Little Black Stonefly is the common name used to describe a prolific early Spring stonefly hatch that occurs on many trout streams around the upper Midwest. The Little Black Stoneflies are small, the nymphs are approximately a size #16 and the adults are generally a size #18. In northern Wisconsin, the hatch generally will occur sometime in February through April depending on how cold or mild the Winter is that year.
The Little Black Stoneflies (also called Early Black Stonefly and Winter Snowflies) are part of the Capniidae family (probably Allocapnia granulata) and are the first stoneflies to appear each year. These tiny Winter stoneflies have anti-freeze compounds in their body fluids to help them stay active during the cold snowy months. They hatch by crawling out of the water to the snowy bank edges to hatch and become adults. Then crawl up onto the snow to look for safe places to hide from the cold elements until ready to mate, lay their eggs and die.
Any small dark nymph pattern in a size #16 fished in the shallows should work for trout feeding on the migrating little black stonefly nymphs working their way to the snow banks. For the adult stoneflies, a size #18, Griffiths Gnat with the hackle clipped the top, add a deer hair wing laid over the top of the body can be an extremely effective pattern to imitate the ovipositing female stoneflies skittering across the surface. Also, a starling wing, black body soft hackle pattern can be effective at times.
Little Black Stonefly Soft Hackle Pattern
The Little Black Stonefly is often confused with the larger Early Brown Stonefly that hatches later in the Spring.