Five thousand Japanese leaf fleas have been released in Amsterdam to combat Japanese knotweed, a once celebrated plant the concrete-breaking roots of which now threaten local biodiversity, impinge on water quality and increase the risk of flooding.
The Dutch government made the unprecedented decision to issue an exemption on a ban on the introduction of alien species in the face of spiralling costs related to the invasive species.
The Japanese knotweed, Fallopia Japonica, is causing major damage to building foundations, pavements and dikes in the Dutch capital, costing millions of euros a year. Laboratory tests suggest that the leaf fleas – Japanese knotweed psyllids, or Aphalara itadori – can kill young shoots and potentially stop the plant growing by sucking up its sap.
An initial 5,000 fleas have been released in three field locations. It is hoped that they will successfully hibernate over winter and establish themselves in the new year. Further specimens will be released next spring.
The Japanese knotweed was introduced and cultivated in the Netherlands as an ornamental plant between 1829 and 1841 by the German botanist Philipp Franz von Siebold. 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.
Its aggressive roots, which can grow up to 20cm a day and break through concrete or tarmac, have since been a major issue across Europe. Amsterdam has previously looked at using fire, hot water and even laser in controlling the plant’s growth without success.
Suzanne Lommen, an entomologist at the Institute of Biology in Leiden, the southern city where the Japanese knotweed was first introduced in the Netherlands, is coordinating the trial.
She said: “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.”
The Netherlands Food and Consumer Product Safety Authority (NVWA) has concluded that the psyllids do not pose a threat to native biodiversity.
Jaike Bijleveld, from the municipality, said there were a thousand sites in Amsterdam where knotweed had taken hold. “It’s a really big problem, but we’re working hard on it,” he told the Amsterdam newspaper Het Parool.
Lommen said there was a chance the fleas would not take to the Dutch climate. “What we do not know yet is how the psyllid will thrive in the Netherlands,” she said. “It comes from an area in Japan where the climate most resembles that of the Netherlands. In the laboratory, it thrives on the interbreed knotweed that grows here. But reality will show whether it can survive in our country.”