EPT Assessment

  • Common Name: Mayflies
  • Found in fast-moving, shallow, well oxygenated, unpolluted streams
  • Different species will eat periphyton (plants attached to underwater objects such as rocks), filter particles out of the water, or prey on other species
  • Go through several different stages of development: egg- larvae- subimago- adult
  • Adults do not eat and live only a few days as they try to reproduce.
  • Mayflies have been around since before the dinosaurs walked the earth!
  • Mayfly may spend a year or more in the larvae or subimago stage before becoming an adult.
  • May flies are the only species of insect that molt once they have developed wings

iStock-1049362746 mayflies on water
iStock-1226539824 (1) stonefly


  • Common Name: Stoneflies
  • Important source of food for fish and other vertebrates
  • Different species will eat shredded leaves or prey on other invertebrates
  • Found in pollution-free, cool, highly-oxygenated, running water
  • Plecoptera are mostly found in cooler, temperate parts of the world
  • Females of some species can lay up to 1000 eggs!
  • Stoneflies are generally not very strong flyers
  • A Stonefly may be in the nymph form for up to 4 years depending on the species.
  • Stages of development: egg – nymph- adult
  • Stonefly eggs are covered in a sticky substance that adheres them to the rocks to keep them from washing away


  • Common Name: Caddisflies
  • Known for building house like structures for themselves out of silk, sand grains or parts of leaves
  • Different species will eat periphyton, dead leaves, filter particles out of the water, or prey on other species.
  • Emergence of caddisflies may be a signal to fisherman that the fish are biting.
  • Each species of Caddisfly builds a unique house that makes them identifiable.
  • Adult Trichoptera do not feed on solid food, only liquids if at all
  • Most Caddisflies are good swimmers, using their legs as paddles rather than wiggling their bodies.
EPT assessment
This is what is looked for in bodies of water when assessing water quality. Ephemeroptera, Plecoptera, and Trichoptera (EPT) are used to study stream health and water quality. The larval stages of these insects are found in freshwater systems. These species are not able to tolerate pollution in their habitats. The numbers of Ephemeroptera, Plecoptera, and Trichoptera found in a sample are added together and then divided by the number of Diptera (order of insects that includes midges, mosquitoes, gnats and flies) found to get a number that can be used to compare to a standard index. A clear, easy to understand explanation of exactly how a EPT assessment is done can be found in NRCS’s technical notes here: EPT Assessment