You can treat both drywall and subteranean termites very effectively yourself using a relatively new method of termite erradication developed by Dr. Tim Myles at the University of Toronto. It's called "Trap, Treat and Release" and it relies on the fact that termites instinctively groom each other. So, you trap some termites in a "trap" which is nothing more than a piece of rolled up cardboard in a metal can (termites love cardboard because it's both soft to chew and the corrugations in it are the right size that the termite feels protected from other bigger bugs that may want to eat it). Once the termites discover the cardboard in the trap, the cardboard will quickly fill up with termites feeding on the cardboard. You catch these termites and put them in a large pan with steep sides (so they cannot escape) and treat them with a slow acting but effective insecticide called "Sulflurin", and put them back in the trap with new cardboard.
The treated termites will return to their nests to tell the other termites where the food is, and will spread the Sulflurin to all the termites in the next because of that instintive grooming they do to each other. Every time the trap fills up with termites, you treat them with Sulflurin and return them to the same location with new cardboard in the trap. Once you no longer find termites in the trap, you can be sure there are none to be found in your house, or in your yard.
This program is currently being used to treat entire neighborhoods in Toronto by volunteers (who undoubtedly have termites in their homes). Check out this web site:
The method is highly effective and can be done by individual homeowners. The only reason it's not being advertised is that it was developed at the U of T with public funding and so no one is making a business out of offering this service to the general public, so no one stands to make money from it as a business. But, you can read the results of trials done in various other cities in southern Ontario where volunteers using this method effectively erradicated termites from entire neighborhoods.