Many times the characters don't seem to notice the visible quantity of ants in their surroundings. Also, it takes time for them to even realize they're being bitten by the ants.
In various scenes, only a handful of real ants are used. A painted matte is used to make the quantity of ants seem as bigger.
As Gordon and Somers embrace , there's visible camera cords lying on the carpet in the adjoining room. No one noticed that these big black cords were in the frame.