Japanese knotweed is one of the most damaging invasive weeds in the UK, Europe and North America. Growing up to a metre a month, it can push through tarmac and concrete. Its effect on native species is often devastating as it out-competes indigenous species covering large tracts of land to the exclusion of the native flora and associated fauna. Fly tipping, inappropriate management and poor control efforts have exacerbated the problem.
The cost of Japanese knotweed to the British economy has been estimated at £166 million.
The psyllid Aphalara itadori is a true knotweed specialist that sucks the sap from the plant. It is about 2mm in length and its nymphs are capable of causing significant damage to the target weed. Click here for more information on the psyllid >>
A six-year scientific research project was coordinated by a project management board including Cornwall Council, DEFRA, the Environment Agency, Welsh Assembly Government, Network Rail, South West RDA and British Waterways to establish whether natural control is a feasible method for the long-term, sustainable management of Japanese knotweed in Great Britain. The research was carried out by CABI.
Out of more than 200 insects and fungi feeding on the plant in its native range, extensive research has shown that Aphalara itadori is the best candidate to help control Japanese knotweed in Britain. The psyllid has been tested on 90 non-target species of plant including those closely related to Japanese knotweed as well as ornamental plants and important crops and these were not affected.
The research has been peer-reviewed by independent scientists and the risk assessment for the study assessed by the Advisory Committee on Releases to the Environment (ACRE) amongst others.
One tiny organism on its own will not have much effect but once it has reproduced for a few seasons with a very large food supply, the population can grow to become a force to be reckoned with. Although the population of a species does not normally expand very rapidly, this is much more likely to occur in the case of a natural control agent, since they too have lost their natural enemies. The psyllid should continually debilitate the plant and prevent it from competing as successfully.
Efficacy studies have been carried out in the quarantine laboratory on potted plants; these showed that the psyllid nymphs, even in relatively low numbers, could reduce the rate of growth of potted knotweed plants, causing stunting and deformation of the upper leaves. However, the proof can only come in the real conditions in the field and the monitoring programme currently in place will reveal efficacy.
Yes. A full public consultation was carried out in the third quarter of 2009.
No. When the psyllid was released it was the first intentional release of a non-native organism to control an invasive non-native plant in Europe. However, there is a long history of using natural enemies to control insect pests in Europe. An example is the release of a specialist predator to control the invasive spruce bark beetle which to this day is still providing a long term solution to this economic pest. In addition bio-controls are regularly used on a smaller scale within the horticultural industry and even in householders’ glasshouses for example to control aphids and other crop pests.
Yes. There is an example regarding an animal species. The non-native predatory beetle, Rhizophagus grandis, was released under licence in the mid 1980’s to tackle the invasive non-native spruce bark beetle (Dendroctonus micans). The predator is now well established and follows the spruce bark beetle as that spreads, keeping the latter under control. Occasional releases are made where distribution of the spruce bark beetle ‘jumps’ e.g. is accidentally transported some distance. This is an example of natural, or biological, control as a non-native species was deliberately introduced in order to tackle another invasive non-native species. Aphalara itadori will be the first such example targeting an invasive non-native plant species.
There have been over 1400 releases of natural control agents against weeds around the world. In all, more than 400 different agents have been released against more than 150 different target weeds. CABI is a world expert in natural control and played a vital role in the development of an International Code of Conduct on the use of natural control. CABI also has a very good international track record of researching and releasing natural control agents. In countries like the US, Canada, Australia and New Zealand natural control is often viewed as the first line of defence when a new and problematic invasive species is identified. See more examples of natural control successes >>
The scientific research into the psyllid has been published to the scientific community and conducted by an organisation with great expertise in this area. Further scrutiny by the UK regulators (FERA and WAG), the independent Advisory Committee on Releases to the Environment (ACRE), and a peer review by three independent scientific experts have all helped ensure that possible risks have been adequately identified and carefully considered so that the release is considered safe.
