Unknotting Canada's knotweed problem

Overview

Results

The team

Related information

So what's the problem?

Japanese knotweed is a fast-growing plant species that is causing a great deal of damage in Europe and North America. This herbaceous plant forms dense, impenetrable thickets comprising of stalks 8 – 12 feet tall. Originally, as the name suggests, the plant was imported from Japan as an ornamental, and in some areas of North America it can still be purchased. The plant soon escaped from gardens and has spread rapidly, out-competing many native species along the way. In Japan the plant has evolved with an array of natural enemies, from fungal diseases to predatory insects, which help to control the growth and spread of the plant naturally. But in the USA and Canada none of these specialist natural enemies are present and so the knotweed is literally out of control.

So far, Japanese knotweed and the closely related (but slightly less virulent) species giant knotweed and the increasingly troublesome “bohemian knotweed” have spread to 41 American states and 8 provinces in Canada. In regions of high precipitation knotweeds are widespread, while in drier regions they occur mainly in the ecologically sensitive riverbanks and low-lying wetlands.

The impacts of knotweed are varied. It can reduce the functioning of an ecosystem by crowding out native vegetation, thus reducing the biodiversity of both plants and animals. Waterways are also affected as knotweed provides less nitrogen to streams compared to the native vegetation that it displaces, in the long run this could disrupt the food chains within the stream and even affect salmon fisheries. Riverbanks become less stable and flood risks generally increase as a result of channels being blocked with dead stems. The knotweeds can also affect the recreational use of waterways by blocking access to rivers and streams and angling and boating can suffer disruption.

One of the biggest financial impacts is the damage to man-made structures. The shoots which stem from the large root system – which can be up to three metres deep and have a seven metre radius – have been known to push through tarmac, building foundations, concrete and drains.

Current control methods have proven extremely costly – usually thousands of dollars per acre. Chemical control is not always effective since multiple applications are often required and there are environmental concerns about the effects of pesticide use close to waterways. The weed has become so widely spread – often in areas that are inaccessible – that current management practices are impractical. A new, innovative approach to tackle this problematic weed is needed.

What is this project doing?

Scientists from CABI have already carried out a considerable amount of research focussing on the same problem in Europe. The main drive of research has been to learn about the biology of the plant and its natural enemies in its native range – Japan. The next step involves selecting those arthropods or fungi with the most potential from the many that are to be found on the target plant. This involves vigorous screening to find candidates that are highly damaging, restricted to the knotweed plants and that do not pose any threat to valued species. The end-goal is to find a suitable species that could be reared and released in North America to help control the spread and growth of knotweed without causing any negative environmental effects.

Results so far

Surveys in Japan identified around 200 natural enemies of knotweed and after extensive testing two species have emerged that both show great potential for use as a natural control agent. Firstly, there is the plant-feeding insect (a type of psyllid) Aphalara itadori. This insect has shown a remarkable specificity with a very high fidelity to knotweed when laying eggs, and any eggs laid on non-targets are unable to develop on the plant species so far tested. A. itadori has killed plants completely in the lab, and even low numbers of the insect reduce the height and leaf area of the plant. The second candidate is a Mycosphaerella species of leaf-spot fungus, which is also proving to be highly specific to Japanese knotweed.

So far most species of plant from North America have been successfully screened against the psyllid and leaf spot fungus. The juvenile psyllid has not been able to successfully develop into an adult on any of the North American plant species tested other than knotweed. The host-range specificity of the leaf spot fungus has shown some encouraging results initially, but problems have been encountered in taking the fungus through its complete lifecycle in the lab. A large scale experiment has been established in Japan to learn more about the fungus and its reproductive process in natural conditions.

Should testing be successful and the release of one of these control agents be approved, then it is hoped that an affordable, self-sustaining and practical form of control will be achieved.

Project Executive