White Fly (Ephoron leukon)

Mayfly, hatching August–September, active in water between 62 and 74°F, and imitated at #12–14. Recorded from 3 of the 8 regions this map covers, across Michigan, Wisconsin and Virginia.

A ghost-white mayfly, size 12 to 14, that hatches in enormous numbers on late-August evenings on warmer, bigger, softer-bottomed rivers, often at the exact moment it becomes too dark to tie one on. The nymph is a burrower in silt. Emergence comes at dusk, and the male moults again immediately, making this the only mayfly likely to be a dun, a spinner and a nymph within the same fifteen minutes. The dun is pure white in fading light; the spinner falls the same evening, white and spent, with the emergence still going on around it. The water goes white for twenty minutes. The active band is 62 to 74 degrees, covering cold-transitional, cool and warm water, and the peak is August and September. A White Wulff in 10 to 14 is the direct match; a Parachute Adams 12 to 20 covers the surface otherwise, and a Hare's Ear 10 to 18 the burrowing nymph.

#12–14Hook size
62–74°FWater, active
Aug–SepPeak months
4Life stages fished

Subbasins with trout water where it is on record: 47 of 326 across the 6 states this map draws.

See the trout water on the live map → Every reach coloured by water temperature — the water these hatches come off.

What it is

Mayfly · Ephoron leukon · nymph → emerger → dun → spinner

Ephoron leukon: a ghost-white mayfly that hatches in enormous numbers on late-August evenings on warmer rivers, often at the exact moment it becomes too dark to tie one on.

Late-summer dusk on the bigger, softer-bottomed rivers. Molts in mid-air; the water goes white for twenty minutes.

When it hatches

Peak is August–September. The pale months either side are the shoulder — the app widens a peak by one month in each direction and recommends the bug there, and no further: that widening is bounded so that a bug peaking in May is never offered in August.

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peak shoulder not recommended

Water conditions

The band is a water temperature, not an air temperature, and it is the range in which this insect is worth fishing — for a mayfly that is the emergence window; for a scud or a midge it is simply the range trout will still eat one in.

62°F 74°F

Active from 62°F to 74°F. On this map that is cold-transitional, cool, warm water — the band crosses 2 of the three breaks the river colours use. Breaks at 58, 64 and 70°F.

Stages

What the insect is doing, and what that means for what you tie on.

Stage	What it is
nymph	A burrower in silt on larger, warmer rivers.
emerger	Emerges at dusk, and the male moults again immediately — the only mayfly likely to be a dun, a spinner and a nymph in the same fifteen minutes.
dun	Pure white, size 12 to 14, in fading light.
spinner	Falls the same evening, white and spent, with the emergence still going on around it.

Matching flies

3 patterns in this book imitate the White Fly, across its stages. Which one to tie on follows the stage the water temperature says it is in — the app makes that call on a live reading; this page lists the whole set.

Where to find it right now

The rivers with a page here that sit in this insect's regions, with a USGS gauge reporting water temperature. This list is re-ranked against the reading each gauge is sending now and today's date, so it is live — the fallback below it, if the readings do not arrive, is each river's measured July mean.

Regional presence

Regions, not collection records. This insect is recorded from the coarse regions below; nobody has surveyed which named river it has been caught on, and this page will not pretend otherwise.

southern lower Michigan MI
the Wisconsin driftless WI
the Virginia mountains VA

Rivers in these regions with a page here

Related hatches

Same family, or overlapping this one's calendar on the same water.

Open the trout map →Every trout stream near you, coloured by today's water temperature.