Could a tiny mite mark the end for Japanese knotweed?

Successful trial may herald release

Homeowners swamped by the Triffid-like Japanese knotweed may have some hope in sight: scientists have established that a tiny mite may be able to control the invasive plant.

Japanese knotweed, or Fallopia japonica, was originally imported to this country in the nineteenth century as an ornamental plant, thanks to its attractive flowers and foliage - it even won awards.

But it has run rampant in Britain's gardens and waterways, growing as fast as a foot a week, forcing its way through concrete and brickwork, causing as much as £170 million of damage to buildings every year.

And an infestation can make a house difficult, if not impossible, to sell. Earlier this year, Elizabeth Abraham, a 91-year-old from Swansea, was horrified to have the valuation of her house near-halved from £80,000 to £45,000 after the estate agent spotted the weed on her property. Some lenders simply refuse to give mortgages on affected properties.

It does, though, have a natural enemy, which following successful trials may now be released in the UK. Aphalara itadori is a psyllid, or plant louse measuring just a couple of millimetres long. And after a controlled released of the psyllids at test sites around the country, scientists at agricultural research organisation CABI say it could be the answer to gardeners' prayers.

The tests have shown that the psyllid does eat knotweed - but not closely related plants such as rhubarb and dock - and has been able to survive the winter in the UK.

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"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," writes Kate Constantine, one of the researchers, in the Ecologist.

"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."

If approved, it will be only the second time that a pathogen has been approved for release against a weed in the EU. This summer, a rust fungus was released into the wild to control the highly invasive Himalayan balsam plant.

Such releases need to be controlled carefully: cane toads, for example, were originally introduced into Australia in an attempt to control the native grey-backed cane beetle and Frenchi beetle, which were damaging sugar cane crops. However, they quickly became far more of a pest than the insects they were intended to control.

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