Japanese knotweed (Fallopia (= Reynoutria) japonica) is a highly invasive species in the UK, mainland Europe, North America and parts of Oceania. Since it was introduced as a prized ornamental from Japan in the mid-19th century, the species has become problematic in its entire exotic range where it has established, with detrimental impacts on native biodiversity and local infrastructure such as buildings and railways.
Due to its ability to form dense monocultures (areas dominated by one plant species) and to regrow from tiny fragments of rhizome (underground plant stems that grow roots and shoots), Japanese knotweed is often governed by special legislation in individual countries. In the UK, it is illegal to cause the species to grow in the wild as stipulated in the Wildlife and Countryside Act 1981.
Currently, the weed can be partially controlled using chemicals. However, this is costly and considered unsustainable because the weed spreads so easily and the use of chemical herbicides (weed killers) is being increasingly restricted.
A biocontrol programme targeting Japanese knotweed began in the UK in 2000. At the same time, research into this control approach started in the USA, shortly followed by Canada in 2007.
Field surveys revealed a range of natural enemies associated with Japanese knotweed in its area of origin. Out of these, the psyllid, Aphalara itadori (a small sap-sucking insect), and the fungal leaf-spot pathogen, Mycosphaerella polygoni-cuspidati, were selected as having the highest potential as biocontrol agents.
Both agents have been thoroughly assessed, leading to the release of the psyllid in the UK in 2010, in Canada in 2014 and in the USA and the Netherlands in 2020. The leaf-spot pathogen has been ruled out as a classical agent due to its potential to damage plants other than Japanese knotweed, but is currently undergoing evaluation as a possible mycoherbicide (a herbicide based on a fungus).
Research to date: psyllid
Aphalara itadori Shinji (Hemiptera: Psyllidae) is a sap-sucking psyllid damaging specifically to F. japonica, F. x bohemica and F. sachalinensis. After the approval for release by Defra in 2010, restricted releases were carried out from 2010-2013 to focus on safety and potential impacts on non-target species.
In subsequent trials set up in cages in the field, the safety and interactions with native invertebrates in the food chain were further investigated. The trials showed no detrimental impacts and, therefore, a new release licence was issued allowing psyllid releases at more appropriate river sites.
From 2015, large-scale releases and monitoring were carried out across England and Wales in coordination with local action groups and local authorities, and then with regional coordinators in conjunction with the EU RAPID LIFE project. Observations revealed that the psyllids reproduced and survived over winter on Japanese knotweed at several release sites. However, long-term establishment and persistent overwintering have proved challenging.
To tackle these issues, a further survey was carried out in Japan in 2019 to collect better climatically-matched psyllids. Extensive and severe leaf curling damage, attributed to psyllids, was found at Murakami in the Niigata Prefecture. Testing is being carried out to evaluate the specific performance of this potentially field-hardier psyllid on Japanese knotweed. The Murakami psyllid has been released in the Netherlands after approval for release by Dutch authorities in 2020.
The aim of this project is to: 1) complete host-range testing of the Murakami psyllid, 2) submit the petition document for release of the new psyllid line from CABI quarantine facility and 3) if approved by Defra, preparation for potential small-scale releases and monitoring will be carried out in the UK. Furthermore, the leaf curling response to the feeding of the Murakami line will be investigated.
Research to date: fungal leaf-spot pathogen
Mycosphaerella polygoni-cuspidati Hara (Mycosphaerellaceae, Ascomycota) is a damaging fungal pathogen of Japanese knotweed which is common and widespread on this plant species in its native Japanese range. While not considered suitable as a classical agent, the pathogen has genetic and biological properties which lend themselves to the potential development of a mycoherbicide, a product which would be applied in much the same way as a herbicide but has a fungus rather than a chemical compound as the active ingredient.
This mycoherbicide would be based on a single-mating type isolate which would prevent the fungus from reproducing, spreading and surviving in the wild, allowing it to be applied in a targeted way to Japanese knotweed, avoiding any damage to native species. This idea has been protected by a European patent in the name of the Secretary of State for Defra. International patent applications have also been filed.
Initial “proof of concept” research, which took place under quarantine in greenhouses, showed that the pathogen can impact the growth of infected Japanese knotweed plants. Following approval for its release from quarantine by Defra and permission for experimental field trials by the Chemicals Regulation Division, the biocontrol agent was found to also cause infection under more natural conditions during trials carried out in 2019 and 2020. A third trial is currently being planned for 2021.
CABI collaborates with a commercial biopesticide company to progress the research into this potential mycoherbicide, which is supported through funding by Dutch waterboards and other Dutch authorities.
