Psyllid bug Aphalara itadori(Photo from CABI website)

Dear Readers, on my Open University course we are currently exploring the world of invasive species, and none arouses my interest more than Japanese Knotweed. I’ve written about this plant before, back in 2015, but I wondered how things were going with the biological controls that were suggested to tackle the plant.

For those who haven’t had the mixed pleasure of encountering this plant, Japanese Knotweed (Fallopia japonica) was imported to the UK as an ornamental by those pesky Victorians. Once here, it decided that it liked the place so much that it has set up shop in many areas, including the edges of Muswell Hill Playing Fields. The plant is actually sterile (there are no male plants in the UK), but the roots spread easily, and the smallest fragment of root can generate a whole new thicket. It is extremely difficult to eradicate, requiring either systematic digging out of every fragment of root for years, or multiple dosing with glysophate. It’s true that Japanese ladies harvest the tender tips and eat them, but you’d need an awful lot of foragers to deal with this lot.

Japanese Knotweed on Muswell Hill Playing Fields

Attention turned to the possibility of finding a biological control for the plant. The scientists had genetically analysed the Japanese Knotweed in the UK, and discovered that it came from a small area near Nagasaki. In the wild, Japanese Knotweed lives on volcanic soil and is a resilient and determined coloniser of these sites, so it’s no wonder that it likes the disturbed soil where it’s often found in the UK.

Once the plants’ genetic inheritance was established, it was a case of investigating what was preying upon it, and then testing the predators to ensure that they had a very tight preference for Japanese Knotweed. The best candidate was a ‘true’ bug (in the sense of being an insect that sucks sap), Aphalara itadori (pictured above). It was chosen because the scientists at CABI (The Centre for Agriculture and Bioscience International) identified it as the ‘bug most likely to’ cause substantial damage to Japanese Knotweed, without transferring its affections elsewhere once the host plant was demolished.

Large scale releases of the insect have taken place under licence since 2015, but results have been mixed. The adult insects overwinter in vegetation near the Japanese Knotweed, and prefer pine trees, which can present problems if there aren’t any nearby. At several sites the bugs did survive the winter anyhow, but long-term establishment and reproduction seem to be a problem.

However, in 2019 a different bug was found eating Japanese Knotweed in the Murakami district of Japan, and there are high hopes that this insect will prove to be a bit hardier in the field. It’s currently being tested to ensure that it only eats what it’s supposed to eat, and to investigate its life cycle in detail. It will be interesting to see if this insect proves to be a better biological control than the previous bug.

Psyllid nymph damage on Japanese Knotweed (CABI)

Another possibility is a fungal pathogen that causes leaf spot on the plant in its native Japan. It would be much too risky to release this as a whole organism, but work is on hand to develop a mycoherbicide: this would use the active fungal ingredient, which has been treated to prevent it from reproducing or spreading. If it works, it could be sprayed onto Japanese Knotweed without any impact on any surrounding plants, or on the waterways that it often grows along. Let’s see how this turns out.

Japanese Knotweed leaves infected with leaf spot fungus

The nineteenth century was a time when the Victorians were cheerfully moving plants and animals from one place to another without any thought for what was going to happen if their ‘finds’ established themselves elsewhere. Himalayan Balsam, Giant Hogweed and Japanese Knotweed are just three of the plants that have escaped, with serious detrimental effects on habitats and native flora. There’s nothing wrong with any of these plants per se – they all have their charms and their uses as part of their native habitats. But having seen how Japanese Knotweed can completely screen out all the other plants that it grows with it’s clear that something has to be done, and in general I’d rather it wasn’t spraying with Round-Up. On the other hand, I also have an instinctive worry about fighting fire with fire. Let’s hope that all the research into the insects and fungi that are currently being mentioned with regard to Japanese Knotweed has been thorough enough to prevent unanticipated problems.

Another fine stand of Japanese Knotweed