There are no examples of biological control agents shifting host to plants that were not within their original physiological host range. Research in the laboratory showed that the psyllid is a specialist and as such has sacrificed its ability to feed and reproduce on anything other than the target knotweeds. This conclusion was supported by results from caged and open field trials in 2010.
Yes. This is a phased release programme with in-built contingency plans. Further releases can only continue if no significant adverse effect on non-target species has been recorded.
It has been released at isolated sites in Spring 2010. There will be a further release across England and Wales during 2011 as part of a phased approach.
The exact location of the release sites has not been published in order to minimise risk of disturbance and avoid compromising the release phase and the monitoring requirements expected to form part of the conditions of the licence. This is not because of a risk or danger to the public but because we want to give the insect the best chance of establishing so that it can successfully survive the next winter, as well as ensuring that the sites are not disturbed and the data compromised. The release represents the culmination of a very long process of research that has taken years and it is important that it goes well if we are to realise the benefits. These requirements have been written into our licence conditions by the regulators.
Release of the psyllid into the wild is on a phased basis, the initial release having happened at three sites, which were then closely monitored to ensure that it behaved as expected according to the laboratory research and to detect any evidence of unexpected behaviour. Further phases of release to specified sites have now been approved in England and Wales and will happen, again subject to close scrutiny, before a final programme of release in 2011. If and when the psyllid is considered ordinarily resident the licence would be reviewed.
At the initial release sites, it is expected that some adults will be released from a container into mesh sleeves on a plant to give them the best chance of settling in, while others will be released more freely to enable their behaviour to be monitored. There are also a number of control sites at which no psyllids will be released but will also be part of the monitoring programme.
The sites are being monitored intensively as required under the licence. Phase one was more about the psyllid settling in, surviving and not doing anything unexpected but the knotweed was also observed to determine any impacts. The adults are going to have to feel ready to lay eggs and it will be the nymphs that emerge that will affect the growth of the knotweed.
A detailed five-year monitoring plan covering sampling, site visits and reporting amongst other things has been prepared and agreed with FERA and WAG. This ensures that FERA is kept adequately informed of developments and it is a requirement of the release license that funding for the monitoring plan and contingency measures was in place from the start.
Natural control is not a quick fix and the benefits can take years to be fully realised. The monitoring programme will generate the data required to determine whether there is significant control of Japanese knotweed at the release sites, i.e. a reduction in size of the plants and the rate of knotweed patch expansion. The ultimate measure of success will be whether the presence of the psyllid leads to a reduction in the efforts and costs required to control knotweed as well as the reduction in its rate of spread. The general consensus is that any contribution to a reduction in current control costs and efforts that outweighs the cost of the programme should be considered a success.
Experience from around the world has shown that bio-control for most species takes between 5 and 10 years from the initial releases to the time significant control is achieved. Whilst more traditional management techniques such as mechanical and chemical control seem to offer more immediate control, one of the most important advantages of natural control is the long-term management it provides with minimal disturbance to the environment and reduced use of chemicals.
No. The psyllid will put natural pressure on it if it establishes successfully in Great Britain – but it won’t make it disappear altogether. It is not in its interests to remove its only food supply. The ultimate aim of this project is to turn the weed from a destructive and expensive environmental burden, into a more manageable plant which poses less of a threat to economic interests and our biodiversity. The advantage of successful bio-control is that knotweed should not spread as fast or establish as well however, we might not see spectacular results for many years.
There is still a risk of spread of Japanese knotweed through bad management which could be harmful to the environment and make the problem even worse. It is anticipated that the biocontrol agent will be integrated with other management techniques and therefore facilitate, not replace, current management techniques.
