Weevils To Tackle Invasive Floating Pennywort

Weevils have been deliberately introduced to waterways in West Yorkshire to try are reduce levels of the invasive floating pennywort plant, the BBC reports. The non-native pond plant grows by up to 20cm per day, forming dense mats on the surface of the water. This chokes other wildlife, and causes drainage problems.

The invasive floating pennywort is native to Central and South America, and the weevils are also native to the region, and have an exclusive diet of pennywort. Therefore, they have been deliberately released into two sites in West Yorkshire which have become blighted by the invasive plant.

Floating Pennywort was originally introduced to the UK in the 1980s as an ornamental pond plant. It has small round green leaves, which clump together to form dense mats on the surface of ponds, lakes, and rivers. It was banned from sale in 2014, when it was recognised that the plant had invaded wild habitats and was out of control.

The plant, which may sometimes be incorrectly labelled as marsh pennywort, reduces the oxygen levels in the water, which makes it more difficult for aquatic creatures to survive, clogs up drainage systems, and crowds out native plants.

The South American weevils have been released in the Aire and Calder Navigation, and one of the tributaries to the River Holme, in an effort to curb the spread of the invasive plant. The weevils have tested for their suitability for release into natural habitats in the UK, and approval has been given by the Centre for Agriculture and Bioscience International (CABI).

Dr Steph Bradbeer, invasive species and biosecurity adviser at Yorkshire Water, said: “Invasive non-native species pose a very real risk to Yorkshire’s environment and wildlife. They can also impact on our ability to treat and distribute water to homes and return wastewater safely to the environment.”

He added: “Floating pennywort, if unchecked, can cause significant problems in slow-flowing watercourses and impact drainage systems. We hope the release of these specialist weevils will provide a way of tackling it without the need for mechanical or chemical intervention.”

Yorkshire Water are acting in collaboration with the CABI, Leeds City Council, River Holme Connections, and a private landowner. It is hoped that the weeveils, which feed on the leaves and stems of the plant, will help to control the rampant growth, and improve the water quality and reduce flood risks.

Djami Djeddour, senior project scientist at CABI, said: “These weevil releases are the culmination of over a decade of collaboration with South American scientists and comprehensive safety and efficacy testing in our quarantine facilities, so it is thrilling to finally get them out into the wild.”

The CABI will continue to closely monitor the impact of the weevils on local wildlife, and on the invasive floating pennywort plant.

Invasive non-native plant species are a significant problem in the UK, causing damage to rare native species and depriving other wildlife of food, nutrients, light, space and oxygen. There is also a great financial cost, with nearly £2bn spent every year to try and reduce and control the spread of invasive species.

There are two classes of regulated non-native invasive species in the UK.  The first class, and by definition the most egregious, are those banned from sale in the UK under The Wildlife and Countryside Act 1981 (Schedule 9) and subsequent EU legislation not repealed.  The second class are not banned from sale but under the same Act and schedule it is illegal to dispose of them or plant them in the wild.

There are 5 aquatic plants listed under The Wildlife and Countryside Act 1981 which are banned from sale in the UK:  Water fern (Azola filiculoides); Parrot’s feather (Myriophyllum squaticum, also known as Brazilian water milfoil); Water primrose (Luwigia grandiflora); Floating pennywort (Hydrocotyle ranunculoides, also known as Floating marsh pennywort); and New Zealand pygmyweed (Crassula helmsii, also known as Stonecrop, Australian swampweed).

In addition EU legislation bans sale of the following: Water hyacinth (Eichornia crasspipes); American skunk cabbage (Lysichiton americana, not to be confused with the white variant Lysichiton camptschatcensis); Curly waterweed (Lagarasiphon major,Elodea densa, often incorrectly called Elodea crispa); and Cabomba (Cabomba carolinia).

Aquatic non-native plants that are not banned from sale but where it is illegal under Schedule 9 to dispose of or plant in the wild are: Duck potato (Sagittaria latifolia, not to be confused with the native Sagittaria sagittifolia); Giant rhubarb (Gunnera tinctoria, not to be confused with Gunnera manicata, also known as Giant rhubarb); Carolina watershield (Fanwort); Water lettuce (Pistia stratiotes); Himalayan balsam (Impatiens grandulifera); and Waterweeds (all species of Elodea except Nuttall’s).

The last one in this list, Waterweeds, can be difficult to avoid as they are widely sold as “Pondweed”.  If in doubt check or ask for the botanical/scientific name and stick to the safer native species like Hornwort, Willow moss, Water crowfoot and Water starwort.

Note that for all of the above, it is an offence to dump them in streams, rivers, and other natural waterways, intentionally or unintentionally. If in doubt always safely compost any suspect water plants or take them to your local recycling centre for disposal.

Carolina watershield, which is native to North and South America, has long stems with feathery leaves, and produces white flowers in certain conditions. The plant became popular for fish tanks, and it was not initially thought to present a threat to native British waterways because of its subtropical origins.

However, cases of colonisation have been found in the Basingstoke Canal, and as the climate warms, there are fears it could start spreading more quickly. The plant has also been discovered to thrive in the Forth and Clyde Canal, where heated water released from nearby factories has enabled it to spread.

Plantlife recommends that rigid hornwort, which is a native oxygenating plant, is a good alternative to use instead. It is also recommended as a good substitute for curly waterweed, which is a native aquatic plant from South Africa. It was commonly sold as a pond plant in the UK, and has become endemic in native waterways for several decades.

Curly waterweed is a serious problem, because it grows rapidly to form dense mats which can choke the entire surface of a pond or lake, and clog drains.

If you are looking for pond plants for sale, please get in touch today.