A colony of beetles are being released into the wild to ensure a river remains navigable to boats.

Azolla filiculoides, better known as the water or fairy fern, doubles in size every four or five days and has rampantly spread over Lincolnshire’s River Witham and its tributary, the River Till.

To ensure boaters can still use the river, which is navigable as far as Lincoln, the Environment Agency has enlisted the help of the azolla weevil, which feeds exclusively on the water fern.

Agency officers have released a colony of the weevils into the river to tackle the weed, which is now 4km long, 15m wide and up to 30cm thick.

Species, such as water fern, could cause the UK to fall foul of EU water-quality targets, as the fern forms on the water’s surface, depriving plants, fish and invertebrates below of light and oxygen. It also damages river banks and increases flood risk.

Like the fern, the azolla weevil is not native to the UK, but it survives in low numbers without threatening native species and has successfully cleared South Africa’s clogged 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 the weevil, we are able to eradicate the weed without the need for dredging and chemicals.’

_{Photo: Rob Reeder – CABI}