Back in the late '80s (
that's 20 years ago!), when I had a poodle-mix (who had terrible fleas where I lived at the time) I used Avon Skin-so-Soft bath oil before I gave her a shampoo. It killed the fleas completely in a few minutes while she was in the tub. Then I used a gentle non-toxic shampoo. I don't know about the formula Avon uses now, though, or if it would irritate the skin of a Yorkie.