We have a sudden ant infestation. Nothing works to get rid of them so we are going to ride it out I guess. Must be the drought in California? I have a question though. They often congregate in our shower. Even when and especially when it is dry in there and no one has showered since the morning. Today, I went in and there were a number of trails leading to perfectly formed circles. The circles had the ants facing inwards and their bodies/tail ends pointing outwards. I wish I would have taken a picture but I was upset and I washed them away. It wasn't a moving circle. It was a stationary size of about a nickel circle. Much like a synchronized swimming event. Is this a meaningful event? Are they talking about leaving my house in this circle? I sure hope so. Thanks for answering me or replying back to my email if you have time. Oh and they are small little black ants - if that matters.
The ant that is visiting your house is most likely the argentine ant. The move inside when it is too dry outside or too wet. As you have noticed there is many advantages to these visits - one is that you are presented with a convenient chance to observe nature in the comfort of your own home. Why are they forming a circle - well there must have been some resource there - residue from the evaporated water (salts most likely) that the ants were feeding on. If inclined you could give them a drops of water with food coloring in them and watch them change colors:
You can read more about the sudden movement of the argentine ants into homes in California at: http://www.sfgate.com/bayarea/johnson/article/Meet-the-bad-ant-that-s-overwhelming-California-5719954.php
With regard to control, I strongly advise a green approach. First, seal the cracks where the ants are entering. Next if the ants in the house are a problem, then just vacuum them up. Outside if you want to control the nest, apply boiling water (lots of it). Chemicals in your house will be a permanent risk to you and others and outside when applies will be a containment to wildlife and water supplies.
Best, Brian Fisher and the Ask AntWeb Team