Paying for protection

Generally, when I inspect a plant in the garden, I do not want to find aphids on it.  Aphids puncture the cells that transport nutrients up and down a plant's stem and then drink the sugar-rich sap that leaks out.  Since aphids reproduce rapidly, they can inflict serious damage to the host plant if left uncontrolled.  Therefore, I almost always remove the aphids as soon as I can.

However, I make an exception when the aphids are accompanied by ants.  The opportunity to observe the ants trumps (at least temporarily) my concern for the plant's welfare.  Instead of just eating the aphids, some ant species will tend the aphids like livestock.  When the ants touch the aphids with their antennae, the aphids excrete a sugary honeydew that is collected by the ants.  The ants aggressively defend this valuable resource, thereby protecting the aphids from attacks by their natural enemies -- or, in the case of this video, a  dry grass stem. 

* To see this video in high definition (1080p), you may need to (1) click "YouTube" to watch on the YouTube website and (2) change the settings at the bottom of the video screen.