Project-specific articles, reports and papers
Djeddour, D. H., & Shaw, R. H. (2010). The biological control of Fallopia japonica in Great Britain: review and current status. Outlooks on Pest Management, 21(1), 15–18. doi: 10.1564/21feb04
Grevstad, F.S., R.L. Winston, R.S. Bourchier, R. Shaw, J.E. Andreas, and C.B. Randall. 2018. Biology and Biological Control of Knotweeds. USDA Forest Service, Forest Health Assessment and Applied Sciences Team, Morgantown, West Virginia. FHTET-2017-03.
Grevstad, F., Shaw, R., Bourchier, R., Sanguankeo, P., Cortat, G., & Reardon, R. C. (2013). Efficacy and host specificity compared between two populations of the psyllid Aphalara itadori, candidates for biological control of invasive knotweeds in North America. Biological Control, 65(1), 53–62. doi: 10.1016/j.biocontrol.2013.01.001
Myint, Y. Y., Nakahira, K., Takagi, M., Furuya, N., & Shaw, R. H. (2012). Using life-history parameters and a degree-day model to predict climate suitability in England for the Japanese knotweed psyllid Aphalara itadori Shinji (Hemiptera: Psyllidae). Biological Control, 63(2), 129-134.
Shaw, Richard H, Bryner, S., & Tanner, R. (2009). The life history and host range of the Japanese knotweed psyllid, Aphalara itadori Shinji: Potentially the first classical biological weed control agent for the European Union. Biological Control, 49(2), 105–113. doi: 10.1016/j.biocontrol.2009.01.016
Fungal leaf-spot pathogen papers
Kurose, D. (2016). Studies on biological control of an invasive alien weed using plant pathogenic fungi. Journal of General Plant Pathology, 82(6), 338–339. doi: 10.1007/s10327-016-0681-8
Kurose, D., Renals, T., Shaw, R., Furuya, N., Takagi, M., & Evans, H. (2006). Fallopia japonica, an increasingly intractable weed problem in the UK: Can fungi help cut through this Gordian knot? Mycologist, 20(4), 126–129. doi: 10.1016/j.mycol.2006.07.021
Kurose, D., Furuya, N., Saeki, T., Tsuchiya, K., Tsushima, S., & Seier, M. K. (2016). Species-specific detection of Mycosphaerella polygoni-cuspidati as a biological control agent for Fallopia japonica by PCR assay. Molecular Biotechnology, 58(10), 626–633. doi: 10.1007/s12033-016-9962-x
Kurose, D., Furuya, N., Seier, M. K., Djeddour, D. H., Evans, H. C., Matsushita, Y., Tsuchiya, K. & Tsushima, S. (2015). Factors affecting the efficacy of the leaf-spot fungus Mycosphaerella polygoni-cuspidati (Ascomycota): a potential classical biological control agent of the invasive alien weed Fallopia japonica (Polygonaceae) in the UK. Biological Control, 85, 1-11.
Kurose, D., Evans, H. C., Djeddour, D. H., Cannon, P. F., Furuya, N., & Tsuchiya, K. (2009). Systematics of Mycosphaerella species associated with the invasive weed Fallopia japonica, including the potential biological control agent M. polygoni-cuspidati. Mycoscience, 50(3), 179–189. doi: 10.1007/s10267-008-0471-z
Seier, M. K., Kurose, D., & Evans, H. C. (2016). From classical to inundative control: Mycosphaerella polygoni-cuspidati as a potential mycoherbicide for Japanese knotweed. In H. L. Hinz, M. C. Bon, G. Bourdôt, M. Cristofaro, G. Desurmont, D. Kurose, H. Müller-Schärer, M. Rafter, U. Schaffner, M. Seier, R.F.H. Sforza, L. Smith, S. Stutz, S. Thomas, P. Weyl & R. Winston (Eds.), Proceedings of the XV International Symposium on Biological Control of Weeds, Engelberg, Switzerland, 26-31 August 2018 (pp. 91–93). Retrieved from https://www.ibiocontrol.org/proceedings/
Shaw, Richard H., Ellison, C. A., Marchante, H., Pratt, C. F., Schaffner, U., Sforza, R. F. H., & Deltoro, V. (2018). Weed biological control in the European Union: from serendipity to strategy. BioControl, 63(3), 333–347. doi: 10.1007/s10526-017-9844-6
Shaw, R. H., Tanner, R., Djeddour, D., & Cortat, G. (2011). Classical biological control of Fallopia japonica in the United Kingdom – lessons for Europe. Weed Research, 51(6), 552–558. doi: 10.1111/j.1365-3180.2011.00880.x