Large black ants are usually carpenter ants, which like to nest in wood. In large numbers, they can cause significant structural damage. To get rid of them, find and destroy the nests.
Know ants' habits
Carpenter ants nest in almost any wood but prefer wood that is moist. They are likely to be found in places where water leaks have moistened wood, so areas near sinks, bathtubs, and window and door frames are good places to look. Moist, hollow places are particular favorites, such as the wall void behind a dishwasher. Carpenter ants push out shredded wood fragments, dead ants and bits of insects they have eaten from their nests, so finding such material is a good clue that a carpenter ant nest is nearby.
Locate the nest
If you find signs that ants are present, tap on nearby wood with the blunt end of a screwdriver and listen for hollow sounds. Sometimes the ants will reveal themselves by making rustling or clicking noises that sound like crinkling cellophane in response to your tapping. You can also use the bladed end of the screwdriver to probe wood that seems weak for the ants' galleries.
Make ant scouts work for you
Don't spray carpenter ants that you see. Instead, set out food for them on the back of small pieces of masking tape. Diluted honey works well. After the ants have fed, trail them back to their nest. This tactic is most effective at night when the ants are most active, so you may need a flashlight and some patience to make this work.
Treat the nest
Once you have located the nest, you need to buy some boric acid and some bulb dusters from a hardware store. Carefully drill 1/8 inch holes into the suspected nest area and use the dusters to puff boric acid inside. Treat at least 3 to 6 feet to the side of where the ants are entering to get the entire nest. Treat around pipe collars and behind the junction boxes for electrical switches and receptacles but never treat inside the boxes. If the main nest is outdoors, you can drench it with an insecticide.
Keep ants from returning
Once you've gotten rid of the nest, remove damaged or rotten wood from inside the house and make any needed repairs. Remove rotting or damaged wood close to your house to prevent ants from invading your house as scouts from outdoor nests.
