In today's Invasive, Non-native Species post we look at Water Fern Azolla filiculoides. It forms a dense green and red covering on the surface of ponds.
Native to the warmer climates of America, it out-competes native species by blocking out light, causing deoxygenation, preventing air-breathing insects from reaching the surface and reducing water temperatures (Defra). Diving ducks like pochards and goldeneyes struggle to dip below the water in search of food. The dense, carpet-like appearance of water fern also poses a safety hazard for humans by appearing solid.
First recorded in the UK in 1886, water fern (also known as 'fairy fern' or 'azolla') was originally used for ornamental purposes in ponds and botanical gardens.
Typically, water fern can be removed by dredging, or by using herbicides. However, as such approaches can harm other wildlife in waterways, an eco-friendly biocontrol method has been utilised in recent years.
The Malvern Hills Trust have taken advice from the Environment Agency on how best to control this highly invasive plant. We have been advised to introduce the Azolla Weevil Stenopelmus rufinasus. The Azolla Weevil was first recorded in the UK in 1921 and it is classed by Defra as an ‘ordinary’ resident. By virtue of its name, the Azolla Weevil feeds only on the invasive Water Fern Azolla filiculoides hence it poses no risk to our native wildlife. Alternative methods of controlling Water Fern include using pesticides however the Azolla Weevil is a more environmentally friendly control method, being a biological control method.
Many other organisations such as the Canal and Rivers Trust are utilising this method with a good success rate, and we are also seeing promising results, see below (untreated pond and untreated pond- different locations).
