Stenopelmus rufinasus, commonly known as the Azolla Weevil, feeds exclusively off the Water fern, Azolla filiculoides. Now the Environment Agency has released a colony of the beetles to destroy a huge mossy slick of the weed in Lincolnshire. The beetle eliminates the need to use chemicals or dredging, which can harm other wildlife.

Azolla, also known as the water or fairy fern, started to pose a problem on the River Till, a tributary of the River Witham, earlier this year. Although Agency officers cleared the weed in June, over the last few months the ultra fast-growing plant, which can double in size every four to five days, has regenerated, spread downstream and invaded the Witham itself.

Invasive species cost UK £1.7 billion every year

The blanket of weed now stretches for an incredible four kilometres (2.5 miles), is 15-metres wide and up to 30 centimetres thick. Invasive species now cost the UK economy an estimated £1.7billion every year.

They cause damage to riverbanks and buildings, increase flood risk, crowd out and kill off native wildlife and become so prolific on waterways that fishermen, boaters and anglers are unable to use them. Despite water quality improving, the presence of invasive species like Azolla could cause rivers in the UK to fail tough new targets on water quality under the Water Framework Directive.

Improving water quality

The Environment Agency currently spends over £2million a year controlling invasive species, and is this year increasing its efforts with partners such as Natural England by targeting some of the £18 milion funding provided by Defra to help more English rivers meet the new EU targets.

Like many other invasive plants, water fern was first introduced to the UK as an ornamental exotic from the Americas in the 1800s. Having escaped from garden ponds into the wild and in the absence of any native predators, the fern quickly invaded rivers, lakes and canals.

The plant forms dense mats on the surface of the water, depriving other plants, fish and invertebrates below the water level of light and oxygen.

As well as creating ‘death zones’ for aquatic wildlife, the mossy carpets can pose a flood risk as the weed clogs the watercourse.

Weevils bred specifically to clear UK waterways

The Azolla Weevil, which also hails from the Americas, arrived in the UK by chance in the early 1920s, probably via imported plants. Unlike the fern, the weevil survives in low numbers in the wild without posing a threat to native species.

It has been used with great success to clear clogged waterways in South Africa. Today, the weevils are bred in Britain by CABI, a not-for-profit science organisation, specifically to clear UK waterways.

Environment Agency invasive species expert Trevor Renals, said:

“The weevils are real specialists and only eat water fern. They don’t harm other plants and often die out naturally once they have eaten their way through the Azolla. Thanks to this weevil, we are able to eradicate the weed without the need for dredging and chemicals.”

CABI invasive species specialist Corin Pratt, added:

“Releasing captive-bred weevils means we can help landowners to control a problematic invasive weed in a natural way. It is cheap, safe and remarkably effective.”