Stonefly Life Cycle
By: Mark Bachmann
Stone flies are a diverse family of insects that are adapted to well oxygenated streams and rivers and cannot survive in polluted water. Stone fly nymphs have fixed gills that can only extract oxygen in clean moving water. If trapped in still stagnant water, they die quickly. Most stone flies crawl out of the water to hatch, usually on rocks or shrubbery. The normal procedure is for stone fly nymphs to migrate to shore to hatch. This activity is attractive to trout and salmonids. Some smaller species will hatch in the middle of the river at the water’s surface much like mayflies. Nymphs like giant stone flies live in the rivers for as long as three years before they hatch into winged adults. They are attractive to trout in several sizes, almost year round.
As the stone fly nymphs grow they must take over a larger territory. The redistribution of these territories usually happens in masses with many aquatic insects changing territory at the same time. This is called a behavioral drift and commonly happens immediately after sundown. The largest behavioral drifts of most stone fly nymphs occur September through May. These mass drifts (or shifts) of insects occur during the twilight hours of the morning and late evening. During these periods stonefly nymphs are moving down stream. Many are caught in the currents and swept downstream out of control. Trout target on these vulnerable treats and will feast until gorged. You need to carry patterns in many different weights to cover different water speeds and depths. The flies we have available will cover all of the subsurface “giant stone fly” nymph activity. There are also patterns such as the Little Golden Stone and the Bead Head Rubber Leg Hare’s Ear that will cover many smaller species as well.
Most anglers are aware of the local hatches of salmonflies and golden stoneflies.Several weeks before the hatch, salmon flies and golden stones will start to migrate to staging areas along the banks. No other occurrence in our rivers and streams will produce a feeding frenzy like the migration of big stone fly nymphs. Even the largest trout find it impossible to refuse a big stone fly nymph fished deep along the bottom. The peak of this activity is the last two weeks before the hatch and we have been fishing these hatches for over 50 years.
Less anglers are aware of the other hatches of stone flies that start as early as February and some hatches are going on in August. These various hatches range in size from less than one-half inch to one and one-half inches in length. Colors range from black to bright yellow, and brown to olive. Some hatches are sparse, but others are measured in blizzards. Most of the varies species create behavioral drifts while nymphs and egg laying flights while adults.
Unlike the better known mayfly and caddis groups of aquatic insects, only the larger stoneflies create mass migrations, or behavioral drifts of nymphs, which meed specific fly patterns. Usually the right size of Gold Ribbed Hares Ear will work. On the average the egg-laying activity is the most attractive part of the stonefly life cycle to trout. Fly selection is usually pretty simple. Stimulator type flies will get you into the game. Use the Chart Below:
Emergence
Common Name
Body Color
Size
When/How
February-April
Little Brown Stone
Brown to Black
#14 – #18
Dry, Egg Laying, Afternoon
April-July
Skwala
Golden Brown-Dark Brown
#10 – #12
Dry, Target Risers, Afternoon
April-July
Little Olive Stone
Olive-Chartreuse
#14 – #18
Dry, Egg Laying, Afternoon
May-July
Golden Stone, Willow Fly
Golden Yellow- Golden Brown
#4 – #10
All Day, Nymph, Dry
May-July
Salmonfly
Orange, Brown, Black
#2 – #12
All Day, Nymph, Dry
June-August
Yellow Sally
Yellow, Yellowish Tan
#12 – #16
Dry, Wet, Egg Laying, Afternoon
Worth while study in the off-season is Bugs of the Underworld: The Natural History of Aquatic Insects, also: Modern Nymphing, European Inspired TechniquesMany flies that are normally used to match mayfly nymphs also mimic smaller stonefly nymphs. For appropriate nymphs: Stonefly Nymphs, Mayfly and Stone Nymphs For appropriate dry flies:text
Fish long and Prosper!
