), normal indoor temperatures, reasonable air humidity and regular watering (just follow the Golden Rule of Watering and water when the soil is dry to the touch). Several are epiphytes, though, so may prefer orchid potting soil to regular potting soil or need to be grown on bark slabs.

Tillandsias

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Tillandsia bulbosa, one of the myrmecophytic air plants. Source: fr.aliexpress.com

Tillandsias, commonly called air plants, are members of the bromeliad family and native to South and Central America where they grow as epiphytes on tree trunks and branches. They’re presently the most popular ant plants grown as houseplants, although only a few species host ants and their ant-hosting capacity doesn’t seem to be a major selling point and is, in fact, rarely mentioned.

Myrmecophytic tillandsias are easily recognized because they produce leaves with an extra wide lower extremity and thus form a kind of bulb at the base of the plant. In the wild, ants pierce a hole in this “bulb” and live there. The two most popular species are T. bulbosa and T. caput-medusae, but there are others.

Myrmecophytic tillandsias may be popular, but they are not typical houseplants. They only produce anchor roots (roots that fix them to bark) and short ones at that. These roots absorb no water or minerals, so you have to water tillandsias by either plunging them into a bucket of water or spraying them. Here are more details on tillandsias and their rather unique needs.

Myrmecodia

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Myrmecodia echinata. Source Michael Wolf, Wikimedia Commons

The genus Myrmecodia, from the Rubiaceae family, includes about 30 species, all myrmecophytes, as their name suggests. They are commonly simply called ant plants, but we’ll call them myrmecodias here to distinguish them from all the other ant plants.

Myrmecodias are epiphytic plants originating from the jungles of Asia and Oceania and produce, at their base, a large swollen, thorny tuber (caudex). If you cut one open, you’d see it’s full of holes and tunnels in which ant species live. The waste given off by the ants provides nourishment in what is otherwise a very mineral-poor environment (there’s not much for plants “to eat” on a tree branch!) and ants also protect them from herbivores. Several species of myrmecodias are grown as indoor plants for their odd shape, but they’re all fairly similar.

Grow them much as you would orchids, either in a pot using orchid mix or fixed to a slab of wood or bark. Their most limiting factor is that they need high humidity at all times.

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Hydnophytum formicarum. Source: Bernard Dupont, Wikimedia Commons

The closely related genus Hydnophytum produces a similar caudex but one without thorns. It requires the same care as myrmecodias

Codonanthe

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Codonanthe carnosa is one of the easiest ant plants to grow. Source: laidbackgardener.blog

The genus Codonanthe, from the Gesneriaceae or African violet family, is a small, pendulous epiphytic plant from South and Central America that produces seeds that resemble ant eggs and that are spread when ants harvest them and bring them back to their nest, an environment rich in organic matter where the plant can thrive. Codonanthes can also live without ants in the wild, but grow more weakly. In return, codonanthes have extrafloral nectaries under their leaves to feed the ants and their berries are also ant fodder.

This is undoubtedly the easiest of the myrmecophytic plants for most indoor gardeners and will grow under average home conditions with no complications whatsoever. It is easy to propagate from cuttings or seeds.

Dischidia

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Dischidia vidalii. The arrow shows the pouch-shaped myrmecophytic leaf. Source: Mokkie, Wikimedia Commons

The genus Dischidia of the Aypocynaceae family comes from Asia and consists of climbing epiphytic plants. Not all are myrmecophytic, but many are. The best known and most widely grown is D. vidalii, still sold under its former name D. pectenoides. You’ll occasionally find it in nurseries in the form of a small climbing or hanging plant. This plant mostly produces small, heart-shaped leaves that offer nothing of interest to ants, but, from time to time, much larger, pouch-shaped leaves that serve as an ant hotel. They actually produce roots inside the swollen leaf so they can absorb ant waste. As an added attraction for the gardener, the plant also produces small red flowers.

This is not the easiest houseplant and suffers from dry winter air unless you take special measures. It’s better to grow it in orchid mix.

D. major, formerly D. rafflesiana, is similar, but bears bigger “pockets” in larger numbers … but is more capricious, needing even higher atmospheric humidity.

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Dischidia astephana in the wild. Its lumpy, bumpy leaves are ant hotels. Source: Clivid, flickr

Other dischidias such as D. astephana produce lumpy leaves that press up against the bark of host trees, leaving room for ants to nest underneath. In the home, they have to be grown on a slab of wood, bark or tree fern, as only leaves that press against a surface will produce the curious bullate leaves that ants so love.

Ant Fern

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One of the curious ant ferns: Lecanopteris pumila. Source: wistuba.com

There are actually several myrmecophytic ferns, particularly in the genera Lecanopteris, from Asia and Oceania, and Solanopteris, from the New World. Epiphytes, they have oddly swollen rhizomes that house ants in the wild. These rather delicate ferns are often grown on bark slabs, like orchids. They need perfect drainage, but also high atmospheric humidity at all times.

Ant-Acacias

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The thorns of bull’s horn acacia have a small hole through which the ants enter and leave. Note the food bodies on the tip of some of the leaflets. Source: Stan Shebs, Wikimedia Commons

In its broadest sense, the genus Acacia is a huge one of usually arid-climate leguminous trees and shrubs that includes over 1,300 species distributed throughout the tropical and subtropical regions of the world. However, the genus has been split up into five separate genera and the genus we are interested in here is now called Vachellia. Of the some 150 species of Vachellia, only a dozen or so, some from Central America, others from Africa, are myrmecophytic, the best known of them being the bull’s horn acacia (V. cornigera, formerly Acacia cornigera).

It’s a small tree of Central American origin with bipinnate leaves and stems bearing pairs of thick spines shaped, as the name suggests, like bull’s horns. The spine is designed to be hollowed out and inhabited by a particularly ferocious species of ant, Pseudomyrmex ferruginea, that not only protects the plant from its enemies, but also kills and removes nearby vegetation, eliminating the plant’s competition.

In addition to providing shelter for ants, bull’s horn acacia produces extrafloral nectaries on its stems and also food bodies at the tip of its leaflets. As a result, it is believed that the ants get all their nourishment from the tree and never need to go elsewhere. No wonder they protect their host tree with a vengeance!

This plant makes a rather stark but attractive houseplant, readily grown from seed.

Cecropia

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Tap on the trunk of a Cecropia obtusifolia in the wild, and thousands of angry Aztec ants will soon charge out to chase you away. Source:  Dick Culbert, Wikimedia Commons

There are about 20 species of Cecropia (now in the Urticaceae family, although this placement is disputed by several taxonomists), fast-growing pioneer trees from the American tropics and most host Aztec ants, tiny but vicious ants that inhabit their hollow trunks. Not only do these nasty little ants protect the tree from herbivores, they also remove any climbing plants and epiphytes that try to settle in. In addition, cecropias produce food bodies on their leaves and thus feed their ant friends.

Cecropias, sometimes called trumpet trees or snakewoods, produce large webbed leaves reminiscent of scheffleras (Schefflera actinophylla) and make interesting houseplants … for those with lots of space, at least!

Other Myrmecophytic Houseplants

There are actually many other myrmecophytic with potential as houseplants, such as Aechmea brevicollis, Hoya imbricata, Macaranga spp., Nepenthes bicalcarata, Pachycentra glauca, Platycerium madagascariense and Tetrastigma voinierianum, not to mention a few orchids such as Myrmecophila and Caularthron. They’re fascinating plants that deserve to be much better known!20171125B The Cliparts

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