Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
Millcreek on Aug 30, 2016August 30th, 2016, 7:16 am EDT
Thought you might be interested in this article on the closure of the Yellowstone River by this parasite. You may have to scroll up to see the article.