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New weapon in war on River Cam's mutant menace: Can good come from weevil?



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Floating pennywort at Grantchester Mill Pool by Mike Foley, Cam Valley Forum
Floating pennywort at Grantchester Mill Pool by Mike Foley, Cam Valley Forum

An alien is in our midst. Like a river-dwelling triffid, it chokes all that goes before it, damaging life and causing flood risk. The scale of the floating pennywort problem on the River Cam and other waterways is shocking, and has prompted Cambridge conservationists to come together to discuss a solution. But now a silver bullet has emerged...

Floating pennywort. Picture: Environment Agency
Floating pennywort. Picture: Environment Agency

An alien waterweed that has mutated, grows at frightening speed and threatens to obliterate normal river life seems the stuff of science fiction.

But to a roomful of scientists and conservation experts meeting in Cambridge, the invasive floating pennywort, Hydrocotyle ranunculoides, is a real and present danger.

The problem is that the floating pennywort is multiplying on the River Cam from Grantchester down to the Denver Sluice on the Great Ouse River, and many patches have been found on waterways throughout East Anglia and much of England. The plant is apparently getting stronger and more virulent as it spreads.

Even at such a sober gathering, the progress of the floating pennywort was likened to the aggressive fictional plant that blinded humans in John Wyndham’s 1951 post-apocalyptic novel The Day of the Triffids.

Floating pennywort. Picture: Environment Agency
Floating pennywort. Picture: Environment Agency

The reason is that despite the best efforts of groups of volunteers pulling the weeds out of rivers and ditches, boats hoisting tons of matted plants onto the river banks and repeated spraying with herbicides, the floating pennywort continues to thrive.

The meeting was convened by the Cam Valley Forum, a volunteer group that works to protect the environment of the river, and the Cambridge Conservation Forum, an umbrella group of 70 biodiversity and conservation organisations.

The extraordinary resilience of the floating pennywort in being able to grow back from the tiniest speck left in the water has so far defeated all efforts to eradicate it from the river system.

Speaker after speaker described their personal battle with the weed, but none of them was able to strike a killer blow – it always bounced back. But as in a novel, an unlikely saviour, or secret weapon, may come to the rescue.

Floating pennywort. Picture: Environment Agency
Floating pennywort. Picture: Environment Agency

This is in the form of a weevil, Listronotus elongatus, which, in its native South America, keeps the floating pennywort under control simply by eating it. If released in Cambridgeshire, the tiny weevil might save the Cam, but the introduction of yet another alien species has yet to get Government approval.

The day began with Lesley Saint, from the Environment Agency, describing how floating pennywort was first recorded in the wild in the UK in 1990. It had been imported from South America as a garden pond plant and escaped. It was now designated by the European Union an ‘Alien Species’ and it had been illegal for shops and garden centres in Britain to sell it since 2014.

The weed builds up huge mats that cover watercourses and cause flood risk by creating dams that also disrupt navigation.

Life in rivers is damaged by lack of light and oxygen, and fish stocks disappear. Fifty tons of weed could be removed at a time from the Great Ouse, but it quickly grew back, even after spraying with herbicide.

Cam Conservators clearing pennywort from the Cam
Cam Conservators clearing pennywort from the Cam

The Environment Agency would like anyone who spots an outbreak of floating pennywort to report it, giving a grid reference and a photograph if possible, so a team could be dispatched to tackle it.

Mike Foley, of the Cam Valley Forum, said he had walked much of the River Cam’s upper reaches and was “slightly appalled” at the infestation. The Bourn Brook had become totally blocked.

He has developed a special long-handled rake for pulling the weed out of the water, but the whole plant had to be removed otherwise it grew back immediately. He said he thought the weed was spread by fishermen, who failed to clean their nets and rods adequately when they moved to a new site, and by boats that caught strands of the weed and carried it to a new place.

One bright spot was the middle of Cambridge. Because the riverbanks were not natural but hard piled to provide a solid wall, the weed could not get its roots into mud and gain a foothold. It therefore did not grow and did not affect punting, which would be disastrous for the tourist trade. However, the weed was likely to spread to ditches and streams in Cambridge because it grew in the river both above and below the city.

Cam Conservators clearing pennywort from the Cam
Cam Conservators clearing pennywort from the Cam

Colin Sparkes, from Cam Conservators, which has a special boat that cuts up and hauls out vast mats of the weed from the river, said he feared that even as they were removing it they were helping to spread it.

He said that the propellers of the boat were cutting up the weed that had been missed from being hauled out. Each tiny piece drifted away and grew new roots.

It was the final speaker, Djami Djeddour, from the Centre for Agriculture and Biosciences International (CABI) in Egham, Surrey, which is owned by 48 countries and whose job is to manage invasive species, that gave the meeting hope.

She had captured and bred the weevil that lives on the weed, lays its eggs on it and whose larvae live inside and hollow out its stems.

The centre is making a case to the Department for Environment, Food and Rural Affairs (Defra) to release the weevil onto the plant in Britain and hope for a decision by autumn this year.

It could be the ‘silver bullet’ that keeps floating pennywort in check, although it would never eradicate it, she said.



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