Japanese knotweed - could a tiny insect tame the monster?

| 17th October 2014
Japanese knotweed makes short work of concrete and tarmac. In its native habitat, it has learnt to crack up volcanic rock. Photo: Rob Tanner.
Japanese knotweed makes short work of concrete and tarmac. In its native habitat, it has learnt to crack up volcanic rock. Photo: Rob Tanner.
Since Japanese knotweed won a gold medal in 1847 as 'interesting new ornamental of the year', it has become far too much of a good thing, writes Kate Constantine. But could the oriental triffid be tamed following the UK introduction of a specialist pest from Japan's volcanic uplands?
In its native range in Japan Japanese knotweed grows on volcanic rock, hence its resilience and concrete-cracking abilities.

You may have heard of, and possibly even seen, Japanese knotweed (Fallopia japonica) growing near you it's so abundant.

This easily spread and problematic non-native weed favours disturbed ground, dominating other vegetation wherever it grows, often to the detriment of our native species.

Growing through concrete, tarmac and other construction materials, it also severely affects urban environments.

It's almost impossible to constrain, but could the specialist Japanese psyllid, Aphalara itadori contribute to reducing Japanese knotweeds' path of destruction?

Brought to Europe in the late 19th century as a prized ornamental, Fallopia japonica was awarded a gold medal in 1847 for the most interesting new ornamental of the year (by the Society of Agriculture & Horticulture).

Its numerous virtues included its vigour and ornamental beauty combined with medicinal properties - it was highly valued in Japanese and Chinese medicine (Bailey & Conolly, 2000).

Japanese knotweed belongs to the Polygonaceae family, along with dock, sorrel and rhubarb. However it is now evident that the plant's (once) perceived positives are outweighed by its negatives. Japanese knotweed has become a serious non-native invasive plant across Europe.

Why is it a problem?

Japanese knotweed is highly invasive and very difficult to control let alone eradicate. The plant grows and spreads via its underground stock of rhizome from which fragments break off and a whole new plant can grow - interestingly Japanese knotweed plants in the UK all originate from a single female clone!

In its native range in Japan it grows on volcanic rock, hence its resilience and concrete-cracking abilities. The plant particularly thrives in disturbed habitats, is associated with human activity and is the bane of developers.

Under the UK Wildlife and Countryside Act (1981) it is illegal to cause Japanese knotweed to grow in the wild and it must be disposed of as hazardous waste to prevent it spreading through contaminated soil.

This hardy perennial dies back over winter when the dead hollow stems can contribute to blocking surface water drainage channels exacerbating flooding.

Traditional control methods are difficult and expensive to implement. When digging up the root system for example, disturbing the rhizome only promotes further growth.

Chemical control is not always favoured or permissible in some environments where knotweed grows, especially near water bodies. In areas where chemicals can be applied, it is best done at certain times of the year and often requires multiple applications, which can be very costly.

What to do to solve this knotty conundrum?

A consortium of concerned sponsors (see below) initiated research into classical biological control in recognition of the need for an alternative and sustainable control option for this problematic plant.

This approach involves importing, releasing and establishing exotic natural enemies (predators, parasites and pathogens) to reduce exotic pest populations to, and maintain them at, densities that are economically and environmentally acceptable.

An essential requirement of any introduced natural enemy is that it is highly specific to the target invasive plant, and does not have any impact on non-target native species.

The development of a classical biological control programme is a time consuming process. Back in 2000, scientists conducted initial surveys of Japanese knotweed in its native range and recorded over 200 insects and pathogens attacking the plant - none of which constrain the plant in the UK.

Of these numerous natural enemies, only a handful were deemed suitable for detailed safety testing under strict quarantine facilities in the UK.

10 years of research in a quarantine facility

To ensure safety, any potential insect biological control agents are extensively tested through feeding, oviposition (egg laying behaviour) and development tests all done in strict quarantine facilities.

A sap sucking herbivorous insect, Aphalara itadori (a psyllid, related to aphids), was brought to a UK quarantine facility for testing to ensure that it only damages and survives on Japanese knotweed.

