The Griffiths Gnat is the standard dry fly to use on midge hatches. These can occur anytime on spring creeks and tail waters but they are often heaviest at dusk. Frequently the large trout cruise the slow currents and sloughs to feed on the midges. Try to anticipate their cruising path by watching their rise forms then cast your fly three feet out in front of them. Virginia has great midge hatches on the mountain streams in the fall.