Mayflies: Why Swarms Hit Lakeside Towns and What to Do

In short, a mayfly swarm is a smelly but harmless sign of clean water that ends on its own within about a week, and the most useful thing you can do is switch off the lights that pull it onto your property.

Mayfly myths and facts at a glance

What mayflies look like

Mayflies are the order Ephemeroptera, from the Greek for short-lived and wings, with about 658 species in North America, according to NC State University.

Adults are soft-bodied, with clear triangular wings and two or three long tail filaments (cerci). University of Minnesota Extension gives a length of 1/4 to 1 1/4 inches, about 6 to 32 mm. At rest the wings are held pressed together above the body, like a sail, which is the quickest field mark. The giant burrowing mayfly, Hexagenia limbata, behind most of the famous swarms, reaches a body length of up to 3 cm (about 1.2 inches), with tails that add about another 3 cm, according to UF/IFAS.

There are two winged stages. The dull, cloudy-winged subimago (anglers call it a dun) moults once more into the shiny, clear-winged adult (the spinner). NC State notes that mayflies are the only insects that moult once they have wings.

Mayfly or something else?

Lengths and features from University of Minnesota Extension and UF/IFAS. All three look-alikes also gather at lights near water; our guide to small flying bugs in the house covers the ones that actually breed indoors.

Are mayflies harmful to humans?

No, not directly. Can mayflies hurt you? They have no means to: UF/IFAS says adults cannot feed with their vestigial mouthparts, NC State notes the adult gut is filled with air, and Minnesota Extension says they do not harm people or property.

There is one genuine health note. Cornell Cooperative Extension (Suffolk County) records that some hypersensitive people develop hay fever and asthma symptoms from inhaling airborne fragments of dead mayflies. If breathing symptoms follow a swarm, see a doctor.

If you are being bitten during a swarm, something else is responsible; biting midges also breed near water, and our page on no-see-ums covers the tiny biters.

What is the problem with mayflies?

The problem is volume. A single night’s emergence over western Lake Erie can produce 87.9 billion mayflies, about 3,078.6 tons of insects, according to a 2020 radar study in the Proceedings of the National Academy of Sciences (Stepanian and colleagues). A 697 km (433 mile) stretch of the Upper Mississippi can produce up to 3.25 billion in one event. Numbers that size cause predictable trouble:

• Slippery roads and bridges. Cornell notes mayflies accumulating around lights make roads, streets and sidewalks slippery and dangerous. UF/IFAS records car collisions and road closures and says snowplows often have to clear the piles.
• Bridge lights. After decorative lamps were installed on the Columbia-Wrightsville bridge over the Susquehanna in Pennsylvania in 2014, Hexagenia bilineata swarms were so heavy in 2015 that the bridge was closed after three accidents and bulldozers removed knee-deep piles, Science News reported.
• Smell. Piles of decaying bodies give off an offensive fish-like odour if they are not removed (Cornell).

Roads attract them for a reason. A 1998 study in the Journal of Experimental Biology by Kriska, Horváth and Andrikovics found that dry asphalt reflects strongly horizontally polarised light that mimics a water surface, so egg-laying mayflies are deceived into laying on the road, where the eggs perish. Darker, smoother asphalt was more attractive.

On weather radar, Mississippi State University Extension says, a heavy emergence looks like a light to moderate thunderstorm.

How long do mayflies live?

As winged insects, usually a day or two; in full, often a year or more. Mississippi State’s Blake Layton notes that most mayflies have an annual life cycle, so most adults are about a year old when they die. The National Weather Service gives 3 months to 2 years as a nymph, depending on species.

The winged part is brief. Edmunds and McCafferty’s review describes the usual pattern as a subimago stage of 8 hours to 2 days and an adult stage that rarely lasts beyond 3 days. In some lineages males spend only minutes as a subimago and a few hours at most as adults. UF/IFAS puts the adult life of Hexagenia limbata at no more than two days.

A female Hexagenia can lay up to 8,000 eggs, about 4,000 on average, onto the water surface, where they sink to the bottom (UF/IFAS). For how this compares with house flies and fruit flies, see how long flies live.

What month do mayflies come out?

Mostly June and July; Minnesota Extension lists them as present May into September. On the Upper Mississippi, the NWS says Hexagenia generally emerge in late June to early July, and Cornell gives late June and early July for most swarming on western Lake Erie. In warmer climates, UF/IFAS notes that Hexagenia limbata can emerge year-round.

Our analysis of iNaturalist research-grade records shows the same summer spike nationally (index of monthly share of all observations, yearly average 100). Mayflies as a whole reach 225 in June and 227 in July; the burrowing mayflies (Hexagenia) are more concentrated, at 286 in June and 336 in July, and almost vanish from November to March.

The NWS office in La Crosse, Wisconsin, logs radar-detected hatches on its Mayfly Tracking page. We compiled every entry from 2012 to 2024, excluding one caddisfly and one mixed-insect hatch. Of 46 logged mayfly events, 27 fell in July and 11 in June.

Source: compiled by The Pest Control Brief from the National Weather Service La Crosse Mayfly Tracking log, which covers the river from Davenport, Iowa, to St Paul, Minnesota. The log records what observers reported, so quiet years may simply be under-reported.

Every event the NWS rated massive or major fell between June 23 and July 20. Since 2019 the first logged hatch has come in early June or even late May, though a volunteer log cannot separate a real shift from a change in who is watching. Emergence flights begin around dusk, the PNAS authors note, which is when most night-flying insects are on the move.

