Ants and Aphids

Although hazardous to humans, the wild parsnip can be a home for other creatures.

This column appeared in The Recorder on July 24.

Last week I introduced you to a plant called the wild parsnip (Pastinaca sativa). Though beautiful, the plant’s toxic properties and the horrible chemical burns it can inflict upon humans (most namely me!) put it on my personal naughty list. Even as I was writing the column, however, I had the nagging feeling that there was more to the story.

I combed through my photo files and discovered that the wild parsnip has been growing in my yard for years. The fact that I haven’t really “noticed” the plant is typically human. I spend a great portion of my waking hours in pursuit of birds, but the study of the plants that surround me (a.k.a. botany) is a pursuit that I am not quite as dedicated to. However, recent events with the wild parsnip have changed my attitude toward plants.

It turns out that in back in July of 2015 I stumbled upon a wondrous example of a symbiotic relationship between two types of invertebrates. The background for this tiny spectacle was – you guessed it – the wild parsnip. It all took place in the weedy margins of my woodpile.

I was walking back to my house after a morning spent in the “thinking chair” when I stopped to admire a beautiful wasp. Its shiny exoskeleton was jet-black and decorated with gorgeous bright yellow stripes. This was a paper wasp that I had seen before, but never fully identified. Two years later, after about an hour of research, I discovered that I had photographed a European paper wasp (Polistes dominulus). The flowers of an invasive plant introduced by European settlers in the 18th Century was being visited by an invasive insect that was introduced by modern Americans in 1981. Can you guess what I’ll be writing about next week?

Anyway, stopping to look at the wasp gave me time to notice a dark patch on the stem of the parsnip plant. Closer inspection showed me that I was looking at a “herd” of aphids that were tapping into the parsnip stem to feed off its fluids. A byproduct of this feeding strategy is a sugary fluid known as “honeydew,” which is forced out of an aphid’s anus due to the pressure of the fluid in the plant stem.

This was quite interesting, but the really amazing sight was yet to come. While focusing my macro lens on the aphids I suddenly noticed that some red-and-black ants were actually shepherding this “flock” of aphids, stopping here and there to feed on the honeydew. But it turns out that they don’t simply exploit the aphids. The ants will also defend them from predators and potentially carry them to new “pastures” (new host plants) if the food supply starts to fail. The ants get food and the aphids are protected and cared for.

This relationship between ants and aphids is very well know and is often used as an example of the symbiotic relationship known as “mutualism” in biology textbooks. As with many such textbook examples, I was content with the idea that this sort of thing happened somewhere out in the big wide world. I was delighted to discover that it was actually taking place behind my own garage.

The natural world is full of such wonders. All around us, in our yards, woods, fields and parking lots there are wondrous spectacles to behold and learn about. I have several weeks of summer vacation left to me and I will pursue the obscure, the unobtrusive, and the practically invisible. Let’s just hope the rain holds off so I can take my camera outside.

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