Species Spotlight: Baetis Parvus

Oct. 2019

By: David Decker

On our western rivers, freestone and tailwater, one of the most common and prevalent mayfly species is the Baetis mayfly group.  The individual species include 6 members that have much in common in terms of habitat and behavior, generally.  Baetis have several common names; Blue Wing Olive, Iron Blue Quill, Snowflies and Invisible Flies.  There are more I’m sure. 

Baetis are a 3 brooded species and nymphal development can be completed in 2 ½ months.  The spring Baetis in the species Vagans can be a #16 and that same species have a mid-summer brood in a size #18 and the fall emergence of Duns is a #22. 

These insects are the bookends to our season and in many cases preferred over larger bugs that might be evident during the same hatching period in a day.  Baetis are noted as a may fly that is on the water for some length of time making them more easily captured by feeding trout during the hatch.  As well Baetis mayflies are more prolific than other bigger bugs like Squawla stoneflies in the spring or October Caddis in the fall.  Also, worth note is the seemed preference by Baetis mayflies to relish hatching in nasty weather.  The spinners seem to prefer more clement conditions to swarm in mating flights, and then oviposit eggs.  The females wrap their wings over their bodies and slip through the waters surface film in quite edge water placing the eggs in water sheltered from strong currents to develop into nymphs. I have had female spinners use my waders to pull themselves beneath the surface.

Baetis mayflies might be considered one of the West’s “super hatches” if for no other reason than they are available for so much of the season.  The nymphal activity is meaningful and make the #18 PT nymph a terrific choice anytime on the water where these little insects live.  As for the dry fly opportunity the fly choices are many and imitations vary considerably.  At times during the heights of the hatch the floating nymph, emerger or cripple patterns are very effective.  If trout are rising to a fly you cannot see chances are good that the fish are on Baetis mayflies primarily feeding on not yet fully developed Duns. 

Using a small sein net collecting and observing these adult mayflies will reveal the correct body and wing shades for the fly tier.   Sometimes big trout prefer a small bug over a larger one. 

Best Fishes.