The cane toad, Bufo marinus, was introduced to Australia by the sugar cane industry in 1935 in an ill-judged attempt to control pest beetles. This was done against the recommendations of scientists at the time. Thousands of toads were released without any host specificity testing and not only failed to control the beetle but turned their carnivorous attention to anything that moved and was small enough to be swallowed. They went on to become a significant problem themselves. Although carried out in the name of biological control, today’s practitioners consider this unfortunate case to have been a highly irresponsible act in an age when there was no real regulation and safety testing. Today’s pest risk analyses should prevent such a thing happening again
Japanese knotweed can be identified by its tall, hollow and arching bamboo-like stems that often zig-zag. The leaves are large, green and spear shaped and stems produce clusters of creamy-white flowers in late summer. In winter the living material dies back leaving brittle brown stems but the rhizomes persist underground with an orange-yellow interior which snaps like a carrot when broken. For more detailed information click here.
An invasive non-native species is any animal or plant that has been introduced (i.e. by human action) outside its natural past or present distribution and has the ability to spread causing damage to the environment, the economy and the way we live.
Invasive non-native species are estimated to cost the British economy £1.7 billion a year. They are one of the greatest threats to the environment worldwide, and their impacts can be far reaching – they have adverse impacts on the native environment by predation, competition and spread of disease. They also threaten economic interests such as agriculture, forestry, fisheries and development.
Invasive non-native species are often introduced either as pets or garden plants, and their pests, or as stowaways within imports of compost, timber, ballast water and other materials.
It is in all our interests that damaging invasive non-native species are not released into the wild, and the priority in the government’s GB non-native species secretariat - GB Strategy is to educate the public about the risk of introducing these species. However where an invasive non-native species is found we need to consider whether to control it, and the possible tools available. Invasive non-natives usually have an advantage over native species as they have often left their natural enemies behind – one tool is the use of natural predators that target only the invasive species.
There have been many notable successes in the natural control of weeds, although of course some work better than others. The general consensus is that any contribution to a reduction in current control costs and efforts that outweighs the cost of the programme should be considered a success. A recent review of natural control programmes revealed a mean cost: benefit ratio in excess of 1:200 and though there was considerable variation, all were found to be positive. A good example of this is the use of the weevil Cyrtobagous salviniae to control Salvinia weed in Sri Lanka. This plant was introduced into the country during the Second World War to prevent enemy aircraft from identifying waterways. It did the job so well that almost all waterbodies in the country were affected. The weevil was released in 1986 and within four years it had destroyed around 80% of weed infestations. Since its discovery Cyrtobagous has been a successful control agent against this weed in more than 10 tropical countries around the world and is still working today.
Of the 1400 worldwide releases against weeds, less than one percent produced non-target effects and all bar two were predicted by scientists prior to release. A famous case where a weevil ended up attacking native thistles in North America, as well as the target thistles, is a prime example. However, the predicted damage to, in some cases rare species, was deemed to be an acceptable risk at the time, given the scale of the problem. However, given the same data and situation, it is highly unlikely that today’s decision makers would authorise such a release.
The care taken prior to releasing a natural control agent is in stark contrast to the wholesale importation of exotic species, either as pets or garden plants and their pests, or as stowaways within many commodities.
Japanese knotweed costs the British economy millions of pounds each year to manage. It damages buildings, delays developments, and forces out native plants.
If left unchecked, it will continue to spread rapidly. Even where there is a concerted effort to control the weed, it is still spreading. Doing nothing is not a low risk option.
The proposed control agents have been trialled on 90 plant species focusing on closely related native species as well as important crops and ornamental species to ensure they do not attack other plants. They will only be released after vigorous testing, peer review and public consultation.
Whilst the natural control agent is not native to the UK, it is “native” to knotweed. Only co-evolved natural enemies are considered as control agents and some of these will have sacrificed their ability to feed on other species in order to specialise on the target weed. By applying internationally-accepted safety testing procedures to a selected agent, it is possible to demonstrate that the risk to UK native biodiversity or crops is negligible and the organism will not be harmful.
