In most cases people are not aware of where the ants are nesting. In your case, you seem pretty sure where they're living. In fact bathroom walls are one of the most common locations for carpenter ants to use. These areas are usually moist and at least humid which carpenter ants love. No doubt this is why they target the bath room in so many homes.

I recommend using the most direct approach possible. That means drilling some holes and pumping the DRIONE dust into the wall void. Drill between the studs all the way around the room. These holes can be above any water proof walls you might have. And don't forget you can access this space from two sides. That means if there is a closet on the other side it might be easier to drill through that wall instead of the closet.

I also suggest doing a good dusting on the outside of the home. This should be done now and then in the spring I'd followup with some CARPENTER ANT BAIT and spray on the foundation. The BIFEN is a good product to use and if done throughout the year, you won't have any chance of a reinfestation.

Here are direct links to the information and products listed above:

Drione:  http://www.bugspray.com/catalog/products/page91.html

Carpenter Ant Bait:  http://www.bugspray.com/catalog/products/page253.html

Bifen:  http://www.bugspray.com/item/bifen_it.html

There are many kinds of ants which will readily infest unprotected trees. Most will forage on trees feeding on smaller insects like aphids, whiteflies and mealybugs. But some are wood destroying ants like the Carpenter and Acrobat species. These ants will usually target older trees or trees that are diseased, deformed, damaged or vulnerable for one reason or another. If left to nest as they wish, the constant drilling and chewing will no doubt bring the end of the trees life much earlier than normal.

Treating the tree with a liquid spray is usually the most direct way to take care of the problem. One of the best products for this treatment is the BIFEN CONCENTRATE. Odorless, easy to use and because it demonstrates no phytotoxicity to plants, it should be broadcast onto the trunk of the tree where you see activity. Most any sprayer will do the job but if you don't have one that can reach high enough, the TROMBONE SPRAYER is perfect for most applications since it can reach so high. Treat every two weeks till you no longer see activity. Retreat during the summer to insure none return. In general, you should get 1-2 months of protection when treating for maintenance (no activity is present). But when ants are active, apply some more frequently till the nests are destroyed.

Here are direct links to the information and products listed above:

Bifen Concentrate:  http://www.bugspray.com/item/bifen_it.html

Trombone Sprayer:  http://www.gotosprayer.com/sprayers/pump-sprayers/trombone-pump