‘Remarkable’ weevils save historic waterway on Wey and Arun canal

A North American weevil, natural enemy of floating water fern

A North American weevil, natural enemy of floating water fern

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Tiny beetles have saved a historic waterway from being choked by an outbreak of invasive weed, the Wey and Arun Canal Trust have announced.

The two millimetre-long North American weevils nibbled away at a carpet of vegetation on the Wey and Arun Canal, clearing the non-native water fern in just a few weeks, the Trust confirmed.

Floating water fern formed a dense carpet on the surface of the Wey & Arun Canal at Tickner's Heath, Dunsfold

Floating water fern formed a dense carpet on the surface of the Wey & Arun Canal at Tickner's Heath, Dunsfold

Twelve thousand weevils were released into the waterway at Tickner’s Heath, Dunsfold, in July, to combat one of the most invasive plants in the country – Azolla filiculoides.

Ian Burton, the Wey and Arun Trust’s conservation adviser, said the impact the weevils made on the fern was ‘remarkable’.

He said: “We are delighted with the work of the little insects.

“The weevils speedily removed most of the water fern and have had a comprehensive impact.

“There is now no sign of the weed.

“We acted to try and clear the weed because it had formed a thick mat on the surface of the water, blocking out the light and threatening the aquatic flora and fauna,
“The canal appeared to be solid ground.

“Floating water fern multiplies rapidly and its area can double in a few days.

“We didn’t want it spreading to neighbouring properties and other parts of the canal.

“There are no organisms native to the UK that can combat Azolla, but research has shown the weevils to be one of the plant’s main natural enemies.”

The Trust will continue to monitor its waterway to ensure that the plants do not return in the spring.

Individually, the weevils consume a small amount of Azolla, but they breed to produce large populations which feed extensively on the weed. Across the UK, entire infected lakes and canals have been cleared using this form of biological control.

The project was a partnership between the Canal Trust, the Heritage Lottery funded Arun and Rother Connections (ARC) and the CABI (Centre for Agriculture and Biosciences International), which supplied the weevils.

ARC works with local organisations to help ensure wildlife can flourish in a thriving river system. 
As part of a wider programme of work to tackle a number of priority invasive species across the whole catchment area, project staff identified the invasive plant and provided the funding to purchase the weevils.

Azolla was introduced into the UK from the Americas in around 1840 as an ornamental garden aquatic.

Some North American weevils also arrived with the imported ferns, and the insect species is now considered naturalised.

They pose no threat to UK ecosystems, unlike the fern itself, which soon escaped into the wider environment, where it now causes considerable problems on ponds, lakes and waterways.

The initiative at Tickner’s Heath is line with the Wey and Arun Canal Trust’s policy of controlling and eradicating invasive species, including Himalayan balsam, giant hogweed and Japanese knotweed, on land that it has responsibility for.

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