We investigate the use of a new biocontrol which is shifting the battle on invasive plant floating pennywort.

Pennywort weevil on floating pennywort. (Credit: Suzy Wood)

The floating pennywort problem

Floating pennywort is a freshwater INNS that is taking over UK waterways.

Rapid growth rates (up to 20cm per day), the ability to thrive in most freshwater conditions and spread through fragmentation have facilitated it’s spread. This results in dense vegetation mats forming – in some cases completely covering waterbodies.

  • Vegetation mats increase flood risk by blocking channels, and impact recreational use of waterways.
  • Reduced biodiversity has been found on sites with floating pennywort infestations due to its ability to outcompete native species for resources.
  • Costs the UK economy millions of pounds each year to manage. Control costs in the Thames region in 2018 exceeded £600k. In Yorkshire these costs are signficantly lower as the population isn’t as widespread.

Floating pennywort on watercourse. (Credit: CCW)

Consequences of floating pennywort. (Credit: Trevor Renals, Environment Agency)

Recreational consequences of floating pennywort. (Credit: GBNNSS)

What is a biocontrol?

A biocontrol is a living organism which is introduced into an environment to control a pest.

These pests can be animals or plants – including Invasive Non-Native Species (INNS). A biocontrol helps to manage populations of pests in a natural way without eradicating them completely. This replicates how populations are managed within natural ecosystems.

Biocontrols reduce the need for intensive human interventions using chemicals and/or machinery. This in turn reduces risks associated with chemical use such as leaching and impacting non-target species. It also reduces the associated costs and complex logistics associated with other management techniques. Reducing the size of an INNS population allows native species to re-colonise affected areas, thereby reducing their impact on the natural ecosystem.

Biocontrol requirements

Species released as biological controls are subject to intensive research and monitoring. A high level of specificity (preference to the pest species) is required to manage the risk of unintended impacts to other species – including humans. The biocontrol must also significantly impact pest species populations to be effective but longer timescales are expected (often between five and 10 years from release).

Impact on floating pennywort

Floating pennywort is difficult to manage as it is found in waterways making use of chemicals and access problematic. Manual removal is successful, but it is time and energy intensive and requires ongoing management. In addition, strict biosecurity is required so that fragments of pennywort are not spread accidently.

In 2011 investigations began into a biocontrol for floating pennywort led by the Centre for Agriculture and Bioscience International (CABI). Within the plant’s native range – Argentina and Brazil – a range of natural enemies were considered. The weevil Listronotus elongatus a common insect on floating pennywort was identified as a good candidate.

Adult pennywort weevil (Credit: CABI)

What do pennywort weevils do?

This weevil shows significant preference for floating pennywort and is very damaging to the plant.

The adults feed on the leaves and lay their eggs on the leaf stalks (petioles). Additionally, the larvae ‘mine’ for food into the petioles, which results in collapse of the plant. The weevil should be able to survive one or two generations in southern England where issues with floating pennywort are greatest.

Pennywort weevil larvae. (Credit: CABI)

Left – Weevil on pennywort leaf with feeding damage. (Credit: Kate Jones, CABI). Right – larva mining. (Credit Djami Djeddour, CABI).

Can weevils be the solution to the pennywort problem?

To determine whether the weevil would be a suitable biocontrol, its partiality for floating pennywort was investigated, as was the risk to native species. A list of native plants was collated by UK stakeholders and botanical experts, against which the weevil was tested for safety and effectiveness. In 2014 and 2019 shipments of the pennywort weevil was sent to the UK to CABI’s quarantine facility, where testing was carried out.

What’s next for the weevil?

Results from testing and the Pest Risk Assessment were submitted to UK regulators in 2017. Both the ministerial and Pest Risk Assessments have now been approved and the pennywort weevil is due for release in southern England this summer for field trials (2021).

Within Yorkshire, the Environment Agency are working with Cabi to identify trial sites in Yorkshire where the weevil can be released and monitored as early as next summer (2022). Staff at the EA have been out collecting specimens of floating pennywort from within the River Aire catchment. From these DNA samples will be extracted to compare the diversity of floating pennywort across the UK. The purpose of this trial is to determine suitability and effectiveness of the weevil in a cooler northern climate.

Surveying along the River Aire (Credit: Angling Trust)


The use of biocontrols, such as the floating pennywort weevil may provide a safer, more cost effective and sustainable approach to invasive species management in the years ahead, benefitting native species, wider UK ecosystems and those that live, work and enjoy these spaces.

We are excited to see how this work progresses in Yorkshire and the wider UK.

Further resources

Ellen Shields

Ellen Shields

INNS Seasonal Assistant, Yorkshire Wildlife Trust


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