from observing bb queens in the spring, they visit the woodland wildflower dutchman's breeches. however, you won't find this flower listed in any literature i have read! so, here is one wildflower/ early bloomer. one plant still wouldn't be enough and it has a short bloom time. i also don't know what you would use b/t the end of breeches bloomtime to the beginning of echinacea bloom time. i am not sure if i remember correctly, but they may have visited my old fashioned bleeding heart. this is a relative of dutchman's breeches so i don't see why not. ajuga/bugleweed is an early blooming groundcover which may fill in the time gap as well.
at least echinacea has a long "season." bb's also love purple blooming varieties, such as lamb's ears, lavendar, and the annual borage. the bb's visit my oregano when it is in bloom and my sunflowers. i am not sure on the type of bee, but i had one on my cucumber vines/blossoms, and last year i had honeybees (i think) on my squash blossoms.