Insect could halt spread of superweed

Scientists tackling the thorny problem of a foreign superweed hope a humble insect could halt its rampage across the UK's gardens.

The sap-sucking psyllid is the natural enemy of the invading Japanese knotweed, which has spread rapidly across our towns and countryside.

If the tiny insect was introduced here, it would be the first time the process of biocontrol has been used to control a plant species in Europe.

Little did the Victorians know that when they introduced Japanese knotweed as an expensive ornamental plant, it would prove the bane of horticulturists more than a century later.

With no natural enemies in the UK and an ability to grow up to three metres in as many months, the weed has flourished unhindered, even affecting the 2012 Olympics site.

Just tiny fragments of the plant can form new shrubs, tearing through tarmac, so the uprooted weeds must be classed as controlled waste.

But scientists at CABI, a not-for-profit environmental research organisation, hope they have found a sustainable solution - which depends on introducing another species alien to our shores.

They identified more than 200 of the weed's natural enemies and rejected all but two which were highly-specific to the plant: the Mycosphaerella leafspot fungus, which devastates knotweed in its native Japan, and the tiny 2mm-long psyllid Aphalara itadori, which drinks its sap.

Neither are found in the UK, but scientists believe that if introduced they would bring down Japanese knotweed numbers and would not be able to feed on other plants.

Once out in the wild, the new species would sustain itself, so long-term control would be "effectively free" in comparison to the £1.56bn bill a Defra working group put on controlling the weed's spread using traditional methods, mainly chemicals.

Progress is currently further ahead with the psyllid, due to difficulties caused by the fungus' complicated life-cycle.

Dr Dick Shaw, who led CABI's research, said: "In the case of Japanese knotweed doing nothing is not an option, so we are applying a century-old technique to a new target and are very hopeful of an effective and sustainable outcome.

"Though it is more famous for its concrete-cracking ability, Japanese knotweed's impacts on our natural habitats are severe, crowding out native plants and seriously reducing opportunities for our native wildlife."

Researchers have been carrying out extensive safety testing under quarantine conditions, following strict international protocols, he said.

They plan to avoid a repeat of the notorious instance when attempts to biocontrol beetles in Australia went disastrously awry, flooding the country with poisonous Cane Toads.

The ugly amphibians were introduced in 1935 by the sugar cane industry in a bid to control pest beetles, but the carnivorous toads ate anything small enough to be swallowed and exploded in number.

For that reason, CABI scientists have been testing the Japanese knotweed's enemies against 79 plant species related to the weed, to make sure the natural controls will target only the knotweed.

Dr Shaw said: "Of course our priority is safety and that is why we have spent the last five years safety-testing the agents, so that we do not have a repeat of the infamous Cane Toad debacle which went ahead against the recommendations of scientists at the time."

CABI has been working to stop the spread of Japanese knotweed with funding from a consortium of sponsors: British Waterways, Cornwall County Council, Defra, the Environment Agency, Network Rail, South West Regional Development Agency and the Welsh Assembly Government.