It looks like they are carrying eggs, they were probably all moved in and thinking they could hollow out that caudex and make a nice home. Of course, they will bring their pet aphids along too.
Ugh! I hate ants, really hate them. We must have dozens of types of ants here and none of them are useful in my mind. The leaf cutters are the worst but the small red ones that bite are pretty bad. Good luck, I say kill them all.
