I hate ants, and I usually discover them the same way you do -- by seeing the cat food moving. My initial recourse is to dump it outside, ants, food, and all, and then feed the cat on top of a chair until things get better.

It used to be that it was just a week or two in spring, but a few years ago I had midwinter ants all over the house, and had to call in professionals. Since then, I've had them back whenever I see more than a couple of them. Most of the spraying is done outside the house, which is the way I prefer it, even if it's supposed to be nontoxic to cats. It's well worth the $100 or so to me every few years.