Norfolk Biodiversity

Fern enough, weeding with weevils!

Azolla (Credited to NNSS)

Azolla, actually quite a pretty plant (Credited to NNSS)

Azolla is one of the most invasive plants in the UK today. Capable of doubling its size in warm weather, water fern (aka, Azolla filiculoides) can quickly form dense mats of floating vegetation which are detrimental to the aquatic ecosystem below.

So how did it get here? Well unfortunately that is a story you will have heard before. Azolla was originally imported from the Americas as an ornamental pond plant back in the 1800s. Due to its invasive nature, it soon escaped the confines of ponds and now can be found causing considerable damage in the wild.

Once established Azolla blocks out light, preventing photosynthesis and reducing oxygen levels in the water. This has a lethal effect on aquatic flora and fauna. The mats grow up to 30 cm thick, impeding water flow and preventing the passage of recreational boats, decreasing the amenity value of the affected water body.

Azolla (Credited to NNSS)

‘Weeding’ through the Azolla (Credited to NNSS)

To make matters worse, as with floating pennywort, Azolla is very problematic to remove. The plant can regenerate from tiny fragments making mechanical removal almost impossible. Furthermore, many herbicides are not suitable for use in the aquatic environment, limiting chemical control options.

So what control options are left?

Luckily for us there is hope!

One of the main reasons invasive species are so successful outside of their native range is because they are freed from their natural predators. In some cases scientists have been able to bring highly invasive plants back under control by introducing a natural enemy/natural enemies into the introduced range. This is known as biological control.

zolla Weevil (Credited to  Rob Reeder - CABI)

Azolla Weevil (Credited to Rob Reeder – CABI)

In the case of Azolla, step up the North American weevil: Stenopelmus rufinasus.  This weevil is not native to the UK but is believed to have been accidentally inftroduced to the UK along with the plant itself, and is thought to have been present since 1921.  Back home, the weevil is very successful at controlling Azolla and these positive results have been replicated in the UK and South Africa.

CABI, one of our partners, have been promoting the use of this weevil as a biological control agent for Azolla utilising funding from the RINSE project.  The work has been carried out under our Field Trials and Demonstration Work Package. The results and evaluation of this work are available, here.

This confirmed that the presence of weevils more often than not, either completely eradicated or highly controlled populations of Azolla. Interestingly, weevil introduction wasn’t required too frequently at project sites, as there were already natural populations there. Despite the need for further demonstrations, this method of biological control has proved effective and worthy of being considered for best practice.

Getting involved

Be Plant Wise (Credited to NNSS)

Be Plant Wise Campaign (Credited to NNSS)

You can help prevent the spread of Azolla and Be Plant Wise. This campaign is about raising our understanding of the damage that invasive aquatic plants can have and providing advice on best practice when halting spread and ensuring our ponds and waterways are Azolla-free.

Don’t forget, you can report invasive species (e.g. Azolla) through the RINSE smartphone app, which is available for iPhone and Android handsets. For more details, please visit:

If you enjoyed reading about invasive plant species, you may like to see our post on Japanese knotweed. Alternatively, you can find out more about Azolla on the Non Native Species Secretariat (NNSS) site (here).




About Norfolk Biodiversity

The Environment Team at Norfolk County Council

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