6th August 2026
Thousands of tiny weevils have been released on the Grantham Canal to help tackle an invasive aquatic weed threatening wildlife and water quality.
The Canal & River Trust has introduced the 2mm-long insects to feed on North American water fern (Azolla filiculoides), which can spread rapidly across the surface of canals during warm weather. If left unchecked, the dense floating mats reduce light and oxygen levels in the water, harming fish and other aquatic life.
The charity says the weevils offer a natural form of biological control because they feed exclusively on the invasive fern and do not damage native plants. Active during the summer months, they breed quickly and can consume large quantities of the weed.
Reanne Bower, ecologist for the Canal & River Trust, said: “Water fern was first introduced to the UK by the Victorians as an ornamental plant for ponds but, over the years, has spread into the nation’s watercourses.
“As the weather gets warmer it’s likely that we’ll see a rapid acceleration in the growth of the weed so now is a good time to release the weevils and prevent it from spreading along the canal.
“We’ve introduced weevils on a number of canals over the last few years and have seen some encouraging results, so we hope that by releasing some at targeted spots on the Grantham Canal we’ll be able to reduce the weed and protect wildlife within the canal.”
The trust has used the insects successfully on other waterways in recent years as part of its ongoing programme to control invasive species across the canal network.
Image: Reanne Bower holding the weevils
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