Japanese Knotweed and Psyllid Aphalara Itadori: The Ongoing Battle
Since Victorian times, when Japanese knotweed was introduced to the UK, scientists and researchers have been studying this invasive plant to understand how it can be controlled.
Around £41million is spent every year on the control and removal of Japanese knotweed, with even more wasted through failed attempts and court proceedings. Currently, the only successful way to remove knotweed is through the application of herbicides by licensed professionals, combined with another form such as burial, or removal of the plants once dead.
As Japanese knotweed is so tricky to remove, and causes so many problems, governments across the world formed the Japanese Knotweed Alliance in 2001. The original partners included primarily UK-based agencies but have since expanded to include the USDA Forestry Service and British Columbia’s County Environmental Health, as well as numerous agencies in the Netherlands.
One of the primary areas of focus for the Japanese Knotweed Alliance has been on biocontrol for Japanese knotweed. Unlike glyphosate, a biocontrol is something like a bug or naturally occurring fungus that affects the growth of a plant, slowing it down or potentially killing it. In Japan, there are numerous biocontrols that limit the spread of Japanese knotweed, but as none of these exists in the Western world, Japanese knotweed has been able to spread uninhibited, overtaking natural plants and impacting vegetation.
The work to control Japanese knotweed
Since the early 1990s, the government has been debating the effective removal of Japanese knotweed. Glyphosate was tested in the late 1990s and found to be a suitable resolution.
While glyphosate is currently the only effective way to permanently remove Japanese knotweed, it is not an ideal long-term solution. Not only due to the need for professionals to apply it, but the fact that knotweed is spreading rapidly across the globe, and glyphosate cannot be sprayed across every inch of the planet. Professional application of glyphosate is successful in removing knotweed, but there is always research into looking for other similarly effective ways.
CABI (Centre of Agricultural Bioscience International), a UK-based research not-for-profit organisation, is aiming to find and release a biocontrol that can limit the spread of Japanese knotweed and survive fluctuating weather conditions.
Introducing Psyllid: Aphalara Itadori
Across Japan, over 200 species of psyllids, which are small sap-sucking insects, feed off the sap in Japanese knotweed, inhibiting its growth. There are also numerous fungal infections that stop Japanese knotweed from growing, which prevents it from taking over the countryside of Japan.
One specific psyllid, Aphalara Itadori, was deemed to be the most effective, as it exclusively feeds on the sap of Japanese knotweed plants. Since the early 2000s, researchers have been studying these tiny insects to see how they affect knotweed plants.
Early studies in controlled conditions proved promising, with the psyllids causing knotweed plant leaves to curl up and die, restricting the plant’s ability to self-reproduce. As all the Japanese knotweed plants in the UK are female and are asexually reproducing, restricting the plants’ ability to photosynthesise and reproduce is a significant step in the right direction.
Overcoming challenges
Unfortunately, an issue arose once the psyllid Aphalara Itadori was released into test locations across the UK. The Japanese weather, where these psyllids had been born was vastly different to the humid and often wet conditions in the UK. As a result, the psyllid population struggled to survive the winter. These psyllids were originally collected from Mount Aso, in the Kumamoto Prefecture in Japan. They then reproduced and survived in the short term at various knotweed sites across the UK. However, long-term, success was limited.
In 2019, a survey was conducted by CABI to find a more climatically suited psyllid. At Murakami, a small seaside city, extensive and severe leaf damage attributed to a psyllid, was found. Murakami is more climatically similar to the UK, and further studies are currently underway across the Netherlands, the UK and Canada. These studies will determine the wintering viability of these insects, as well as their long-term viability in the wild.
Are biocontrols the answer to our knotweed problem?
Essentially, no. Even if biocontrols like psyllids were able to survive in the various climates across the UK, they won’t actually kill Japanese knotweed. Instead, they’ll limit how much it can spread.
For example, the psyllid population in Japan hasn’t killed off knotweed, instead, they’ve halted the spread of the plant and found a way to naturally control it. It’s hoped that the psyllid population will act as a natural predator and reduce it to nothing more than a shrub.
Biocontrols, in general, are a more long-term solution. Often, taking natural predators from one habitat where certain factors exist and migrating them to another unfamiliar habitat causes problems, such as other native species being affected. Long research studies in controlled areas are required, which means that, unlike chemical testing, biocontrols can take many years to develop and roll out.
So, how do we eradicate Japanese knotweed?
Right now, the most effective way to eradicate Japanese knotweed is to use a herbicide with careful removal and disposal, conducted by professionals. As a small fragment the size of a thumbnail can cause knotweed to spread, simply hacking it back makes the problem worse.
In the future, it’s hoped that biocontrols, combined with highly targeted herbicide treatments, will help to rein in this invasive species. Using biocontrols will help reduce the environmental impact of removing Japanese knotweed, as well as protecting the plants around the knotweed from dangerous herbicides.
If you’ve got Japanese knotweed, we can help
Here at Japanese Knotweed Specialists, we’re experts in Japanese knotweed removal and are always looking for ways to be more sustainable in how we work. We’re excited by the prospect of more eco-friendly ways to eradicate knotweed.
If you suspect Japanese knotweed on your property, get a survey with one of our specialists.