CABI scientists have been researching the possibility of using natural control methods to curb the onslaught of Japanese knotweed in the UK since 2000.
The first phase of the project saw a team of British scientists from CABI and Leicester University visit Japan to carry out an initial survey of the plant in its native range. With considerable assistance provided by Japanese scientists, the team visited 31 sites across nine regions. Altitudes ranged from sea level to 1,550 metres above sea level and in all, a total of 3,400 km were covered.
This research revealed that the plant was much less prolific in Japan than in the west due in part to the considerable stress it was under from a whole host of natural enemies. Many of these insects and fungi were collected for identification and initial testing in CABI’s UK quarantine facilities.
An essential step in natural control programmes is to carry out a thorough review of any previous literature on the subject. This ensures that all current knowledge on the plant and its known natural enemies is captured so future research will add to this. This review of both the Japanese and English language literature revealed that a large number of natural enemies present in Japan, were not present in the UK.
Following this initial research, a four year programme began in 2003. Regular field studies were carried out in Japan across the seasons to increase the knowledge of natural enemies in the field and ensure that none were missed. This involved crucial assistance from the team at the Biological Control Laboratory of the University of Kyushu in Fukuoka.
In all, more than 200 insects and fungi were found to feed on Japanese knotweed in Japan. A small proportion were prioritised based on their behaviour in Japan and what scientists believed would be their likely impact on Japanese knotweed in the UK.
In order to check whether the insects or fungi were specific to Japanese knotweed (ie didn’t feed on or attack other plants), a comprehensive test plant list for the UK was compiled using standard International procedures. This consisted of more than 70 closely-related species. These included representatives from 23 families, of which 33 are natives, 15 are introduced species, 3 are native to Europe, 13 ornamentals were tested against, as were 10 closely related economically important species. 51 of these species are in the Polygonaceae, the same family as Japanese knotweed.
Work was carried out in CABI’s strict quarantine facilities exposing natural enemies to non-target plants in order to ensure the safety of any future release. Find out more >
Using the test plant list, host specificity can be determined in the laboratory. Techniques include starvation, development and choice tests for insects and infection and spore production studies for fungi. Any fungus or insect being considered for use as a natural control agent that threatened any of these plants would be dismissed. For example, during early studies a sawfly and a rust fungus were found to attack a few species of dock and were therefore instantly rejected as potential control agents.
From these studies a sap-sucking Aphalara psyllid, and a Mycosphaerella leafspot fungus where identified as potential control agents. These are both restricted to Japanese knotweed in Japan and as lab studies show, here in the UK too.
The psyllid release programme has a very thorough monitoring component that involves the regular survey of the knotweed, associated vegetation and invertebrates inside and outside knotweed stands at all eight pairs of release and control sites. At present, there is no obvious impact of the psyllid due to small population size but we hope to see positive impacts soon once population sizes increase.