You can use Termidor to treat drywood termites and it is very popular for that purpose. Termidor will not penetrate through wood, so it will not work just by spraying it on the outside of the wood. It has to touch the termites inside the wood, so this means you have to drill and inject it into the wood. To do this you should drill whatever size hole you need to into the center of the wood, typically you should drill every 6 inches apart in the area you suspect termite activity. If you can find a kick out hole where they kick out their pellets, you should drill directly into this hole. You should caulk over the holes when you are done treating them so water does not get in if it rains. The line on the termidor bottle is .8 ounces, which is how much you are supposed to mix per gallon of water. .8 ounces per gallon of water. The drill hole can be any size, whatever size you need to make it to get the Termidor in the hole from whatever you have to apply it with. The holes should also be at a downward angle into the center of the wood so when you pour in the Termidor it will stay in the wood and not run out.
As long as the Termidor touches just a few termites, it will kill them all.