The wrong sort of weed for canal weevils

British Waterways had planned to release the tiny 2mm-long organisms into the Dearne and Dove canal in Swinton, near Rotherham, to tackle the invasive Azolla fern.

But when they arrived they realised the canal was actually infested with duckweed, which is not edible to the weevils, so had to quickly transport them to an affected canal in Huddersfield instead.

Although the individual Azolla plants are only about 10mm long, they can multiply rapidly, covering the surface of a waterway with thick mats in a matter of weeks.

The weed, which was introduced to canals after being imported from North America, reduces light and oxygen levels in the water, killing fish and other wildlife.

Individually the weevils consume a relatively small amount of the fern. However, the creatures breed to produce very large populations which, as a whole, feed extensively on the Azolla mat until sections start to die and sink.

Given time to reproduce and spread, the weevil "Stenopelmus rufinasus" is capable of clearing entire lakes or canals of Azolla.

Jonathan Hart-Woods, British Waterways environment manager, said: "Water fern was first introduced to the UK by the Victorians as an ornamental plant for ponds. Despite looking attractive, this weed is actually a serious threat to waterway wildlife."