caulk. Exterminator.

hi there, first of all you need to find out what type of ant we are dealing with. Second we need to check if the windows are rotting if they are made with wood. What I would recommend is to check the exterior to see if any dead trees or tree stumps are on the property that would be conducive to the ants wanting to potentially setup a satellite nest under your window frame. If all else fails contact an exterminator that can take a closer look at the problem. However, I would not caulk the crack until an exterminator has taken a closer look at the problem.
Peter Power
Power Pest Control