Riverflies

Riverflies include three groups of insects; mayflies (Ephemeroptera), caddisflies (Trichoptera) and stoneflies (Plecoptera).

Riverflies are at the heart of freshwater ecosystems and are a vital link in the aquatic food chain as a food source for fish and birds. They live most of their lives as larvae in freshwater habitats before emerging as adult flies. They are good indicators of habitat health as they’re sensitive to changes in water quality.

More than 280 species of mayflies, caddisflies and stoneflies are present in Britain.

Riverfly declines

There is strong evidence that over at least the last few decades there has been a widespread decline in the numbers of riverflies in some British rivers.

These declines are worrying because they reflect the health of our freshwater habitats – which are vital for the plants, animals and ultimately, people who depend on them.

Many factors threaten riverfly survival:

• Habitat loss. Many ponds have been lost in recent years through in-filling or draining. Modification of river channels, e.g. straightening or widening, can cause the loss of important bankside and shallow in-stream habitat.
• Pollution. Water with high nutrient levels and other pollutants in run off from farmland. Excess nutrients can also lead to large algal growths, which smother vegetation and reduce water oxygen levels, making conditions unfavourable for riverflies. Toxic insecticides can be present in run off from agricultural land or from industrial sites, for example, damaging acidic runoff from conifer plantations and disused mines. Just a teaspoon full of cypermethrin pesticides can devastate riverfly populations for some miles of river.
• Soil erosion. Damage to soils on farmland can lead to unnaturally high levels of silt in watercourses which smothers the river bed, clogging the gills of larvae and preventing plant growth.
• Abstraction. Reduced flow rates can affect riverflies. Not only does that simply mean less habitat for riverflies, but also causes pollutant levels to become more concentrated.
• Light pollution. Artificial light can cause adult flies to become disorientated and attract them away from the water.

Rare and threatened species

Eight riverfly species have conservation status assigned by the Government and are listed on the UK Biodiversity Action Plan (BAP):

• Mayflies – Southern iron blue, Yellow mayfly
• Stoneflies – Northern February red; Rare medium stonefly
• Caddisflies – Small grey sedge, Window winged sedge, Scarce grey sedge, Scarce brown sedge

To find out more about the eight riverfly BAP species click here.

What can you do?

• If you are part of an angling group, get your group involved with the Riverfly Partnerships Riverfly Monitoring Initiative, which gives anglers and others the skills to monitor their rivers.
• Get involved in managing your local river by joining a local rivers group. Contact your local Wildlife Trust or the Association of Rivers Trusts to find existing groups – or if you don’t have one, you could set one up.
• Stand up for your local river through the ‘Love Your River’ campaign
• Make a pond as part of the Million ponds project. Ponds are important habitats for a number of mayfly and caddisfly species. For more information on making ponds, see the Freshwater Habitats Trust website
• Don’t dispose of unwanted chemicals carelessly – chemicals washed or poured into drains often end up in rivers and other water habitats. Follow the Environment Agency’s Pollution Prevention Guidelines