In Japanese, itadori actually means 'Japanese knotweed' indicative of the insect's closely co-evolved relationship with the plant. In fact most psyllid species are specialists.

Since it is impossible to test all plant species in the UK, the selection of plants to test is an essential component of any biological control programme.

The Japanese knotweed test plant list was composed of 90 species including all native plant species from the same family (Polygonaceae) as the weed, as well as species sharing a similar habitat and plant species of economic and social importance.

So in case you're wondering - yes, rhubarb growers are safe from infestation by an unwelcome new pest. And no, the psyllids will do nothing to stop dock invading your flowerbeds.

The tests consisted of exposing test plants to psyllids.

  • First they were given a single plant to lay their eggs on, revealing if eggs were laid on any non-target plant;
  • Second, the psyllids were given a choice of several plants to lay their eggs on, to see if they chose Japanese knotweed.
  • Then any eggs that were laid were followed through in development tests to see if the recipient plant was a true host.

The results confirmed the psyllid to be a highly specialized natural enemy of Japanese knotweed. Females are highly unlikely to lay eggs that stood any chance of developing into adults on any species other than the invasive knotweeds (Shaw et al., 2009).

Building up psyllid populations

The research culminated in an application to release the psyllid in the UK under UK Government license and permission was granted in 2010.

Releases in 2010 were made at three sites, and in the following years at a total of eight sites in England and Wales. All of which are still closely monitored. For each release site there is a control site with no psyllids, and repeat visits each year documents any impacts of the psyllid on the weed and the surrounding environment.

The challenge now is to get the psyllid to establish in the natural environment which, as for any programme of this type, isn't easy. Aside from establishment it takes a new species time to build up large enough populations to have any significant impact on the target plant and to persist. This is why biological control projects often take years to show any effect.

To date, some overwintering of the psyllids at release sites has been observed, and, as expected, no negative effects on any native species have been recorded but unfortunately populations have remained too small so far to give conclusive results.

The EU's first officially sanctioned release

Despite the currently limited impact on Japanese knotweed in the UK, the release of the psyllid, Aphalara itadori, was a huge step in the natural control of invasive plants, being the first officially sanctioned release of an insect against a weed in the European Union.

Very recently the highly invasive non-native Himalayan balsam has had a rust fungus released to control it - the first instance of a pathogen against an invasive weed in the EU.

A key finding from invasion biology is that there is usually a time-lag between the introduction of a non-native species and the time it takes for its impacts to become apparent (supporting the notion that preventing introductions of potential pests and weeds in the first instance is so important).

The same can also be true of biological control agents - at first nothing appears to have changed but with time the situation could be reversed as history starts to catch up with these aggressive invaders.

And then a more natural equilibrium is restored. For biological control doesn't aim to eradicate the target plant, but rather to reduce its vigour and invasiveness, whilst maintaining the control agent population.

Keeping the invasive plant in check ideally allows native plants to re-establish and flourish, increasing biodiversity and reducing environmental and economic impacts.

And while it's unlikely to win any more horticultural gold medals, who knows - in years to come a tamed Japanese knotweed may be seen as no more than a mild nuisance, as the once notorious weed takes its modest long term place among Britain's naturalised flora.



Kate Constantine is a Project Scientist at CABI, an international not-for-profit organization that improves people's lives by providing information and applying scientific expertise to solve problems in agriculture and the environment.

More information: Japanese knotweed alliance.

Eat it: 'Chef cooks up recipes for Japanese Knotweed'.


  • Bailey, J.P. and Conolly, A.P. (2000). Prize-winners to pariahs - A history of Japanese Knotweed s.l. (Polygonaceae) in the British Isles. Watsonia, 23, pp. 93-110.
  • Shaw, R.H., Bryner, S. and Tanner, R. (2009) The life history and host range of the Japanese knotweed psyllid, Aphalara itadori Shinji: Potentially the first classical biological weed control agent for the European Union. Biological Control, 49: 105-113.

include the Department of Environment, Food and Rural Affairs (Defra), Environment Agency, Welsh Assembly Government, South West Regional Development Agency, Network Rail and the Canal & River Trust (previously British Waterways) all coordinated by Cornwall Council.


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