Pest profile

Mayflies

Order Ephemeroptera (e.g., Hexagenia limbata, the giant mayfly) · Ephemeroptera: Ephemeridae[3]

Mayflies spend most of their lives as aquatic nymphs, then emerge together as winged adults that live only a day or two. Big hatches near lakes and rivers can coat walkways, roads and lit buildings, but mayflies are medically harmless.[1][2]

Adult giant mayfly with a yellow body, large upright see-through wings with fine veins and two long tails, standing on a white surface
Adult giant mayfly with a yellow body, large upright see-through wings with fine veins and two long tails, standing on a white surface. (c) DiegoH, some rights reserved (CC BY) — via iNaturalist, CC BY 4.0

How to identify it

  • Winged mayflies hold their large, triangular front wings upright and together over the body; the hind wings are much smaller or missing.[1]
  • The body is delicate, bare and often shiny, and the front legs are the longest and are held forward at rest.[1]
  • Nymphs, called naiads, have long legs, short antennae, plate-like gills along the abdomen and usually three long, thin tails.[1]
  • Colors vary by species, including yellow, green, white and black; the giant mayfly, Hexagenia limbata, is yellow or yellow-green and nearly an inch long without its tails.[1]

Where it lives

Thousands of species occur worldwide near streams, rivers, lakes and ponds; big emergences are typical of large lakes and reservoirs, where burrowing species like Hexagenia live in the mud.[1][2]

Biology and life cycle

Nymphs live in water, breathe through plate-like gills on the abdomen and feed on algae and bits of organic matter.[1][2]

Most species take about a year to develop, and a few take several years, so a mayfly that lives only a day as an adult may be a year old.[2][1]

Mature nymphs rise to the surface and molt into a dull-winged pre-adult, which flies to nearby plants and molts again into the clear-winged adult.[1]

Mayflies are the only insects that molt again after they have wings.[1]

Adults have non-working mouthparts and do not feed; they mate in swarms, lay eggs on or in the water and die within one or two nights.[1][2]

Damage and signs

  • Mass emergences can involve millions of insects, enough to show up on weather radar like a light to moderate storm.[2]
  • Piles of mayflies on walkways, roads and buildings near large lakes can create safety hazards and general mess, especially around electric lights at night.[2][1]

Health risk

Mayflies are medically harmless, and the adults have non-working mouthparts, so they cannot feed.[1]

Control

Prevention

Managing lights is the main defense, since most problems start when adults are drawn to electric lights at night.[2][1]

Keep them out

Physical exclusion, such as keeping mayflies out of buildings and screened areas, is more effective than insecticides during a mass emergence.[2]

Traps and physical control

Clear piles with leaf blowers, vacuums, or brooms and dust pans; these physical methods usually work better than spraying.[2]

Common questions

How long does a mayfly hatch last?
Mass emergences are short-lived and usually last about a week. They happen every year, but some years produce far more mayflies than others.[2]
Do mayflies bite?
No. Adult mayflies have mouthparts that don't work, so they can't feed or bite, and they are medically harmless.[1]
Are lots of mayflies a bad sign for the lake?
Usually the opposite. Mayfly nymphs are an important food for fish, and healthy mayfly populations are a sign of a healthy aquatic ecosystem.[2][1]

Sources

  1. Texas A&M AgriLife Extension, Department of Entomology, “Mayfly (Field Guide to Common Texas Insects).” texasinsects.tamu.edu/mayfly/ (accessed 2026-10-06)
  2. Mississippi State University Extension Service, “Mayfly (Bug's Eye View, Vol. 4, No. 11).” extension.msstate.edu/newsletters/bugs-eye-view/2018/mayfly-vol-4-no-11 (accessed 2026-10-06)
  3. iNaturalist, “Hexagenia limbata taxon page (order and family).” www.inaturalist.org/taxa/51116 (accessed 2026-10-06)

Updated 2026-10-06. Built from sourced records. Spot an error? Tell us.