No. Genetic modification involves human intervention to provide an organism with certain genes that code for desirable traits. In the case of natural control 'Mother Nature' has done the modifying for us through the process of evolution. Natural control aims to allow a natural balance to be restored by the re-association of an invasive plant and its natural control agent.
Most insects and pathogens are restricted in what they are able to feed on and some are monophagous, meaning they will only attack one species of plant. This is not as surprising as it seems. Many endangered insects are under threat because their only host plant or its habitat has become rare and they are unable to feed on anything else. Of course some insects and pathogens will eat all kinds of plants but these are rejected as potential natural control agents early in the safety testing regime.
A natural control agent does not do itself any favours if it completely eradicates its only host plant; it will be faced with certain death given that it cannot survive on anything else. Fluctuations in the pest and predator populations are normally observed until equilibrium is reached. This is when the weed population is pushed below a necessary threshold level. It should stay like this indefinitely, providing constant and perpetual control without any further cost. Because the selected natural enemy will be specific to knotweed it will not be able to move onto and affect another plant.
The harlequin ladybird (Harmonia axyridis) was introduced as a biological control agent to several European countries in the 1990’s (not the UK). The aim of the releases was to provide temporary control of aphids in greenhouses and on orchards and other crops outdoors. However, the ladybird has since managed to spread into the wild across more than 15 European countries including the UK. The harlequin ladybird is a voracious predator and is out-competing native ladybird species through direct predation and competition for food.
Although this is not an example of classical biological control (as unlike the proposed release of the psyllid, the harlequin ladybird was never released for permanent establishment, nor subject to safety testing), this is an example of the need for appropriate regulation of the introduction of biological control agents. CABI supports international initiatives to ensure that biological control is carried out safely, most recently as a partner in the EU Policy Support Action REBECA (Regulation of Biological Control Agents) to develop more balanced procedure for risk assessment and regulation of all types of biological control agents in Europe.
No. Eradication is not the normal outcome of natural control since it is not in the agent’s best interest to eradicate its only food source. Long-term control, below an economic or environmental threshold, should be anticipated.
In some way yes although it was not introduced as part of a natural control programme in the way it was in South Africa. The azolla weevil Stenopelmus rufinasus was an accidental but fortuitous introduction that has become ordinarily resident and is now currently being redistributed to control the problematic aquatic weed A. filiculoides on ponds and slow-moving rivers. Click here for more information.
No. Natural control projects are activities for the 'public good' and there is no money to be made. CABI is a not-for-profit organisation and has only received the funding required for the research phase of the programme from the consortium of sponsors (see 'About us').
Yes Japanese knotweed is a major problem in many European countries. It would be relatively easy to extend the release to countries on the continent as we would just need to add their test plants to the safety testing regime and consider the use of natural enemies.
The agent relies on knotweed for its survival and therefore any spraying regime will need to be carefully integrated with the lifecycle of the agent so as not to deplete its food supply and to maximise the impact on the weed in a synergistic way. Spraying knotweed in the autumn should allow the agent to have maximum impact throughout its growing season, draining nutrients, whilst then ensuring that systemic chemicals are taken down to the rhizome just before deterioration begins.
Natural control could be considered for any non-native invasive species providing that its impact is deemed sufficient for the research to be justified. In the plant world, CABI is currently working on Himalayan balsam,floating pennywort, and Australian swamp stonecrop. CABI’s team in Switzerland are also considering using natural control against insects such as the Lilly leaf-beetle and the horse chestnut leafminer as well as Ambrosia weed.
On 28 May 2008, England, Scotland and Wales launched a co-ordinated strategy to reduce the threat to Britain’s native biodiversity from invasive non-native species.
The GB Non-Native Species Secretariat is the focal point for non-native action by the English, Welsh and Scottish administrations.
Further details on the strategy can also be found on the Defra website at
There is also a report on the economic impact of non-native species on the economy of Great Britain on the Defra website at