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.

After monsoon

After the monsoon ends the weather turns unbearably hot again; that’s what an Indian summer is. In the sweltering heat of October it is a minor disaster if you forget to water plants. The rose bush has been putting out flowers through the monsoon, because the rains keep it from drying up. Today I saw that two days of not watering it has begun to affect it.

Methi, fenugreek

Many plants are beginning to bud. I look at the methi (fenugreek) shrub. Every stalk is budding new leaves. The hairy surfaces of the leaves catch every piece of lint which floats by. You have to carefully wash the leaves before you use them in the kitchen.

Hibiscus bud

But really this is the time of the year for insects. The hibiscus bush is beginning to push out flower buds. As soon as one opens, ants swarm over it. Soon they will bring their aphid cows up the stalks. The vegetation below the spectacular flower will be thick with aphids, as ants run up and down their farm milking them.

Dotted moth

Moths have pupated too. I saw this lovely October visitor on the wall today, sitting out in full sight. The lore about bright and visible butterflies and moths is that they are poisonous. Many birds would see this yellow on the wings of the moth more brightly than we do, so it is definitely signaling that it is inedible.

Green lacewing

Well back on the wall I found a few green lacewings. They are nocturnal and have probably come here to eat the aphids from the ant farms. Lacewings are not poisonous: birds and bats will happily eat them. That’s the reason this one was sitting far back on the wall, under an overhang. In another month all these showy insects will be gone. That’s when migratory birds begin to arrive.