Pennywort was introduced to the UK as an ornamental pond plant, but with an ability to grow up to 20cm per day, it has become a biohazard forming dense rafts over rivers. The plant cuts oxygen levels in the water, harming native plants, fish and invertebrates, and also disrupts water-based activities such as fishing, canoeing, and exacerbates flood risk.
The weevils were released over the winter with more scheduled for this summer, and according to DEFRA, this is the first use of a biocontrol to tackle floating pennywort in the world.
Lord Benyon, minister for biosecurity, said that “we all have a role to play” in stopping the destruction caused by floating pennywort, and that the release of the weevils would “boost our efforts to eradicate this pest from our waterways once and for all”.
It is unusual for one non-native species to be deployed against another, but the move follows years of research carried out by the Centre for Agriculture and Bioscience International (CABI), who carried out safety and efficacy testing.
The insects work as biocontrol as the adult weevils feed on the floating pennywort leaves, and the females lay eggs into the stalks. The larvae then eat into the stems of the plant, which reduces its ability to grow and spread further.
CABI senior scientific officer, Djami Djeddour, said that she was “pleased and excited to have released the weevil into the wild”.
She continued: “Field studies by CABI scientists and collaborators in South America found this weevil to be the most promising agent and after comprehensive screening in CABI’s quarantine facility in the UK, it was confirmed to be highly specialised, only feeding and developing on this damaging weed.”
The release of weevils is set to be strictly controlled, and will require a licence from the Environment Agency according to reports in the Guardian.
According to evidence submitted to the Environmental Audit committee in 2019, the INNS posing the greatest threat to human health are mosquitoes and ticks.
The same committee found that the UK had missed its legal targets on invasive species.
In the government’s response to the committee in 2020, it acknowledged that the impact and risk from INNS in the UK “remained significant” and that the number of INNS established in Britain had “remained constant in terrestrial environments and increased in freshwater and marine environments.”