Entomology Reference

Hexagenia / Burrowing Mayfly

The marquee Lake Erie open-water mayfly hatch -- doppler-radar-visible swarms, a 68 deg F at-depth emergence trigger, and a documented 84% western-basin decline between 2015 and 2019.

Identity

Common name	Hexagenia (Hex); also Burrowing Mayfly, Giant Mayfly, Canadian Soldier, Lake Fly
Scientific group	Hexagenia limbata + Hexagenia rigida (genus-level scope; species-level Lake Erie disaggregation `[source-required]`)
Family	Ephemeridae (common burrowing mayflies)
Order	Ephemeroptera (mayflies)
Profile type	Hatch entity
Source status (overall)	source-backed
Confidence	high
Pattern match ref	`hexagenia`
USGS / agency record	USGS Bridgeman et al. 2006 + Madenjian et al. 1998 (western Lake Erie Hexagenia recolonization record)

At-a-glance biology

Size: adult body length 20-30 mm (0.8-1.2 in); fly tying typically hooks size 6-10 for dun/spinner, size 4-8 for nymph patterns. Quantitative Lake Erie-specific size measurements [source-required] at Tier 1-3.
Presence window: mid-June through mid-July open-water emergence, maximum June 20-30 (Tier 2 Ohio Sea Grant). Adult lifespan 24-36 hours -- adults do not feed.
Thermal trigger: 20 deg C (68 deg F) at depth (Tier 2 Michigan Sea Grant). The trigger is benthic water temperature, not surface.
Lifecycle scope: egg -> burrowing aquatic nymph (1-2 years in U-shaped lakebed-sediment burrows) -> winged subimago (dun) -> imago (spinner). Mayflies are the only insects that molt again after they have wings.
Habitat: Lake Erie open-water benthic. Nymphs burrow in soft sediment; recruitment is hypoxia-sensitive during summer stratification (Tier 3 USGS Bridgeman et al. 2006).
Indicator role: "canary in the coal mine" for Lake Erie water oxygen levels and ecological condition (Tier 2 Ohio Sea Grant).

Source-disciplined claim summary

Hexagenia is a Lake Erie open-water phenomenon with a well-documented population trajectory. Per Tier 2 Ohio Sea Grant (2011), Hex disappeared from western Lake Erie for approximately 30-40 years (1960s-1990s) under industrial pollution and eutrophication; the population rebounded in the mid-1990s as water quality improved under the Great Lakes Water Quality Agreement. The rebound did not hold: Tier 3 Stepanian et al. (PNAS 2020) used US weather radar to track an 84% decline in western Lake Erie Hex emergence biomass between 2015 and 2019, alongside a 52% decline in the northern Mississippi River basin over a similar window. Any current-year angling forecast assuming 1990s-2000s emergence density is no longer defensible.

The emergence timing anchor is among the most precisely sourced in the SHA pilot: Tier 2 Ohio Sea Grant documents peak emergence mid-June through mid-July with the maximum during June 20-30, and Tier 2 Michigan Sea Grant documents the 20 deg C (68 deg F) at-depth temperature trigger. Adult lifespan is 24-36 hours per Tier 2 Ohio Sea Grant; the entire purpose of the adult stage is mating and oviposition. Tier 3 Bridgeman et al. (USGS publication record, 2006) documents larval recruitment as severely impaired during years of frequent June-September stratification because hypoxia kills nymphs in their lakebed burrows.

For predator-forage role, Tier 2 Ohio Sea Grant and Tier 3 Bridgeman et al. anchor Hex as an important food resource for the economically valuable Lake Erie yellow perch fishery and a documented walleye food source during the brief annual emergence window. Smallmouth bass and freshwater drum extensions during the spinner-fall window are translation heuristics, not quantitative diet-selectivity citations.

Honest gaps

Species-level distribution between H. limbata and H. rigida within Lake Erie basins [source-required].
Quantitative predator size-class selectivity (which fish targets which Hex life stage) [source-required].
Variation in peak emergence date by latitude across Steelhead Alley (Ohio + PA + NY shores) [source-required].
Tributary-mouth Hex applicability for steelhead/trout fishing -- structurally limited; Hex is essentially open-lake-only.
Current-year emergence intensity forecasts come from ongoing Stone Lab + OSU Sea Grant + agency monitoring, not from this profile.

Where this matters to anglers

The single most important translation note is structural: Hexagenia is NOT a Steelhead Alley tributary steelhead/trout hatch. The 68 deg F at-depth emergence trigger sits at or above the trout/steelhead heat-stress threshold. By the time Hex is emerging, target-species advice on Lake Erie has shifted to warmwater species -- walleye, yellow perch, smallmouth bass, freshwater drum. Hex patterns commerce-mapped under a steelhead/trout target risk promoting steelhead/trout fishing during heat-stress conditions, which is why the SHA pattern-mapping for Hex targets the warmwater set explicitly.

