The Green River does not have a very diverse insect population. However, when a hatch occurs it can be unbelievable how many bugs are on and in the water. Sheer numbers of bugs can be in the millions, making it hard to breathe without sucking some in.
When such a hatch occurs the trout go wild and fishing can be fast and furious. There are three main hatches on the Green:
Midges - Midges hatch year-round. In the winter they are commonly referred to as snow flies because they hatch even on the coldest days of the year. They are extremely prolific and can hatch by the million, generally during the warmer part of the day during the winter and almost any time during the summer.
Imitations should include nymphs, emergers and adults in size 16 to 22. Griffith's Gnat, Spent Midge, Larva Lace, Midge Larva, and Parachute Midge Emergers will all catch fish.
Mayflies - Two Mayfly hatches occur on the Green. Bluewinged Olives begin hatching in March and continue into early June. Again in the fall, generally in late September or early October they begin hatching again. The fall hatches continue into December.
Pale Morning Dun hatches begin in mid May and continue through July.
Imitations should include nymphs, emergers and adults and you should be prepared with an arsenal of each.
Caddis - Caddis hatch intermittently from early May through the end of August. In his book on the Green River (River Journal: Green River) Larry Tullis says, "If caddis are hatching in the evenings, fish caddis larva and pupa patterns during the day. Chamois Caddis and LaFontaine Sparkle Pupas are effective."
Nymphs - The Green River is crawling with scuds (tan, olive, pink and orange), cranefly larva and small red worms (San Juan Worms). Any or all of these can be fished effectively year round.
Terrestrials - During the summer and fall it's great fun to fish on top with hopper, beetle, ant, cricket and cicada imitations. These patterns can be deadly once the fish start looking for them.
Streamers - Wooly buggers, Zonkers, Leaches, minnow imitations, Muddlers, Spruce, and Wiggle Bugs are all effective, especially for big brown trout. Fish streamers year round. They are especially effective when the rainbow fry start to emerge from the gravel in early spring.
A rule of thumb on the Green: "Smaller is better." Typical nymphs and drys range from sizes 16 to 20.
The exception to this rule is the terrestrials. Sometimes a big, gaudy imitation cast so it "splats" when it hits the water will be most productive.
For nymphs and small dry flies a three to five weight rod is best. For big terrestrials or streamers, or on windy days, a heavier, six or seven weight rod is best.
Generally a longer leader is better, especially late in the summer after the trout have been fished over heavily all summer. Fish with an eight foot to 12 foot leader when fishing dry and a six or seven foot leader when fishing wet.
So, when is the best time to fish the Green? Whenever you can and as often as you can. I don't think it would be possible to get tired of fishing the Green River.