Invasives Blog

Damage to Japanese knotweed caused by the psyllid – credit: Eline Schuur.

A natural enemy – identified and reared by CABI scientists – to fight the scourge of Japanese knotweed in the Netherlands is showing early signs of success, a meeting of stakeholders heard recently.

The starting population of the tiny psyllid – Apalara itadori – was reared by CABI and was then transferred to Koppert Biological Systems to upscale the rearing for release against Fallopia x bohemica which, technically, is a hybrid of Japanese and giant knotweed.

There is evidence that some of the first release of 5,000 psyllids have successfully overwintered. Further psyllids have been released in the spring and summer of 2021 and are reproducing, spreading beyond their initial locations and have visibly damaged knotweed leaves.

Project Coordinator Dr Suzanne Lommen, of the Institute of Biology, Leiden University and Koppert Biological Systems, told partners, financers and other interested people at their final project stakeholder meeting –– that even lateral shoots of Japanese knotweed plants are being damaged by the psyllid.

Cheap and environmentally-friendly solution

Crucially, she added that there was no evidence of damage on non-target plants in the vicinity of the release sites reinforcing the results of the CABI team in the UK. She was later interviewed by national media in the Netherlands including evening news programme NOS Journaal who are keen to report on the progress of the fight against Japanese knotweed.

Dr Lommen said, “If the psyllid can establish, reproduce and spread, and do the damage we see in the greenhouse trials, it can hopefully inhibit the growth and spread of Asian knotweed in nature. Then we will have a very cheap and environmentally-friendly solution with many years of effect that you can combine with the more expensive methods.”

The field trials, as part of a consortium that also includes the knowledge institutes Probos and STOWA, the Pest Species Working Group of the Dutch Water Authorities, CABI and Koppert Biological Systems could take longer to see more lasting results as the psyllid needs to establish in larger quantities to really push the Japanese knotweed back. The project also has the support of 18 Dutch water boards and, among others, Rijkswaterstaat, ProRail and various municipalities including Amsterdam and The Hague.

Damage to Japanese knotweed caused by Aphalara itadori – credit: Eline Schuur.

Japanese knotweed was deliberately introduced to the Netherlands in the 19th century as an ornamental plant by the German botanist Philipp Franz von Siebold where he cultivated it in the Hortus Botanicus in the Dutch city of Leiden. Discovered by the side of a volcano, it was named as the “most interesting new ornamental plant of the year” by the Society of Agriculture and Horticulture in Utrecht in the late 19th Century.

Plant outcompetes native species

However, the plant has aggressive roots – that can grow up to 20cm a day – and outcompetes native species and can even break through concrete and tarmac by exploiting cracks. Not only is it considered a threat to local biodiversity but Japanese knotweed can also impinge on water quality and heighten the risk of localised flooding, the UK Guardian newspaper reported in October last year.

Johan van Valkenburg of the NVWA reiterated at the meeting of stakeholders how CABI started with the exploration as to whether such a biological control could be relevant to the Netherlands and was pleased to see how far the project has come.

Dr van Valkenburg was also keen to learn more about CABI’s work finding biological controls for floating pennywort and crassula.

Dr Janny Vos, CABI’s Partnerships Development Director, also outlined to the stakeholders CABI’s work on the damaging leaf-spot fungus Mycosphaerella polygoni-cuspidati as another potential weapon in the arsenal against Japanese knotweed.

The fungus attacks the plant in its native range but was found not to be suitable as a classical biocontrol agent. However, the pathogen is considered to hold potential as a mycoherbicide. The project ‘Evaluating the mycoherbicide potential of a leaf-spot pathogen against Japanese knotweed’ is investigating its potential in this regard in collaboration with private industry.

Previously a range of methods have been tried to tackle Japanese knotweed. Officials in Amsterdam, for example, have tried using fire, hot water, electrocution and even laser to control the plant’s growth but without success.

It is hoped the humble Japanese psyllid Apalara itadori – either in isolation or integrated with other sustainable tools – will prove effective in the long term and if so, the scientists may advocate its use in more places to help prevent the spread of Japanese knotweed.

Additional information

Main image: The psyllid Aphalara itadori at a Dutch field site (Credit: Janny Vos, CABI).

Nos Journaal TV report

You can see Dr Suzanne Lommen of the Institute of Biology, Leiden University, talk about the early success of the psyllid Aphalara itadori as a natural enemy to Japanese knotweed in the Netherlands (in Dutch) on national TV here.

Project page

For more information on CABI’s work on Japanese knotweed see the project page ‘Establishing the psyllid: field studies for the biological control of Japanese knotweed.’

Relevant news story and blogs

See also the news story ‘CABI teams up with colleagues to pioneer the biological control of Japanese knotweed in the Netherlands’ and the blogs ‘Biological control for Japanese knotweed comes into sharp focus during second release of psyllid’ and ‘CABI supports first Dutch field trials with exotic insect to combat Japanese knotweed.’

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