Hello and welcome to YT.
Check your dog food bag and see if corn is listed first.
She maybe allergic to it.
My older yorkie has this problem.
I have her on a lamb and rice food and it seem to have taken care of the itching.
Also where you live.. has a lot of fleas
so make sure you have her on a flea treatment like frontline,advanage..etc.
One flea can make them scatch like crazy.
You can give yorkies a bath once a week.
You might try an oatmeal based treatment shampoo.
I always use baby shampoo on mine.
They can become itchy when they get to dirty as well.
Good luck, I hope you can find out what is causing her to itch and scatch so much.
Also, good luck in finding a new addition to your family.
Yorkie's are like potatoes chips ..you can't have just one.
I have 6
