Competition and mutualism

In the last days of September I wrote about the strange flowers of a century plant (Agave americana). I’ve been visiting it regularly since then, looking at how the flowering progressed week by week from the bottom of the stalk to the top. While the flowers dropped off the bottom of the stock, parasites found a place on it. White woolly aphids began gathering on it, tended by their guardian ants. This was interesting, since the century plant is a recent import, and neither the aphids, nor the plants would have adapted to each other. So this species of aphids must be generalist feeders, attacking plants opportunistically. And this plant would have no specific defense mechanism against the pests.

This was also obvious from the rate at which the aphid colony grew. In a week and a half I saw them grow into a mass of white covering the remnants of the cymes of fallen flowers. The mutualism of aphids and ants is very interesting, with ants providing protection and cleaning services to these aphids blooms. It seems that the honeydew secreted by aphids often limits their growth, but the ants harvest this excretion, full of sugars and essential amino acids, while fending off aphid predators. They also build retreats for the aphids (I must look for these aphid pens). At the same time, there are predators which seem to have developed many kinds of tricks to fool ants into leaving them alone. How many years have gone into making these webs of life? Aphids seem to have evolved about 280 million years ago, and ants around 100 million years ago. Ants evolved around the same time as flowering plants and moths, and butterflies evolved around 55 million years ago. The mutualistic and antagonistic relationships between them cannot be older of course, but there seem to be no clear indication of how old these relations are. It is such an unsatisfactory state of affairs!

This infestation had bloomed very quickly. Before the flowers had fallen off, the first aphids had begun to climb into the stem of this plant; you can see the first aphids arriving on the stem of this cyme, shepherded by ants. I’d discovered earlier that the flowering of the century plant depletes the leaves of stored moisture. This means that there is a huge flow of sap through the flowering stalk. This is the reason the sap feeding aphids flock to this plant. How did they find the plant? By smell perhaps? That would explain why they arrived only after the first flowers began to drop off; sap must have oozed out of the breaks and attracted these parasites. What a wonderfully complex ecology is revealed in the single flowering of a plant in a garden! The world is full of questions.

By I. J. Khanewala

I travel on work. When that gets too tiring then I relax by travelling for holidays. The holidays are pretty hectic, so I need to unwind by getting back home. But that means work.


  1. Fascinating relationships, and on such a small scale where they’re challenging to observe. I’d never heard of guardian ants. Your great photos and explanations help me understand the relationships between plant, aphid and ant.

    So many questions!

    Liked by 2 people

  2. Very intersting topic. Aphids are distressing to me, they relentlessly attack my hibiscus plants. Is it also possible that the ants which are fast moving and discover potential plants then shepherd any aphids they find onto the plants to establish the colonies?

    Liked by 1 person

    1. This aphid species, which also attacks hibiscus, doesn’t seem to be among those “domesticated” by ants. The interactions you see here seem to be mutualistic. So I think it is unlikely that they were brought to the site of infestation by ants. If you are worried about them, it might be more useful to concentrate on removing the aphids. The ants will go away.


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