Blue Winged Olive is a generic name for mayflies from the Baetidae family and includes multiple genera and species. These prolific Mayflies emerge on the Yakima from late winter through late fall. The best hatches occur in March and April and again mid-September though early November, ranging in size from #24 to #14. A smaller emergence occurs during the summer months in sizes 22 and 20. BWOs nymphs are swimmers and live in a variety of moving waters, but prefer slow to moderate runs and riffles.
Size: 14-22 / Colors: Nymphs: mottled brown. Duns: (underside) light to dark olive.
Western March Brown nymphs are clingers found in moderate to fast flowing runs and riffles. Emergence generally begins in late March and continues through early May. These are large Mayflies which can produce some of the Yakima's best dry fly fishing for large fish.
Size: 12-14 / Colors: Nymphs: mottled brown. Mature nymphs have a reddish tinge in the gills. Duns: (underside) tan
Ephemerella excrucians, Ephemerella dorothea infrequens
Pale Morning Dun activity begins in May and continues through July. Infrequens emerges first, followed by and overlapping excrucians. This can be an important late spring, early summer hatch, but varies from year to year. PMD nymphs are classified as crawlers and inhabit slow to medium sections of the river.
Size: 18-16 / Color: Nymphs: muddy olive to light rust. Duns: (underside) pale yellow to olive yellow to rusty yellow. Spinner: rust.
Mahogany Dun nymphs are crawlers found in moderate to fast runs and riffles that migrate to slower water as they mature. Mahogany Duns Emerge in the spring in April, May and June, and again in the fall in September, October and November, generally in slower water.
Size: 16 / Color: Nymphs: dark dun to black. Duns: (underside) rusty brown to rusty cream. Spinner: dark rusty brown.
Light Cahill nymphs are crawlers that are found in moderate to fast riffles and runs. Emergence occurs in April and May and again in September and October.
Size: 16 / Color: Nymphs: light to medium yellowish cream. Duns: (underside) yellowish cream
Drunella grandis, Drunella doddsii
Green Drake nymphs are crawlers that are found in moderate to fast riffles and runs. Emergence occurs in late April through early July. The strongest hatches are in June in the upper river.
Size: 10 / Color: Nymphs: medium to dark brown. Duns: (underside) light green to light brown.