Formosan termites are a type of subterranean termite. They are just native to the former republic of Formosa, now Taiwan. And you still have to acknowledge, surface treatment is the best way to protect a structure from drywood termites. I don't want to argue with you anymore because you will have more stamina than me on it I am sure (blacklistedbully style), but go read the link. Or better yet. Read the below. I pasted it for you.
Snapshot
Active ingredient: Bifenthrin
Works in: 24 hours
Lasts for: 90 days
Bifen XTS is a fast-acting, oil-based termiticide that can kill termites in 24 hours by attacking their nervous systems. This treatment is not recommended for use indoors. Spray it around the perimeter of your home to kill termites.
Bifen XTS also works as a preventative measure—after spraying, it leaves residual material for up to eight weeks, killing any termites that come into contact with it. Bifen XTS is also an effective pest control solution for dozens of other kinds of pests.
We should note that Bifen XTS is oil-based, so if you aren’t careful spraying it, you may harm your garden.
And by the way, it's not a big if. You either have a slab foundation or you don't.** If you want to argue further, please take it up with the guys at This Old House.