Why some years are worse, and why mayflies matter

The long-term story is about water quality. How often do mayflies come out? Every year, but numbers swing, Mississippi State notes. The PNAS team describes how, by 1970, mass emergences had largely disappeared from many midwestern waterways, including western Lake Erie, because of farm runoff, low oxygen and pollution, before recolonisation in the early 1990s. Radar now shows a fresh decline: an 84 per cent drop in Lake Erie from 2015 to 2019 and 52 per cent on the Upper Mississippi from 2012 to 2019. The authors point to low oxygen, algal toxins and pesticides as likely pressures.

UF/IFAS says agencies routinely sample Hexagenia nymphs to monitor water conditions, and the NWS notes their presence means pollution such as sewage is not present in large amounts. What are mayflies for? Mostly, food: Cornell lists their chief importance as food for fish, dragonfly nymphs and birds, and the PNAS authors estimate a year’s Lake Erie emergence carries trillions of calories into shoreline food webs.

How to get rid of mayflies, or at least live with them

You cannot remove mayflies at the source, because they breed across whole lakes and rivers. Mississippi State’s advice is that physical controls, meaning light management, exclusion, leaf blowers, vacuums, brooms and dust pans, are usually more effective than insecticides.

1. Turn lights off on hatch nights. The NWS says adults are attracted to lights and can fly several miles inland. Switch off porch and dock lights on June and July evenings, or put them on motion sensors.
2. Use the least attractive bulbs. In the 156-night porch bulb trial we summarise in why moths are attracted to light, warm white LED and yellow bug bulbs caught the fewest insects and incandescent bulbs the most.
3. Close curtains on lit windows that face the water, so indoor light does not act as a beacon.
4. Clean up within a day. Sweep, blow or hose dead mayflies off steps, docks and driveways before they rot and smell. NC State notes some European communities sell the dead insects as bird food, fish bait or fertiliser, so composting them is fine.
5. Skip the spray and the zapper. The PNAS authors describe Hexagenia as among the most sensitive aquatic insects to commonly applied pesticides, and the swarm ends on its own anyway. On electric traps, see do bug zappers work.

When to call a professional: rarely for a home. Waterfront businesses and marinas with large lit areas may want a lighting and cleanup plan, and if you are unsure whether you have mayflies or biting insects, a licensed operator or your county extension office can identify them.

Why are mayflies called mayflies?

The English name is simply May plus fly, first recorded in the mid 1600s, according to Dictionary.com. The National Wildlife Federation notes that, despite the name, mayflies are active through the warmer months, not just May.

Mayflies is also Andrew O’Hagan’s 2020 novel, adapted by the BBC, which the Royal Television Society describes as a tale of male friendship. This page is about the insect.

Frequently asked questions

Do mayflies bite?

No. Adult mayflies have vestigial mouthparts and cannot feed, according to UF/IFAS, and Cornell Cooperative Extension states plainly that they do not bite or sting. They have no stinger. If you are getting bitten during a swarm, the likely culprits are biting midges or mosquitoes that emerge from the same water on the same calm evenings.

How long does a mayfly swarm last?

A single emergence is over within days. The PNAS radar study describes dead mayflies littering the shore within two days of an initial emergence, and Mississippi State Extension says mass emergences usually last only a week or so. Near large lakes and rivers there may be several separate hatches through June and July, as the NWS La Crosse log shows.

How often do mayflies come out?

Every year, usually once per generation. Most mayflies have an annual life cycle, so the same waters produce a hatch each summer, but some species take up to two years as nymphs. Mississippi State notes that populations are higher some years than others, so a quiet summer does not mean the mayflies have gone.

Are mayflies a sign of clean water?

Generally, yes. Mayfly nymphs are sensitive to pollution, and agencies sample Hexagenia nymphs to monitor water quality, according to UF/IFAS. The National Weather Service notes that their presence means pollution such as sewage is not present in large amounts. Big swarms vanished from Lake Erie when it was badly polluted and returned after clean-up in the early 1990s.

Can mayflies infest your house?

No. Mayflies breed only in water, and Cornell Cooperative Extension notes they do not feed on homes, furnishings or food. A few may come through an open door or gap in a screen and die indoors within a day or two. If small flying insects keep appearing inside week after week, they are breeding indoors and are a different insect.

Sources

• University of Florida IFAS Extension, Featured Creatures EENY-726: Giant Burrowing Mayfly, Hexagenia limbata (Orfinger and Lucky, 2019, reviewed 2022).
• University of Minnesota Extension, What’s that insect: Mayflies; Chironomid midges; Caddisflies; Crane flies.
• NC State University, General Entomology: Order Ephemeroptera.
• Cornell Cooperative Extension of Suffolk County, Mistaken Identities (mayflies and midges).
• Mississippi State University Extension, Bug’s Eye View: Mayfly, Vol. 4, No. 11 (Layton, 2018).
• National Weather Service La Crosse, Mayfly Tracking log, 2012 to 2024, and First Mayfly Hatch of 2016.
• Stepanian PM et al. Declines in an abundant aquatic insect, the burrowing mayfly, across major North American waterways. Proceedings of the National Academy of Sciences 117(6), 2020.
• Edmunds GF and McCafferty WP. The mayfly subimago. Annual Review of Entomology 33, 1988.
• Kriska G, Horváth G and Andrikovics S. Why do mayflies lay their eggs en masse on dry asphalt roads? Journal of Experimental Biology 201, 1998.
• National Wildlife Federation, Wildlife Guide: Mayflies.
• Science News, Confused mayflies wreak havoc on a Pennsylvania bridge (2018).
• Dictionary.com, mayfly; Royal Television Society, Mayflies: how the BBC adapted Andrew O’Hagan’s novel.
• Justice MJ and Justice TC, household bulb light-trap comparison, 2021 preprint (as summarised in our moth guide).
• The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.