Natural born killers – biocontrol of invasive non-native plants in Scotland

Biological control is the use of living organisms to help control pest populations. In this post Marion Seier, Senior Plant Pathologist at CABI (Centre for Agriculture & Bioscience International), describes the work being carried out by CABI’s Invasive Species group to develop biological control agents for use on the UK’s non-native plants…

Control of invasive non-native plants in Scotland is typically carried out by hand – cutting, pulling and removing seed heads – and with chemical herbicides.  These types of control can help reduce the impact of invasive species in limited areas but they are expensive and unsustainable. There is also concern about the environmental impacts and toxic effects of the widespread use of herbicides, such as glyphosate.

Classical biological control – or biocontrol – has been used successfully to control invasive non-native plants species in other parts of the world.  It is based on the principle that in their new locations, non-native plants grow in freedom from the natural enemies that in the countries from which they originate – their native range – keep them in check. By introducing selected host-specific natural enemies to the introduced range the plants lose the vigour that makes them so invasive. Scientists at CABI are currently undertaking research and development of biocontrol agents for three invasive non-native plants causing problems in Scotland, and one that isn’t present in Scotland yet but is expected to arrive in the future.

Japanese knotweed Japan

Japanese knotweed in its native Japan (C)CABI

Japanese knotweed

In its native range of Japan, Taiwan and Korea, Japanese knotweed is found growing on hills and high mountains.  In Scotland it can form dense stands on riverbanks, outcompeting native species and causing an increased risk of flooding. Two biocontrol agents are being investigated for use in the control of Japanese knotweed; a bug and a fungus.

Biocontrol for INNP - Scotland's Nature blog - psyllid on JK image credit CABI

Psyllid on Japanese knowtweed (C)CABI

The bug, Aphalara itadori, is a sap-sucking psyllid that feeds on Japanese knotweed. Initial trials have had limited success as the bugs haven’t thrived in UK weather conditions. However hardier bugs were collected last year from a more northern area of Japan and subject to final safety testing and approvals it looks hopeful that these hardier bugs will be suitable for release in the wild.

Biocontrol for INNP - Scotland's Nature blog - psyllid impact native range Japan_Murakami_Niigata - image credit CABI

Psyllid impact on Japanese knotweed (C)CABI

A fungal leafspot pathogen was evaluated as a classical biocontrol agent but during safety testing it was found to cause disease symptoms on a couple of UK native plants and so is not suitable for release in the UK.  The fungus does, however, have potential for use as a mycoherbicide (herbicide based on a fungus).  This would allow targeted spray application against Japanese knotweed without the fungus being able to persist and spread in the field to infect any non-target plants.  Field trials are ongoing and it is hoped that ultimately a product will be developed that could be applied much in the same way as a conventional herbicide but without the risks associated with chemicals.

Mycosphaerella leafspot Japan

Mycosphaerella leafspot on Japanese knotweed (C)CABI

Himalayan balsam

Native to the foothills of the Himalayas, in Scotland this plant can form dense monocultures in damp semi-natural woodlands and along waterways, replacing native vegetation and destabilising river banks.


Himalayan balsam (C)GBNNSS

In 2016 CABI scientists discovered a rust fungus (Puccinia komarovii var. glanduliferae) infecting Himalayan balsam in its native range.   Extensive testing confirmed that the fungus infects only Himalayan balsam and was therefore safe for release in the UK.  Two strains of the rust have been released at test sites in England and Wales and the results have been promising.  However, not all populations of Himalayan balsam are susceptible to these two rust strains and so CABI is currently working to locate new rust strains in the hope that they will be more compatible with the resistant Himalayan balsam genotypes.  This year, with funding from the NatureScot Biodiversity Challenge Fund,  a Tweed Forum project with CABI is extending the field trials, releasing the rust into Scotland for the first time.

New Zealand pygmyweed

A native of New Zealand and Australia, this plant can form dense mats in lochs and marshy ground.  It can choke out native species and reduce water oxygen levels making the habitat unfavourable for aquatic animals.

Crassula helmsii smothering pond - taken by Stewart Angus_m195124

New Zealand pygmyweed being invasive in Scotland, (C)Stewart Angus

CABI scientists have discovered a gall-forming Eriophyid mite from Australia which causes stunted growth of the plant in its native range.  Extensive testing confirmed its safety for release in the UK and it has subsequently been released at five sites in England and Wales during 2018-19.  Results have been promising with mites developing self-sustaining populations. Further field testing is ongoing and it is hoped that in the future the mites may help control New Zealand pygmyweed in  Scotland.


Gall-forming mite on bud (C)CABI

Floating pennywort

Native to the Americas, floating pennywort is widespread and damaging to waterbodies in England and Wales.  Thankfully it has not yet been recorded in the wild in Scotland, however, should it be in the future, we might be glad of some weevils form South America. The weevil Listronotus elongatues is currently being investigated for its suitability for release in England and Wales.


While biocontrol will not provide a silver bullet to rid us of invasive non-native plants, it is hoped that it will become another useful tool to be used as part of integrated pest management.


This article is published in memory of Dr Carol Ellison who recently passed away after a prolonged illness. Carol was a passionate and dedicated scientist, a tremendous contributor, mentor and coach to CABI’s science team and to our broader work on invasive species. She leaves a legacy of significant scientific achievement, including on the biocontrol of Himalayan balsam.

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