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Plant hires ant bodyguards

Ah, the beautiful little prairie at work! I've written about this one-third acre transformation of barren turf grass into biodiversity boiling over before, HERE. Today, I trotted outside for a brief 15-20 minute photographic interlude, and was rewarded with something rather cool.

Among the many native plants in our prairie patch, all provided by Ohio Prairie Nursery, is this little gem. It's Partridge Pea, Chamaecrista fasciculata, a showy pinnate-leaved beauty. It is easy to grow, quick to arise, and pleasing to the eye. Insects also find it pleasing to the palette, as we shall see.

The rich lemony blossoms are offset by chocolate-brown stamens dangling from a crimson bulls-eye. An arrangement sure to lure winged pollinators. And any would-be pollinating insect would be wise to arrive by air. Partridge Pea has made a long-term co-evolutionary pact with some serious six-legged tough guys to keep ne'er do wells out of its foliage.

An ant (I don't know the species) clambers about a Partridge Pea plant. It was one of many on the plant that I chose to photo-document. We have lots of these plants in our prairie, and I suspect all of them were inhabited by ants.

We are not here, in this photo, to look at this ant. We'll look at ants in a bit. I draw your attention to the tiny cuplike appendage on the leaf's petiole, upper lefthand corner of the photo. Note the glistening reddish syrup within. The petioles (leaf stalks) of all the leaves are similarly adorned with these cups, which are known as extrafloral nectaries. A great many flowering plants are laden with intrafloral nectaries - sweet nectar rewards hidden within the flowers. They are there to entice pollinators into the bloom, where they will be dusted with pollen and thus complete the plant's pollination process.

Extrafloral nectaries are far scarcer, and arguably far more interesting.

Ah! One of our ants has found an extrafloral nectary, and is lapping the secretions within like a dog at a bowl. Extrafloral nectary nectar is basically supercharged plant sap, rich in fructose, glucose, various proteins, and amino acids. Utterly irresistible to ants. Plants such as the Partridge Pea, that are stippled with extrafloral nectaries, are apt to loaded with ants. The ants scurry busily from nectary to nectary, and in between they race about the rest of the plant in their quest for more of the sweet stuff.

The impact of this ant army? An incredibly effective deterrent to any wannabe defoliators such as caterpillars or other insects that would damage the plant. If the ants detect a threat to their spoils, they will launch a brutal attack and drive off or kill the interloper. This behavior, of course, greatly benefits the plant and is an excellent example of a mutualistic relationship: both organisms in the partnership benefit.

Numerous studies have been done on ants and extrafloral nectaries, and most have found that plants with ants suffer less grazing damage, and also generally produce better fruit crops, than plants without ants. Some investigations have suggested that plants with extrafloral nectaries may also benefit by keeping ants away from the flowers. Ants are probably not great pollinators in general, and can spook legitimate pollinating insects from the flowers, rob nectar with no reward for the plant, and possibly damage reproductive tissues. But probably the main goal of possessing extrafloral nectaries is to in essence hire a team of ferocious, fearless six-legged body guards to safeguard the plant.

Plants protect themselves in numerous ways: production of various compounds that ward off herbivores; thorns; development of difficult to digest fibrous products such as lignin, etc. But few protective strategies top the complex active defense system brought on by development of extrafloral nectaries. This defense system isn't particularly common; for instance, of the 330 or so species of Chamaecrista worldwide, only about 26 have evolved extrafloral nectaries.

While ants are clearly the primary target of the nectaries, they also attract other predators such as jumping spiders. The spiders will apparently sip at the nectaries, and are drawn to plants that sport them. Having a jumper or two hanging out in your foliage is a good way to help keep plant-damaging insects at bay. I photographed this jumping spider (species unknown) on Tuesday in a prairie with plenty of Partridge Pea close at hand. I didn't see it visiting nectaries, and didn't know that they would do so until doing some research for this piece. I'll be more alert in the future and try to photo-document a jumper sipping from a nectary.

Partridge Pea is in full bloom now. If you're around some of these plants, have a look to see if ants are working the extrafloral nectaries.


Gaia Gardener: said…
Really interesting post, with great photos. Thank you. I've known about extrafloral nectaries, but your photos help me visualize them much better than I could before.


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