Two of my four Valencia tomato plants have been looking sickly for the better part of a week. First the lower leaves turn yellow, then this seemed to spread up the plant until the whole thing was yellow and wilted.

Fearing that these two were a lost cause, I pulled them out today but noticed when I did so, ants scurrying in every direction. There were dozens of them! The area of the garden where the tomatoes are planted is covered with red plastic mulch, so until I pulled these guys out, I hadn't observed any insect activity.

I've heard that ants will keep aphids like a rancher keeps a herd of cattle, but I wasn't able to find aphids anywhere in the garden. Nor was any other insect activity evident. Could the ants be eating the roots or is something else going on?