Flyfishing Entomology


Re: Little BWOs


From: Michael
Received: Friday, May 06, 2005 9:09 AM

I've just started doing some research on a particular local stream and I'm a bit confused. For example; some of the research indicates that Little Blue Winged Olives (Baetis tricudatus) typically hatch in size 16-20 April 1st. It also shows Little Blue Winged Olives hatching in size 18-20 on June 1st, but the scientific name is different, Baetis levitans. If the pattern name and size is the same, what are the differences that the Species name indicate? Is it that the color might be slightly different, or....?

Thanks for the help. Love the site.

Hi Michael-

Am delighted to hear from you.  Thanks for your kind words concerning my website.  It is not surprising that you may be a bit confused attempting to sort out Little Blue-winged Olives, as that is a hopeless task.

Perhaps 25-30 years ago, there were many taxonomic consolidations within the Baetidae family.  As a result, among other changes, many formerly recognized species were consolidated into Baetis tricaudatus and B. flavistriga.  This shortly followed the publication of many classic flyfishing entomology books, so they didn't reflect these taxonomic changes.  One of those changes was that B. levitans was reclassified as B. flavistriga

Baetis tricaudatus is polyvoltine, which means the species has more than one generation per year.  In Mayflies, An Angler's Study of Trout Water Ephemeroptera by Malcolm Knopp and Robert Cormier, the East-Midwest Species Emergence for B. tricaudatus is graphed as: 1st quarter of March through the first quarter of April; 3rd quarter of May through the 4th quarter of June; 3rd quarter of July through the 3rd quarter of September. This species only requires 9-12 weeks to attain larval maturity in fertile waters.  Some polyvoltine Baetidae species (like Callibaetis sp.) are smaller with each succeeding generation per year, but I don't know whether (or not) that principle applies to other polyvoltine Baetidae species, like Baetis tricaudatus and Diphetor hageni.

Baetis flavistriga (B. levitans) is univoltine, which means the species has one generation per year.  Knopp and Cormier graphed East-Midwest Species Emergence for B. flavistriga as 3rd quarter of April through 4th quarter of August.

The common (fisherman's) name Blue-winged Olive is used to describe many genera and species of mayfly.  For the most part, the differences between these genera and species are extremely subtle, and often require dissection and a microscope to differentiate.  With regard to your question about whether color differences result in different species, the answer is no, not any longer.  However, that is the primary reason there are so many (now invalid) taxonomic synonyms.  Several years ago, Dr. W. Patrick McCafferty responded to me via email indicating mouthpart morphology is the primary differentiating factor between the Baetidae species.  However, I expect DNA will be playing a greater role before long.

Hope this helps with the BWO confusion.  However, if it just triggers more questions, feel free to ask.


Created: 08/13/2005   Last modified: 08/25/2006    www.FlyfishingEntomology.com