Biocontrol of Water Fern

Nick Carter, Conservation Officer (Fens)

Water fern in storage lagoon

Water fern, Azolla filiculoides, a native of Central and North America was introduced into the UK in about 1840 as an ornamental aquatic plant but has since spread into the wider environment. It can quickly form dense, thick mats of green or red vegetation that block out light, cause de-oxygenation of the water, kill aquatic flora and fauna and interfere with water management. It spreads vegetatively - making mechanical control impossible - and by spores in the autumn.  

It was first noticed in autumn 2012 in the storage lagoon at Hilgay but following the cold winter of 2012/3 it was not noted in 2013. It reappeared in autumn 2014 again in the storage lagoon either from survivors of the original infestation or re-introduction by wildfowl. It grew rapidly to form dense mats which survived the winter and spread to other parts of the site in 2015. The warm weather during June and July resulted in the formation of dense mats again.

Stenopelmus rufinasus adult,
photo by Corin Pratt CABI

Natural help is at hand however in the form of a tiny, native North American weevil, Stenopelmus rufinasus. It was first recorded in the UK in 1921, presumably being present on imported water fern plants. It is considered by Defra (Department for Environment, Food and Rural Affairs) to be ordinarily resident so there are no licensing requirements to release populations where they are not present naturally.

CABI (Centre for Agricultural and Biosciences International) is a not-for-profit organisation which develops solutions to agricultural and environmental issues and is a world leader in the development of biocontrol solutions. It rears the weevil and we have recently bought and released several batches of it in order to get it established at the site before the winter.  As the weevil feeds exclusively on water fern there is no risk to other plant species and is thus a very specific control measure, an advantage over more broad-spectrum chemical control measures.

We will monitor the situation in 2016 to check that the weevils have survived the winter and do further introductions if it looks as though the water fern is getting out of control again. In the longer term it is hoped that the permanent presence of the weevil on site will keep the water fern distribution in balance so dense, extensive mats do not form.