Fighting an exotic with an exotic: that may not sound smart. After all, that exotic fighter can also become a pest itself. Nevertheless, a consortium in the Netherlands received a permit to import a natural enemy of an Asian plant from Japan and release it here. It concerns the Japanese psyllium, which should help keep Asian knotweed in check. The sap-sucking insects of two millimeters in size survive the winter here, reproduce and inhibit the growth of knotweed. The consortium, called #uitde1000node, announced this this week.
They are a nightmare for site managers: Asian knotweed. Two species made their way from Asia to Europe in the past two centuries – the Japanese and the Sakhalin knotweed. The hybrid or Bohemian knotweed was created here by crossing. All three are doing well in our country. “Far too good,” says Suzanne Lommen of Leiden University and Koppert Biological Systems. She is coordinating the project with the Japanese psyllium. “Asian knotweeds are among the most invasive plant species in the world. They grow at lightning speed, displace other plant and thus animal species, and with their vigor they can make existing cracks in concrete and asphalt bigger.”
Electricity or boiling water
Administrators now use several methods to control the plant: grazing, mowing, digging, and killing the roots with electricity or boiling water. “But all those methods are expensive and labour-intensive,” says Lommen. “Besides, they don’t work well enough. Often the knotweed comes back in full force after you try to remove it.”
This is partly because new plants can grow from tiny pieces of roots. Working or moving soil therefore unintentionally causes extra spread of the exotic species. This happens, for example, with construction projects and the redevelopment of nature reserves. “That is probably also the reason that the Asian knotweed has only really become a problem in recent decades,” says Lommen, “while it has been in our country for a century and a half.”
In addition, the exotic knotweeds grow along ditches and canals and on dikes. Water boards suffer from this: the plants can clog pipes and weaken dikes. But municipalities are also concerned. “The question came from those two quarters: isn’t there another solution?”, says Lommen. “We have formed a consortium with a number of research institutes and funding parties, including eighteen water boards, a few large cities, but also Rijkswaterstaat and ProRail, for example.”
We had to work out all the possible risks
In 2019, the consortium applied for an exemption from the government to be able to use the Japanese psyllid to combat Asian knotweeds. Lommen: “It took months before the permit was finalized: we had to show extensively what we were planning and work out all possible risks.”
It’s not that crazy to fight exotics with exotics, says Lommen. Everyone has the specter in their head of the Asian ladybug, which was introduced in Europe to combat aphids, but since then has mainly caused a lot of nuisance in buildings, outcompetes native species and damages vineyards. “A lot of people don’t know that such stories are really exceptions,” says the researcher. “For more than a hundred years, people have used natural enemies to combat exotic plants, and they almost always succeed.”
Weevils and moths
In North America, for example, there are good experiences with controlling European ragwort with the European ragwort flea; introduced weevils and moths help control the South American water hyacinth in Africa. Lommen: “Europe is still very hesitant about these kinds of measures.”
For their license application, the researchers meticulously listed all the options and risks. “In Asia, the knotweed have thousands of natural enemies,” says Lommen. “Insects, mites, fungi. With a literature study and years of experiments, we have found out which of these we could best use. Ideally, the biological control agent only eats that one plant species.”
From that extensive analysis, the Japanese psyllid emerged as a candidate. “This species can only survive and build a population on the three Asian knotweeds,” says Lommen. Just to be sure, researchers tested how well it does on more than 140 Dutch plants, including all native members of the knotweed family. These include viper root, redshank, pig grass and all kinds of sorrel. “On none of these can the Japanese psyllium complete its life cycle.”
This strain is extremely specialized
Suzanne Lommen biologist
The risk is never zero, Lommen emphasizes. For example, you can never rule out the possibility that a species will adapt evolutionarily in the next tens of thousands of years in order to survive on other species. “But that seems very unlikely to me, because this species is extremely specialized.”
These findings were sufficient for a permit. That came in June 2020. And so the researchers placed Asian knotweed infested with psyllids at three test locations where a lot of knotweed grows: at a wastewater treatment plant in Zeist, along a bicycle path in Amsterdam-South and on the Wellenseind estate in Lage Mierde in Brabant. And there the insects are doing fine, as it turned out in 2021. Survival in the winter was good and the psyllids reproduce. The researchers found them up to 375 meters from where they had been released.
Three meters high
But how good are the tiny sap suckers really at tackling plants that can grow up to three meters high? “Once the plants are fully grown, the psyllids can do little more,” answers Lommen. “They mainly suck on the leaf buds, very young leaves and shoots.” If there are enough, the plants will only develop misshapen leaves and root growth will be inhibited. Then you can prevent a group of plants from expanding. Reducing an existing group is probably not realistic.
Leaf fleas are therefore not the solution, but an addition to other methods, Lommen emphasizes. How effective they really will be will depend on how quickly the introduced psyllids increase in number and how quickly they spread. “We will monitor this closely in the coming years. That is also necessary, according to our permit.”
The consortium hopes to be able to introduce psyllids in other places as well. “Administrators are eager for it. Also in Belgium. There were already people there who said: can’t we just come and get some psyllids from the estate in Brabant?”