LOL. Good one, John.
For anyone who might be interested in more information about one of my favorite hatches (OMD/cornuta), here are some observations from more than 30 seasons of fishing this hatch in PA and NY:
1. On mainstem freestone streams in northeastern PA, I generally see them from about the last week in May through the first week in June. They typically emerge in the morning, and emergence usually coincides with a water temperature of about 58 degrees. Under "normal" conditions (in quotation marks because these conditions seem increasingly rare of late), the hatch lasts a little over an hour, often peaking around or before 10am. My favorite floral "indicator" of this hatch is the blooming of lavender and white wild phlox along stream and river courses. I call it "cornuta weed."
2. During heat waves (when the water temperature does not cool below 58 degrees overnight), they will emerge around dawn. Under unusually cool conditions, they can trickle out through much of the day. On one cool overcast day, the hatch peaked around 2:30 in the afternoon.
3. On the Upper Delaware (esp. W. Branch), a late spring release of very cold water oftens turns hatching schedules topsy turvy. After the release period, cornuta can show up later in the season and later in the day (in the evening).
4. In relatively cool watersheds with a good drop rate (fairly extensive fast water throughout), cornuta can often be found in good numbers from big mainstem streams and rivers up through tiny tributaries and headwater streams. Anecdotally, it does seem to be sensitive to organic pollution. On one favorite stream, the population has virtually disappeared in recent seasons after an upstream waterpark dumped a slug of untreated sewage into a tributary. Some other mayflies (like the sturdy E. invaria) seem to have survived this event more successfully.
5. Emergence is later on cold tribs. On one especially cold headwater, I usually see them emerge around the second week of July. This presents the opportunity to follow the hatch as it moves upstream. In some memorable seasons, I have fished this hatch for nearly two months.
6. Rises to the emerging duns in fast water are dramatic, pyramid-shaped slashes, similar to rises to some emerging caddisflies. Because one often sees mating flights of male Hydropsyche in the morning at this time, it can be easy to mistakenly attribute the rising to the caddisfly activity. When I first encountered this hatch many moons ago, I wasted precious time fishing caddis imitations to the cornuta emergence before I realized my mistake. On the streams I fish, there is usually little or no competition for the trout's attention during the cornuta emergence, making fly selection easy if you are prepared for the hatch.
7. The overall emergence period coincides with the first appearance of the Paragnetina media and Haploperla brevis stoneflies and the Isonychia mayflies. (You'll see the evacuated shucks of all of these in evidence on streamside rocks during cornuta time.) In the evenings, a host of mayflies are emerging, and it is this abundance of evening hatches that often means that fewer anglers focus on the morning cornuta activity: The activity of Ephemerella invaria (Large Early Sulphurs/Light Hendricksons) is lingering and Ephemerella d. dorothea (Pale Evening Duns/Little Sulphurs, when present) is just beginning. On streams that have them, Ephemera guttulata (Green Drakes/Coffin Flies) and Ephemera simulans (Brown Drakes) command the attention of big fish hunters. M. vicarium (March Browns/Grey Foxes) and Epeorus vitreus (Yellow Quills/Pink Ladies) provide somewhat sporadic, but often worthwhile opportunities. One of the largest wild browns I've caught during this period, a hefty 22" male, came to an emerger (a wet fly to match the underwater emergence) during the unpredictable vitreus hatch. That fish edged out a number of 16-21" browns that came to the cornuta emergence.
8. Although the fast-water dry-fly fishing to cornuta can be as exciting as any I've experienced, it is a mistake to neglect the nymph. The nymphs fish well throughout the period before and (perhaps more surprisingly) after the actual emergence on the surface. I'd estimate that three-quarters of all the fish that I've caught during this hatch (and most of the larger fish) came to the nymphs.
9. Other Eastern Drunella can provide similar, though more limited opportunities. D. cornutella, D. lata, and even D. tuberculata and D. walkeri can be found in many of the same streams. Their later emergence often means that marginal water temps can complicate things, and none of these species seem nearly as abundant in the streams that I fish. Of them, D. cornutella is the most abundant species I encounter, but it still pales when compared to cornuta. For our purposes, cornutella is nearly identical to cornuta, but smaller (6-8mm, about the size of E. dorothea or D. lata), and it emerges later in the season (often in July).
10. I'm not really betraying any cherished secrets here. Few can tear themselves away from the more popular evening hatches at this time of year, and I write all of this knowing that the emergence is over on most (larger) streams and this information will probably be forgotten by this time next year. :)