It's debatable whether this period produces the season's best fishing, but these anglers are dry-fly enthusiasts who know that each year at this time they will find hatches of Green Drakes on the Henry's Fork and giant stoneflies on the Madison. The hatches of these two insects certainly bring more large trout to the surface than any others. It is no secret that on most of the better rivers and lakes, the best fishing begins as the salmonfly hatch on the Madison winds down. Hebgen Lake has to rank as one of the world's best stillwater dry-fly fisheries, with August being the prime month. As the caddis and mayfly hatches taper off in August, terrestrial insects become an important food source for trout.
Grasshoppers are the main attraction.
When trout are feeding on hoppers, the action can be fast and furious. August through mid-September is the best time to fish terrestrial imitations on all the streams in the area. The following chart is not intended to be comprehensive. It lists only the most important hatches of aquatic insects for dry-fly fishing in Yellowstone National Park and the surrounding area. Hatch dates can vary from year to year depending on duration of runoff and water temperature.