Invasives Blog

For the first time in the Netherlands, an exotic insect species is released into the wild to combat a harmful plant species. The Japanese knotweed psyllid should offer relief against the rampant Asian knotweed. Suzanne Lommen of the Institute of Biology Leiden is coordinating the field trials as part of a consortium which includes CABI, Leiden University, Koppert Biological Systems, Probos, STOWA and the Pest Species Work Group of the Dutch Water Authorities. The project also has the support of 18 Dutch water boards and, among others, Rijkswaterstaat, ProRail and various municipalities.

Here we re-blog an article which appeared on the Leiden University website to share the good news!

Worldwide it is prohibited to release exotic species into nature. For the first time in Dutch history, the government has issued an exemption to introduce an alien species in the Netherlands to combat a plant. Project #uitde1000knoop, in which Leiden University participates, starts field experiments this week with the Japanese knotweed psyllid (Aphalara itadori) as a weapon against the Asian knotweed. Dr Lommen coordinates research on the Japanese knotweed psyllid in the Netherlands. She answers some questions about the project.

Why do you want to fight knotweed? Isn’t that an ornamental plant?

‘This is about the Asian knotweed, and by that we mean the Japanese knotweed, the Sakhalin knotweed and the interbreed of the two. The first two are indeed ornamental plants that were deliberately introduced to the Netherlands. In the 19th century, the researcher Von Siebold brought the Japanese knotweed from Japan to Leiden, where he cultivated it. However, these species appear to be invasive here, crowding out native plant species. Additionally, their vigorous roots can cause damage to foundations, pavements and dikes, amongst other things. They are difficult to control and that costs millions of euros per year.’

Visible effect in the laboratory: the Japanese knotweed psyllid slows down the growth of the Asian knotweed. Plant with psyllid on the left and plant without psyllid on the right (Photo: CABI).

Why is a new method needed?

‘All sorts of things have been tried, but complete pest control is extremely difficult and very expensive. We will have to combine various methods to get the Asian knotweed under control. We know from the Japanese knotweed psyllid that it can kill young shoots and slow down or even stop the growth of the plant by sucking up sap – nutrition – from the plant.’

‘If the psyllid can establish, reproduce and spread, and do the damage we see in the breeding trials, it can hopefully inhibit the growth and spread of Asian knotweed. Then you have a very cheap and environmentally-friendly solution with many years of effect that you can combine with the more expensive methods.’

Are there any risks associated with the release of the psyllid?

‘You want to rule out that the knotweed psyllid attacks other plants. There are very reliable methods to determine this. For example, extensive risk analyses have been performed for plant species in North America, England and Northwestern Europe. This shows that the Japanese knotweed psyllid does not pose a risk to native plants or plants of economic value. All native Dutch species of the knotweed family have also been tested, and none proved to be a suitable food source for the Japanese knotweed psyllid. The Netherlands Food and Consumer Product Safety Authority (NVWA) has concluded that the Japanese knotweed psyllid does not pose a threat to native biodiversity.’

Japanese knotweed psyllid (photo: CABI)
Japanese knotweed psyllid (Photo: CABI)

‘What we do not know yet is how the psyllid will thrive in the Netherlands. He comes from an area in Japan where the climate most resembles that of the Netherlands. In the laboratory, he thrives on the interbreed knotweed that grows here. But reality will show whether he can survive in our country.’

When do you expect the results of the field tests? And how will it continue after that?

‘We will release the knotweed psyllid on three field locations. After the winter we hope to find specimens there as proof of successful wintering. Next spring, we will release them again to see if they establish and reproduce. Only when the psyllid appears in large quantities can we tell if it is doing enough damage to the Asian knotweed to push it back. It will take a few years before we see any results. I do not expect any side effects and hope for a quick establishment. If we can confirm that, we hope to release the knotweed psyllid in more places in the future to help its spread.’

Header image: Asian knotweed proliferates in Lage Mierde, next to river de Dommel (Photo: Suzanne Lommen).

Additional information

Project #uitde1000knoop (‘untangling the knotweed’) is carried out by a consortium of the Center for Agriculture and Bioscience International (CABI), Leiden University, Koppert Biological Systems, Probos, STOWA and the Pest Species Work Group of the Dutch Water Authorities. With the support of eighteen Dutch water boards and, among others, Rijkswaterstaat, ProRail and various municipalities, the consortium investigates how biological control may combat the proliferating Asian knotweed. Biological control uses a specialised enemy from the pest’s area of origin, in this case, the Japanese knotweed psyllid (Aphalara itadori). The goal is to control the invasive species and restore ecological balance.

Project page

Find out more about CABI’s work ‘Establishing the psyllid: field studies for the biological control of Japanese knotweed’ from the project page.

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