Sunday, April 21, 2013

Down on the Aphid Ranch

Originally this post was just going to be about vines.  There are four basic plant forms: trees, shrubs (small, branched trees), herbs (non-woody, soft plants), and vines.  Vines are woody but not strong, and they grow as tall as trees by climbing up other structures, often trees, and they usually have roots that are good for holding on, like this:

English ivy with roots that are good for holding on to buildings or trees.
 Some vines are evergreen like the English ivy on the back wall of our classroom:
English ivy doesn't lose its leaves in winter.
And some are deciduous like the milkweed vine on the lamppost by the playground:
Milkweed vines lose their leaves in winter.
Like I said, I was originally going to post about vines.  BUT when I was looking through my vine pictures, I noticed a very lucky but accidental detail on this picture of the English ivy:
New growth on the English ivy with some curious dots on the stem.
Look very closely at the stem, and you will notice there are ants walking on that stem:
Ants going up and down the ivy stem.
I started to wonder what ants were doing walking on an ivy stem, since there is not likely to be much ant food at the top of an ivy vine.  Then I looked even closer and I saw this:
An farmer ant and her aphids.
Now we have a story!!!  The picture is a little blurry, which means you're going to have to go out to the outdoor classroom and see this for yourselves.  The dark spots are insects called aphids (which can also be whitish or greenish), and the reddish brown spot is an ant.  What the ant is doing is called aphid farming.  It's a bit gross, but it's so amazing that it's completely worth learning about.

To explain aphid farming, we have to go back to plant sap.  Remember plants do photosynthesis and make sugars, which are dissolved in plant sap, making plant sap slightly sweet?  Aphids have pointy, needle-shaped mouthparts they poke into soft plant tissues, and they suck the plant sap out of plants for their own food - much like the psyllids we learned about back in the fall.  Since aphids live closely with plants, they are said to be in a relationship called a symbiosis.  In this relationship, the aphids are harmful to the plants because they 'sap' their energy.  The aphids benefit by getting food.  A symbiosis where one organism benefits and the other is harmed is called parasitism. 

Here's where it gets slightly gross.  Aphids drink a lot of plant sap, and they digest most of the sugar in it, but not all.  The leftover sap with a tiny bit of sugar in it goes on through and out the other end of the aphids' digestive system.  In all other organisms, this substance would be called feces or poo, but in aphids, the substance is a clear and sugary liquid, so it's called honeydew.  (Do NOT confuse this kind of honeydew with the delicious green melon you find in the produce section.)  If you have ever noticed sticky, clear spots on the hood and windshield of your car if you park it under a tree in summer, you have seen the results of the mist of honeydew that rains from the aphids in the tree.  Take a deep's really only plant sap run through an aphid!

OK, here's where it gets really gross, but also really, amazingly neat.  The sugars in the honeydew are technically a food source (like any other sugar), and ants eat sugar.  Put those two facts together, and you know what that ant is doing with the aphids on the leaf in the picture above.  Yes, some ants eat honeydew.  The ants know a good food source when they find it, so they protect the aphids and fight off aphid predators.  They even move the aphids around to better sap sources if their honeydew production slows down.  Some ants even keep aphid eggs in their ant nests in the ground during winter and place them on new plant growth in the spring so the aphid eggs have food when they hatch.  The ants are said to farm the aphids - just like humans farm cattle!  Dairy cattle ranchers protect the cows and move them around to new pastures with more food.  Cattle farmers also protect calves and raise them to adults.  And cattle are farmed to provide a liquid food source: milk! 

It's amazing to me that humans are not the only type of organism that does farming.  Ants live in groups and have very complex behaviors, as exhibited by the aphid farming.  Their behaviors often mimic human behaviors: they have 'jobs', fight battles, farm aphids (and fungi - neat story for another time), build large complicated structures and much more.

Ants and aphids are in a symbiosis - they live very closely together.  Their symbiosis involves both organisms benefiting from each other.  The aphids get protection, and the ants get food.  A symbiosis where both organisms benefit is called a mutualism.  Humans and cattle are in a similar mutualism.

The new growth on our ivy is very likely to have a constant supply of aphids and ants, so you should be able to find them any time!

1 comment:

  1. Whoa! I never viewed at any stem like this before. The detailing and information is just superb. Very helpful write up. Hope to learn more from your garden.