cd,
Rule of thumb here in northeast Ohio is the full moon in June & they will return to the beds (although not as heavily) each full moon throughout the warm months. I believe there is a full moon 5/27 & that may be the one since the full moon in June is very late in the month.
Favorite flies: yellow woolly worms (unweighted of course), white foam spiders, red butt epoxy ants, yellow gurglers, black foam beetles.
Mike