Invasive Non-Native Species Part 2

Using biological control to fight invasive plant species in the Test and Itchen River Catchment

Courtney Brain

This week we are celebrating Invasive Species Week (15th - 21st May)!

To raise awareness of the harm invasive species are having on our native ecosystems, we are releasing a three part blog series - so you can learn all about the steps we are taking to keep invasive species under control. Read on for part two... 

The Test and Itchen Invasive Non Native Species Project (T&I INNS Project) aims to reduce the impact of invasive non native species on the special chalk stream habitats of the Test and Itchen. An element of this is educating all stakeholders about utilising the best available management techniques, including those which are currently under development and not yet available.  
Biological control is the use of living organisms, such as insects or pathogens, to control invasive or pest populations. By re-introducing a natural enemy that would be present in the invasives native range, a more beneficial equilibrium can be reached. The selection of these species is based upon years of research, discovering potential candidates as well as extensive quarantine periods and testing to ensure this biological agent has no negative impact on native vegetation or ecosystems. 

At present there are two biological control species which can be purchased from CABI (Centre for Agriculture and Bioscience International). The weevil Stenopelmus rufinasus which is a biological control agent for Water fern (Azolla filiculoides) and a rust fungus species which is a biological control agent for Himalayan balsam (Impatiens glandulifera). Currently only the weevil is being used by this project, but that’s not to say that the rust fungus wont be used in the future, but we will explain why later.

Water fern – Weevil Stenopelmus rufinasus
Water fern Azolla filiculoides, is a free floating surface dwelling fern which can form mats up to 20 cm thick. This species is particularly suited to nutrient rich, slow flowing river sections or still waters and can have significant impacts on local biodiversity, hydrology and water chemistry. 

Treating Water fern Azolla filiculoides with traditional control methods should be avoided where possible. Manual removal can exacerbate spread and the use of harsh chemicals in sensitive aquatic environments can have devastating effects on watercourses if applied incorrectly. So it is highly recommended that the biological agent, the weevil Stenopelmus rufinasus, be used.

Stenopelmus rufinasus is an azolla-feeding species of weevil. It has been classed as resident species in England and Wales since it was first recorded in the 1920’s, it is suspected it was imported as a stowaway on Water fern plants and has survived here ever since. Although its spread was likely limited by cold weather, it has likely been providing background control of this invasive since it was introduced but is naturally slow spreading so unable to catch up with new outbreaks.

It was first tested as biological control agent in South Africa, after being released in 1997 it was proven as a great biological control agent providing ongoing high-level control of Water fern. In the 2000’s CABI began investigating the potential to mass rear and re-distribute this biological control agent to target outbreaks in the UK. Testing showed that Stenopelmus rufinasus is a monospecific weevil, meaning its entire life cycle is dependent on Azolla, with a strong preference for Azolla filiculoides (the particular Water fern species we are concerned with) so poses no threat to native biodiversity.

Another key attribute of this species is its ability to reproduce extensively over the course of two or three months, building populations capable of effectively eliminating its entire food source, after which adults must then fly in search of further infestations.

There are several advantages to using this biological control agent:
- Minimal effort is required as the release process is simple
- Highly specific so does not cause damage to the surrounding species or environment
- Capable of over-wintering so multiple applications are not always necessary
- Resident species so no permits or species permissions required for application

On the other hand it should be noted that control is not instantaneous and it can take roughly 2 months to be effective, and in this time it may appear as though the problem is actually getting worse. In some instances additional applications may be required the following growing season to address overwintering spores which are released in June/July. But despite the potential risk of needing to reapply, we believe that the advantages above greatly outweigh any other methods.

These weevils have already been released at several points along the Test and Itchen and we will continue to do so over the coming years. In 2021 we helped release these weevils at 3 sites, in 2022 we released them at a further 5 sites. We already have 3 releases planned for 2023, so are ready keep building on this and helping riparian owners utilise this amazing treatment method!

Himalayan balsam – rust fungus Puccinia komarovii var. glanduliferae
Himalayan balsam Impatiens gladulifera is an invasive annual herb which was originally introduced in the 19th century as an ornamental garden plant. This species is capable of growing in moist and semi-shaded environments and is capable of forming vast monocultures along the banks of watercourses.
Treating Himalayan balsam with traditional methods such as manual removal and chemical control can be labour intensive, expensive and ineffective over large areas. For long term management of this species there is now another option available, the biological control agent rust fungus Puccinia komarovii var. glanduliferae.

This rust fungus is an autoecious macrocyclic species, meaning it completes all five spore stages of its life cycle on a single species. More details on the incredibly intricate and interesting five stage life cycle can be found here.
The fungus infects the stem of seedlings during germination in spring and then infects the leaves later in the growing season inhibiting growth and seed release.

A CABI project seeking to identify the safest and most effective biocontrol agent for Himalayan balsam began in 2006. Surveys were conducted in India and Pakistan to identify natural enemies of the invasive that could be utilised as biological control agents in the introduced range. Many fungal and arthropod species were rejected as they were able to attack other closely related species making them unsuitable biological control agents.

This project identified the rust fungus Puccinia komarovii var. glanduliferae as a potential biological control agent and exported to a quarantine facility in the UK in 2010 to undergo extensive testing. After rigorous testing DEFRA ministers approve the rust for release in 2014.

However during trail release studies it was noticed that not all infestations were vulnerable to the fungus. From this a molecular study was carried out, this study showed that there were three variants of Himalayan balsam present in England and Wales, that had been historically introduced from multiple locations within its native range. In 2017 the second strain of rust fungus was approved for release and in 2021 a study began to locate the third variant in the native range so that all three variants of Himalayan balsam can eventually be controlled using this rust fungus.

The rust fungus has since been released at more than 50 sites, with the assistance of landowners and local action groups who are trained in the rust release protocol and monitoring procedures.

There are several advantages to using this biological control agent:
- Highly targeted only effecting the particular strain of Himalayan balsam means no damage to surrounding ecosystem or other plant species .
- Shown to successfully overwinter and establish near release site.
- Very little intervention required after initial release.

Unfortunately there are also several reasons making this method currently unsuitable for release in the Test and Itchen river catchments, but that could change in the future.
- The majority of infestations in southern England are composed of the third strain of Himalayan balsam for which there currently isn’t a rust fungus available.
- Relatively large initial cost incurred to train individual or group on release and monitoring protocols, as well as to rear the specific fungus required for release.
- Very slow rate of spread as the rust fungus, might only spread 5-10m from introduction site in the first year, it could be 5-10 years before it reaches any significant distances downstream. During which time surrounding plants can not be removed within a designated buffer zone otherwise the rust fungus can not spread.
- Control not eradication, so further effort is still required to reduce the presence and impact of this invasive .

Using the Puccinia komarovii var. glanduliferae rust fungus in the Test and Itchen
To date this biological control agent has not been used in the Test and Itchen catchments. This is mainly relating to the cost of the release protocols and it is also suspected that only the third variant is present the Test and Itchen catchment, for which there is yet to be a suitable rust fungus available.
This project will keep this as a possible control option for the future, should a suitable rust variant and a perfect release site be found.

In summary, we believe utilising biological control agents to reduce the impacts of invasive non native species on the Test and Itchen river catchments is the way forward. Particularly considering how ecological important and significant our chalk stream are. So we must reduce our reliance on harmful control methods (manual and chemical) and turn to more sustainable options which will only benefit these precious ecosystems more.

To find out more about our project please click here