Five years after thousands of Japanese bugs were released into Britain to control the spread of a superweed, scientists say more time is needed to assess whether the ambitious project will succeed.

The psyllids — Aphalara itadori — were reared in laboratories from a sample brought over from their native habitat in Japan where they suck the sap from Japanese knotweed and keep the plant in check, along with a range of other bugs.

In Britain, there is no natural enemy to knotweed and, as a result, the invasive plants have gotten out of control in some areas.

A lot of money is spent on taming the knotweed using chemicals. The weed can push through concrete pavements and brickwork and causes millions of pounds worth of damage each year.

Since 2003, scientists from CABI, a nonprofit organization that provides scientific expertise, have worked on finding a more natural solution. They have been releasing the bugs on to knotweed at several trial sites since 2010.

The project, which is the first time a foreign insect has been deliberately released into the European Union to tackle a weed, has had mixed results.

Kate Constantine, one of the scientists at CABI, told Kyodo News that the trials confirm earlier lab tests that the bugs are safe and do not have a negative effect on other plants.

However, one of the problems appears to be maintaining a sufficient number of adult psyllids at the trial sites. Some are surviving the winter but not in the numbers necessary to establish a population which will keep on growing and have an effect on the knotweed.

Constantine said: “We have found some psyllids overwintering (it is thought that psyllids shelter in evergreen trees in winter) but not in the numbers we would expect. The psyllids were reared from a sample originally taken from a site in Kumamoto Prefecture that has similar climatic conditions to the United Kingdom.

“However, the humidity levels at the Japanese sites are higher and our trial sites are relatively dry. We suspect that humidity could be important for maintaining the population over winter and subsequently building up numbers.

“This spring we are hoping to release psyllids at more suitable sites, such as close to rivers.”

Constantine said that if the psyllids take hold on knotweed it may take up to 10 years for effects to be seen on the growth of the plant. The hope is that several communities of psyllids could take hold and then spread across the country to tackle the superweed.

She said the idea behind the project is for the psyllids to feed on the sap and make the knotweed weaker. This, in turn, will make the chemicals more effective on the plant should they be needed.

In addition, the knotweed becomes less of a competitor to native species and they can re-establish themselves and achieve a biological equilibrium.

In Japan, knotweed grows nearly all over the country but has never been thought of as ornamental.

KEYWORDS bugs, weed, knotweed