Yellowstone River supports a variety of aquatic and terrestrial insect species that in turn support a world-famous trout fishery. Insect hatches are consistent and prolific—as are the fish they feed. The Yellowstone River is best known for it annual hatches of salmonflies that emerge in mid to late June. The largest hatches on the river include salonfly, stonefly, mayfly and caddis.
The chart below provides a list of the most common fly and insect hatches that occur each year on the Yellowstone River. Many of these hatches are also representative of hatches occurring in other tributaries, rivers and streams throughout the Yellowstone River drainage.
| Fly Hatch | Size | Date |
|---|---|---|
| Midges | 16-22 | Late February - April; Mid September - October |
| Blue Winged Olives (BWO) | 14-16 | End of March - mid May; End of Sept - October |
| Western March Brown | 12-14 | Mid April - end of May |
| Western Greek Drake | 10-12 | June - mid July |
| Mother's Day caddis | 12-14 | End of April - end of May |
| Pale Morning Dun (PMD) | 14-20 | Mid June - end of July |
| Salmonfly | 4-8 | Mid June - beginning of July |
| Yellow Sally stonefly | 10-12 | End of June - July |
| Golden stonefly | 4-10 | End of June - mid July |
| Caddis (general) | 12-20 | Mid June - August |
| American Grannom Caddis | 12-16 | July - mid August |
| Little Brown Caddis | 14-18 | Beginning of July - beginning of August |
| Plain Brown Caddis | 14-18 | Mid July - mid August |
| October caddis | 6-10 | October |
| Trico mayflies | 18-22 | End of July - beginning of September |
| Western Golden Stonefly | 2-6 | August - mid September |
| Ants | 16-20 | Beginning of July - September |
| Beetles | 12-18 | Beginning of July - September |
| Grasshoppers | 8-14 | Mid July - beginning of October |
| Streamers | 2-8 | March - April; July - October |
Annual and seasonal variation in weather conditions may impact insect emergence and timeframe from the dates provided. While our Yellowstone River hatch chart covers the majority of hatches, we recommend having a variety of sizes and patterns for each of the insects listed above. And don’t forget to top of your tackle with a good selection of attractors.
Yellowstone River Hatches and Fly Patterns
The following are the most popular fly hatches and patterns for the Yellowstone River.
Midges
Midges are the first early fly hatches to appear on the Yellowstone River. Dry fly fishing with midges takes off in late February and really picks up in March extending through early April. Getting trout to bite during midge hatches is hit and miss. Your best bet is to fish dry flys when the weather is nice and there’s minimal wind. Target slow currents and eddies.
Blue Winged Olives (BWOs)
Blue Winged Olive mayflies hatch every spring and fall between March and May and then again between September and October. The majority of BWO hatches occur between Gardiner and Billings, Montana with the epicenter of hatch activity occurring upstream from Columbus to Yellowstone Park. BWO hatches occur any day during hatch season by are strongest when the sky is overcast. Spring BWO hatches produce smaller specimens (size 14-16) than fall hatches (size 16-22).
March Browns
March Brown hatches are inconsistent on the Yellowstone. Conditions must be perfect for March Browns to emerge. Water temperatures between 45 and 50 degree Fahrenheit with overcast skys set the perfect stage for March Brown emergence. When hatches do occur, it’s usually between mid-April and May along the entire river. Hungry trout love March Brown mayflies. Fly patterns in the size 10-14 range that match the hatch are ideal for targeting trout.
Caddis
Several caddis hatches occur on the Yellowstone River between mid-June and August. The Mother’s Day caddis hatch is the first to emerge toward the end of April. Fishing the Mother’s Day caddis hatch is a bit of moving target from year to year. Other noteworthy caddis hatches include Ameican Grannom, Little Brown, Plain Brown and October which run from July through August, and in October. Water flow and temperature impact caddis hatches. Fishing with caddis patterns can be hit and miss.
Salmonflies
Salmonfly hatches typically begin in late June but may not get underway until the beginning of July depending on the year and runoff. From Gardiner downstream to Livingston, salmonfly hatches occur over just a few days when water temperatures rise to 55 degrees Fahrenheit. As salmonfly nymphs emerge they make their way to the river bank where the remove their outershell (shuck) and re-emerge as a fully formed adult mayfly. Both Salmonfly nymph and adult dry fly patterns (sizes 4-8) will catch trout on the Yellowstone River.
Golden stoneflies
Planning your Yellowstone fishing adventure around golden stonefly hatches is a great idea if you enjoy dry fly fishing. Golden stoneflies emerge toward the end of June just after salmonflies. Golden stonefly hatches run through mid July. Golden stoneflies are relatively large (size 8 to 12), but not as large as salmonflies. Yellowstone river trout will readily take Golden stonefly dry fly patterns.
Yellow Sally stoneflies
Yellow sally hatches occur right on top of Golden stonefly and salmonfly hatches from the last week in June through end of July—with adults hatching into early August. Nymph yellow sallies can range in size from 8 to 16, the most common size being 12. Yellow sally nymphs make up a large part of Yellowstone trouts’ diet, but adult Yellow sallies are rarely taken by trout. Fish yellow sally stonefly nymph patterns under the surface.
Pale Morning Duns (PMDs)
PMDs are one of the most abundant mayflies on Yellowstone River. They emerge in mid-June through the end of July. The are primarily found in the riffles and longer runs along the river between Gardiner and Columbus. Target trout using size 14-15 PMD immitation nymph and fly patterns. Fishing the riffles and runs should provide more catches.
Trico mayflies
Trico hatches begin at sunrise and are over in the blink of an eye—usually within just a few hours. Hatches occur along the banks where there is structure, and in riffles. Trico mayflies are small, emerge in late summer, and don’t garner the attention of most anglers. However, savvy anglers will use trico dry fly patterns to target trout during the late summer. Every fall Trico mayflies begin to die off while flying above Yellowstone river. As they fall, they cake the water’s surface creating a smorgasbord for hungry trout. Casting a well placed trico pattern on the surface during this mass exodus is the perfect time to entice a strike.
October Caddis
October Caddis is a late season hatch that takes place—you guessed it—in October. However, October caddis hatches usually well underway by the end of September. Most anglers don’t pay much attention to October caddis hatches. In comparison to salmonfly and Golden stonefly hatches, the October caddis hatch is a bit paltry. October caddis fly patterns in the 8 to 10 size range can be fished during October caddis hatches.
Grasshopper, Ants, and Beetles (Terrestrials)
By the end of July, most major hatches have died off and trouts diet is made up primarily of a few aquatic insects and a lot terrestrial insects—including grasshoppers, ants and beetles. Strong winds and regular breezes ensure a large number of these insects find their way into the river for trout to prey on. Use terrestrial patterns that match the hatch to entice trout to strike. Trout aren’t as particular about color as they are size. For color, use natural tones. Grasshopper patterns range from size 8 to 12. For ants and beetles use fly sizes 14 to 18.
