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Alun Davies, Minister for Natural Resources and Food

First published:
3 June 2014
Last updated:

This was published under the 2011 to 2016 administration of the Welsh Government

Japanese knotweed is one of the most damaging invasive weeds in the UK.  It can grow up to a metre a month and can push through tarmac and concrete.  Its effect on native species is often devastating as it out-competes indigenous species covering large tracts of land to the exclusion of the native flora and associated fauna.

Japanese knotweed is both difficult and expensive to control and I am acutely aware of the problems it causes for homeowners and landowners, particularly in South Wales.  Tackling this issue requires innovative solutions which is why I am pleased that the Welsh Government with other UK administrations has been investigating whether natural control is a feasible method for the long-term, sustainable management of Japanese knotweed.  The associated scientific research is being conducted by the Centre for Agriculture and Biosciences International (CABI) who investigated more than 200 insects and fungi that feed on Japanese knotweed in its native range in Eastern Asia.  From this initial research the sap-sucking psyllid Aphalara itadori was identified as the best candidate to help control knotweed in the UK.

After gaining the necessary approvals, the first psyllids were released at two sites in Wales and a number in England in spring 2011 and periodic releases have since taken place.  The project is currently in its fourth year of five and involves a detailed monitoring plan, including sampling, site visits and reporting.

Since the first release in 2011, low densities of adults have been found at some of the release sites indicating successful overwintering.  However, as yet the organism has had difficulty establishing self-sustaining populations and so this year CABI will conduct caged field trials that will run concurrent to the main trials.  The caged trials will involve releasing larger numbers to establish higher population densities with the aim of demonstrating effective damage to the knotweed.

Natural control is not a quick fix and the benefits can take some time to be fully realised.  Experience from around the world has shown that bio-control for most species takes between five and ten years from the initial release until the time significant control is achieved.  The early signs are encouraging for the establishment of this highly specialist psyllid.  Despite poor summer weather since its release, the psyllid has shown that it can survive in small numbers and overwinter in the wild here.  The next challenge for the project is to encourage the psyllid to achieve survival in larger numbers that are capable of the greater levels of control.

The success of this project would provide numerous benefits to Wales in terms of reduced herbicide use and cost, as there is virtually no need for further expenditure once the psyllid is established and a sustainable method of control.

The Welsh Government is also supporting a two-year trial at Swansea University examining the chemical control of knotweed.  These knotweed field trials are the largest of their kind ever undertaken in Europe or North America and should provide additional information to aid the management of this weed.