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Bugs that feed on an invasive water weed are being used to tackle an infestation on a canal.

The weevils have been released into the Monmouthshire and Brecon Canal in Torfaen to combat non-native water ferns.

The 2mm-long beetles feed solely on the azolla plants, originally from North and South America.

It follows action by local anglers to pull half-a-tonne of duck weed from a stretch of the canal at Pontnewydd.

They said the weed there was choking the waterway, killing fish.

But the azolla plants pose an even greater risk to the canal, as the tiny ferns rapidly multiply and can entirely cover a canal surface, depriving it of oxygen and light.

The council has released what it described as "an army" of the azolla weevils into the Llantarnam section of the canal, which has undergone a £1.4m restoration.

"It was great seeing the weevils being released," said Fiona Cross, who is the executive member for environment at Torfaen council.

"Nature helping nature is a great way to solve some environmental problems."

The azolla, which was introduced into Britain in 1886 as an ornamental plant, became established in waterways within 20 years, and is now regarded as one the UK's most invasive species.

Not only can it completely cover water, the carpet of plants can be up to 30cm (1ft) thick - stopping all other amphibians, fish and invertebrates reaching the surface.