For the brief annual window when Hex matters: pre-hatch nymphs along the bottom in 6-20 ft of water; emerger and dun patterns at dusk through dark; spinner fall and topwater feeding the morning after emergence nights. Light yellow + cream + tan with brown markings is the color baseline; rusty for spent spinners. Treat efficacy as conditional on current-year emergence quality -- the Stepanian 84% decline is the load-bearing reason not to assume historic-density swarms.

Reference questions about Hexagenia

What is Hexagenia?

Hexagenia is the genus of large burrowing mayflies (family Ephemeridae, order Ephemeroptera) often called "Hex" or "the burrowing mayfly." In Lake Erie, USGS-listed western-basin recolonization literature identifies Hexagenia spp. as including H. limbata and H. rigida. Adults are large (20-30 mm body length), light yellow to cream with brown markings, large clear wings, and long forked tails. Nymphs are also large and burrow U-shaped tunnels in soft lakebed sediment.

When do Hexagenia hatch?

Peak Lake Erie emergence runs mid-June through mid-July, with the maximum during June 20-30 per the Ohio Sea Grant College Program (Tier 2). The trigger is water at depth reaching 20 deg C (68 deg F) per Michigan Sea Grant (Tier 2). Adult lifespan after emergence is 24-36 hours -- adults do not feed; the entire purpose of the adult stage is mating and oviposition. Michigan Sea Grant notes the broader mayfly season can extend June through August because different cohorts and the two main Lake Erie species emerge at slightly different times.

Are Hexagenia in Lake Erie?

Yes, and they are the marquee Lake Erie mayfly. Hexagenia disappeared from western Lake Erie for roughly 30-40 years (1960s-1990s) under industrial pollution and eutrophication, then rebounded in the mid-1990s as water quality improved under the Great Lakes Water Quality Agreement (Tier 2 Ohio Sea Grant). That rebound has since reversed: Stepanian et al. (PNAS 2020, Tier 3) used US weather radar to document an 84% decline in western Lake Erie emergence biomass between 2015 and 2019, alongside summer hypoxia concerns and harmful algae blooms (Tier 2 Ohio Sea Grant). Hex remains an important food source for the Lake Erie yellow perch fishery and a "canary in the coal mine" indicator of lake oxygen levels.

What patterns imitate Hexagenia?

For the brief annual emergence window, light yellow + cream + tan with brown markings + rusty for spent spinners is the color baseline. Hook size 6-10 for dun and spinner patterns; size 4-8 for weighted burrowing-nymph patterns fished pre-hatch on the bottom in 6-20 ft of water. Important translation note: Hex is a Lake Erie open-water hatch, NOT a tributary steelhead/trout hatch. The 68 deg F at-depth emergence trigger is at the trout/steelhead heat-stress threshold, which is why Hex pattern commerce-mapping at SHA targets walleye, yellow perch, smallmouth bass, and freshwater drum -- not steelhead or trout.

Source list

Source	Tier	Use
Ohio Sea Grant (OSU), Small Lake Erie Mayfly Population (2011)	2	Peak emergence mid-June to mid-July, max June 20-30; adult lifespan 24-36 hours; canary-in-coal-mine indicator role; food source for walleye + yellow perch; 30-year mid-century absence + mid-1990s rebound.
Michigan Sea Grant, When will the mayflies arrive? (2024)	2	Hex emergence trigger: water at depth 20 deg C (68 deg F); season can continue June-August through different cohorts/species.
Stepanian et al., PNAS (2020)	3	Western Lake Erie Hexagenia population declined 84% between 2015 and 2019 (US weather radar tracking + sediment nymph validation); 52% decline northern Mississippi River Basin 2012-2019.
Bridgeman et al., USGS publication record (2006)	3	Larval recruitment severely impaired during stratified June-September seasons via hypoxia; Hex an important food resource for yellow perch fishery; major indicator species of Lake Erie ecological condition.
Madenjian et al., USGS publication record (1998)	3	Western Lake Erie burrowing mayflies documented as Hexagenia spp. including H. limbata + H. rigida; recolonization began during the 1990s.
NC State University ENT 425, Order Ephemeroptera	2	Order-level mayfly lifecycle terminology -- incomplete metamorphosis, aquatic nymph/naiad, subimago-to-imago molt, angler terms dun/subimago and spinner/imago.

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