Invasive Non-Native Species Part 3

Exploring future biological control agents which could be used in the Test and Itchen

Courtney Brain

These biological control agents are still in the early stages of research or releases so may not be available for a few years yet, but we can’t wait to explore the options!

In this article we will explore the potential future biological control agents which are currently being researched and developed by CABI  (Centre for Agriculture and Bioscience International), which could potentially be used to target Japanese knotweed, Floating pennywort, Parrot’s feather and New Zealand pygmyweed in the near future.

Japanese knotweed
Japanese knotweed is a highly invasive plant that was introduced in the mid 19th century. Infamous for causing damage to infrastructure, increasing flood risks and spreading quickly. Currently this invasive can only partially be controlled using herbicides, but this is a costly control method that isn’t always successful.

In 2001 the Japanese Knotweed Alliance was formed in 2001 to explore the potential for a biological control of Japanese knotweed in Great Britain, CABI was contracted to undertake the research for this project.

Initial field studies in Japan found the psyllid Aphalara itadori (small sap sucking insect) and the fungal leaf-spot pathogen Mycosphaerella polygonia cuspidate. These two species were considered good potential biological control agents but after thorough testing only the psyllid was approved for release. The leaf-spot pathogen was ruled out due to its potential damage to other plants making it an unsuitable biological control agent.

Psyllid research to date:
The sap-sucking psyllid Aphalara itadori Shinji (Hemiptera: Psyllidae) which is damaging to F. japonica, F. x bohemica and F. sachalinensis was approved for released by DEFRA in 2010. After which several restricted releases were carried out to determine the risk posed to other non-target species. These trials showed this psyllid had no detrimental impacts on native invertebrates, so a new release licence was issued. Several large scale releases occurred in 2015 across England and Wales, monitoring of which showed overwintering to be an issue which would impede long-term establishment of this species.

In 2019 further surveys were carried out in Japan to find more climatically-matched psyllid to overcome overwintering issues. Psyllids in Murakami exhibited severe leaf curling damage so testing is being carried out to evaluate the specific performance of this potentially field-hardier psyllid on Japanese knotweed.
As of 2022 the project aims to complete host-range testing of the Murakami psyllid and acquire approval for small scale monitoring released in the UK.

Fungal leaf-spot pathogen research to date:
Interestingly even though this species was ruled out as a traditional biological control agent due to the threats posed to native biodiversity, research has instead focused on suing this species to develop a mycoherbicide. A mycoherbicide is a fungus based herbicide rather than a unnatural chemical based herbicide. In theory this product would then be applied similarly to a herbicide, but developed so the fungus could not reproduce and spread or survive in the wild beyond the application site, which would avoid damage to non-target plants.

Following DEFRA approval for release from quarantine, experimental field trials were conducted from 2019 to 2021 which showed the agent can infect Japanese knotweed and to a lesser extent Bohemian knotweed. These experiments shows that application earlier in the season leads to better disease development due to higher susceptibility of the plants.

Collaboration with private industry is being explored to hopefully develop a mycoherbicide which can be applied much like chemical herbicides to control Japanese knotweed.

New Zealand pygmyweed
New Zealand Pygmyweed Crassula helmsii, which is also known as Australia Swamp Stonecrop is a voracious invasive aquatic species that was first introduced to the UK from Tasmania in 1911, as a good oxygenating plant commercially available in the aquaria trade. This species is often associated with iron rich waters, this adaptable species is able to colonise a variety of slow-flowing water ways with soft sediments from lakes, coastal wetlands, marshes and river margins.

In 2010 CABI, commissioned by the UK government, began investigating the potential for using biological control to reduce the impacts of this species. Fields studies in the plants native range found many natural enemies which were unsuitable biological control agents as they were able to attack non-target species. After further studies, the mite Aculus crassulae was identified as the most suitable agent. Mites from this family are known for host specificity and ability to reduce the plants reproductive success.

Safety testing was carried out between 2012 and 2018, during which host specificity was determined – this species will only feed and develop on Crassula helmsii. The mite feeding was shown to cause the growing shoots or terrestrial and emergent forms of New Zealand pygmyweed to develop galls or swelling, which significantly reduced growth of the plants. Further studies showed the species was capable of surviving UK environmental conditions.

This species was granted ministerial approval for release in 2018. Since 2018 the mites have been released at 12 sites around England and Wales, which have been closely monitored to increase the understanding of field results before upscaling can occur. These studies have shown that the mites can develop populations under UK environmental conditions, so the project now aims to ensure the mites can establish robust sustainable populations in the UK.

Floating pennywort
Floating pennywort Hydrocotyle ranunculoides was introduced in the UK in 1980s from Northern and Central America as part of the aquatic nursery trade, as an ornamental pond plant. This widespread species is capable of growing in a variety of marginal habitats, rooting in the muddy margins of slow-flowing streams, ditches, lakes and ponds and has even been recorded in gravel pits.

In 2011, a CABI project began to identify potential biological control agents to target Floating pennywort. Several studies were undertaken in the species native range of Argentina, after rejecting several natural enemies which were unsuitable biological control agents, the weevil Listronotus elongatus was prioritised for further studies. The weevils were approved for export from Argentina and Paraguay in 2014 and 2019 respectively, for quarantined testing.

During these tests, the weevil was found to have a significant preference for the target invasive and cause significant levels of damage to infected plants. The adult stages feed on the leaves of the plant and then lay their eggs in the petioles of the plant – the stalks that attach leaves to the stems. The emerging larvae then mine the petioles for food which causes the plants to collapse.

This species was granted ministerial approval for release into the wild in Autumn of 2021. Preliminary fields trials at 2 sites in England were closely monitored, showing that adult weevils are capable of persisting through winter and feeding and egg-laying activity increased in spring. The studies also showed that the immature stages are capable of having significant impacts on plant biomass at lower temperatures.

More releases were planned for summer of 2022, and the mass rearing of these weevils is now the focus for the project. It is hoped that this biological control agent will provide a long-term sustainable solution which contributes to the national strategic approach to the management of floating pennywort – GB Strategy for Managing Floating Pennywort.

Parrot’s feather
Parrot’s feather Myriophyllum aquaticum, also known as water-milfoil, is indigenous to South America and was first introduced into the UK as part of the aquatic trade in 1870s as good aerator of garden ponds. This species is mainly found in low-lying areas, amongst the muddy substrates of shallow slow flowing waters and typically favouring nutrient rich eutrophic conditions. Capable of tolerating moderate stress conditions, including saline influenced waters up to 10 ppt (above which is toxic to the plant) and periods of desiccation.

In 2021 a feasibility study began to assess the potential for finding a biological control agent to target parrot’s feather. Initial studies have involved a leaf-feeding beetle Lysathia sp, which has previously been released in South Africa and was shown to exhibit good control over the parrot’s feather there. In September of 2021 it was imported to the UK for safety testing which is currently underway at CABI’s quarantine laboratories.

Another species being researched is the stemming weevil Listronotus marginicollis. This research is being undertaken by CABI in collaboration with a team in Argentina. The team of scientists at Fundación para el Estudio de Especies Invasivas (FuEDEI) is investigating this weevils potential as a biological control agent for parrot’s feather in the UK.

Although in the very early stages compared to the other potential biological control agents soon to be available, good progress is being made. So for now we will continue to follow updates on this project and see how it progresses.

Click to learn more about our Test and Itchen Invasive Non Native Species Project (T&I INNS Project), which aims to reduce the impact of invasive non native species on the special chalk stream habitats of the Test and Itchen.