Cleaning teeth is a lot harder than picking strawberries. And picking strawberries is hard enough for robots that it makes news when they do it.
I suspect, if dental hygienists are in danger of unemployment, it's not robotics, but rather advances in medicine that make dental hygiene optional (ick!).
Nano technology and other innovations will soon enable most dental cleaning to easily be done at home.
And meanwhile, the country has an oversupply of trained dental hygienists, many of whom went to school to become dental hygienists because their prior jobs were eliminated by technology or offshoring enabled by technology.