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Azolla is one of the UK’s most invasive water plants. CABI’s work on Azolla control - researching and testing a biocontrol agent to control the weed naturally in the UK - has led to a programme that rears and supplies weevils to customers affected by Azolla.

Here you can find out about Azolla, current control methods and how to purchase weevils.

About Azolla

What is Azolla?

The fairy fern or floating water fern (Azolla filiculoides) is an aquatic plant with fern-like foliage that originates in the Americas. It was first recorded in the UK in the 19th century and became naturalized. It is now a popular garden aquatic.

Why is it a problem?

Azolla filiculoides has a remarkable ability to multiply. Fronds grow rapidly and elongate until fragments break off to form new plants. Mats that form on the water's surface can be 30cm thick and can double in size in hot weather.

The mats block out light, kill rich and diverse aquatic flora and reduce available oxygen which can lead to the death of fish and invertebrates, impede flood defences and water-based recreation and block irrigation pumps.

Azolla has escaped from gardens into the wider environment, becoming a problem on ponds, lakes, rivers and canals throughout the UK. Azolla was one of five aquatic weeds banned from sale in the UK in 2014.

Azolla control methods

Mechanical and chemical control

Fragmentation of the fronds makes mechanical control virtually impossible. This is worsened by the annual production of millions of tiny spores which are released in autumn and grow into new plants the following year.

There is one herbicide that is licensed for use on water in the UK but environmentally-friendly alternatives are preferred.

Biological control

Biological control has been used effectively against invasive species for over 100 years. It is a natural way of controlling invasives using living organisms, such as insects or pathogens.

For Azolla, research has shown the tiny 2mm long North American weevil (Stenopelmus rufinasus) to be one of the weed's main natural enemies. In South Africa, it has proven to be an outstanding example of biocontrol, providing ongoing weed control.

Why are weevils the best solution for controlling Azolla in the UK?

The weevil can only feed on Azolla, it will not harm other plants. When 'en masse,' the weevils consume large quantities of the weed and they can be targeted at very specific sites where the weed is considered a problem.

Following extensive research, CABI studies have shown that Azolla infestations can be brought under complete control using only the weevil in a matter of weeks and without the need for chemicals or further control measures.

The weevil is mass-reared in the UK and shipped throughout the summer months to customers affected by Azolla.

How to purchase weevils

To order packs of weevils or to discuss control options for Azolla infestations, please contact us.

Weevils are available in standard, medium and large packs.

Standard weevil pack (sufficient to control approximately 10 square metres) - £100 each (plus VAT)

Medium weevil pack (sufficient to control approximately 50 square metres) - £270 each (plus VAT)

Large weevil pack (sufficient to control approximately 100 square metres) - £395 each (plus VAT)

For postage costs and queries about controlling larger areas, please contact us.

Key contact

Corin leads the Azolla control project. He mass-rears the weevil, Stenopelmus rufinasus, and conducts ongoing research to improve our understanding of the weevil and its effectiveness as the natural control agent against Azolla in the UK.

CABI In United Kingdom

Corin Pratt

Invasive Species Management Researcher

T: +44 (0)1491 829083 E: c.pratt@cabi.org

Frequently asked questions

Related Projects

Find out more about CABI's work on invasive weeds and biocontrol in the UK

Azolla control

One of the UK’s most invasive plants, the fairy fern or floating water fern causes problems for anglers and water managers. It forms thick mats on the water’s surface which can double in size in a few days, blocking out light and killing aquatic flora and fish. Fragmentation of the fronds makes control by mechanical means virtually impossible. This project involves researching and testing the use of a 2mm-long North American weevil¬†(Stenopelmus rufinasus) as an agent to control the weed naturally.

Biological control of Himalayan balsam

Himalayan balsam has rapidly become one of the UK’s most invasive weed species. A lack of natural enemies allows it to successfully compete with native plants for space, light, nutrients and pollinators, reducing biodiversity and contributing to erosion. Traditional control methods are inadequate. This project involves identifying an insect or plant pathogen that exclusively attacks Himalayan balsam, which can be released into the UK to control the plant while leaving indigenous species intact.

Controlling floating pennywort in a safe and sustainable way

Floating pennywort is an invasive aquatic plant that can over-run water bodies in the UK, and is threatening habitats, native plants, fish and insects. Also a problem across much of Europe, this plant has rapid growth and can regenerate from small fragments. Management is mainly limited to mechanical clearance which is expensive and often ineffective. Through comprehensive host range testing, this project aims to identify the safest and most effective biocontrol agent to keep the plant in check.