WE ARE under attack. An alien is overrunning our gardens and breaking into our homes – cracking concrete foundations, smashing through brickwork and rearing up between floorboards. The invader is Japanese knotweed, and it is no ordinary weed. This plant, it’s fair to say, is one of the most hated in the world.
And the headline-grabbing horror stories of infested homes and plummeting property prices are just part of it. Out in the wild, knotweed is choking waterways, undermining flood defences and smothering native habitats across large swathes of Europe and North America. The plant has even found its way to Australia and New Zealand. “Outside its native range, knotweed has the biodiversity value of concrete,” says entomologist Dick Shaw. “Nothing lives on it. Nothing eats it.”
“It has the biodiversity value of concrete. Nothing lives on it. Nothing eats it”
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Shaw is mounting a counter-offensive. This spring, I visited his lab in a leafy corner of London’s stockbroker belt. Outside, all over the country, pink shoots of Japanese knotweed were bursting through soil, tarmac and concrete and heading skywards. The bamboo-like stems can grow as much as a metre a month. Yet the ones in Shaw’s lab weren’t growing much at all. They were stunted and draped with thin waxy white ribbons.
The wax, Shaw explained, is secreted by small sap-sucking insects called psyllids. He points to the new leaves at the tips of the shoots. In every case, the leaf was dying or dead, the life drained from it by newly hatched psyllids. Thanks to these 2-millimetre-long insects, these plants would never grow to be 4-metre monsters like those outside.
Over the past four years, hundreds of thousands of these insects have been released at eight secret sites in England and Wales, as part of Europe’s first trial of a biological control against a weed. Much hinges on the results. If the psyllids live up to expectations, they will provide a desperately needed weapon in the global war on knotweed and could go some way to countering the popular misconception that biological control inevitably leads to disaster.
For while other parts of the world have a long and mostly successful track record in the use of biological controls against weeds, Europe has long resisted such measures. The knotweed experiment could open the door to the UK and other European countries using biocontrols to tackle a slew of alien weeds rampaging across the continent.
The knotweed invasion when doctor and botanist Philipp von Siebold acquired a specimen of Fallopia japonica from Japan. By 1848, his horticultural firm in the Netherlands was supplying cuttings to buyers in Europe and in 1850, von Siebold sent a specimen to the UK’s Royal Botanic Gardens at Kew. It was an immediate hit, admired for its attractive leaves and sprays of creamy flowers – and for its vigour. Cuttings soon found their way to country estates and nurseries across the UK, and later to the US.
Before long, however, the plant began to show its darker side. By 1898, the Victorian gardener William Robinson was warning that Japanese knotweed was “weedy, and in light soils springs up everywhere”. By the 1930s, it had acquired the nickname Hancock’s curse in Cornwall, where one infestation was said to have slashed the price of a house by £100. The invasion was under way.
Volcano resistant
Knotweed thrives along rivers and places disturbed by people, such as roadside verges and railway embankments. In its native Japan, it colonises exposed substrates, from river gravel and sand to the still-smoking ash-covered slopes of volcanoes.
In winter, the leaves and stems die back, but the tough underground rhizome survives and extends further and deeper with each cycle of summer growth and winter dieback. One plant’s rhizome can eventually form an extensive network 14 metres across and 3 metres deep. The rhizome can survive even burial by volcanic lava – sending up rock-piercing shoots once the surface has cooled. It’s not surprising, then, that it can seriously damage roads, walls and foundations. “A plant like that will laugh at concrete foundations,” says Mike Clough of Japanese Knotweed Solutions in Manchester, UK.
It grows so fast and tall that it crowds out native plants. “Little light or rain get through the canopy and very little grows beneath,” says Sean Hathaway, knotweed officer for Swansea, UK, which with around 100 hectares of knotweed has been dubbed the knotweed capital of the world. That means far less food for the animals that feed on native plants, with knock-on effects on the rest of the food chain.
The one thing the invading knotweed cannot do is produce seeds. Remarkably, , all clones of von Siebold’s original specimen. The secret of knotweed’s astonishing spread is that new plants can grow from the tiniest fragments of stem or rhizome, all too easily carried by floodwaters or by human activities such as the dumping of garden waste. What’s more, pieces of rhizome can remain dormant for years before sprouting.
By the 1970s, knotweed was established across the UK (see above). Finally recognising it had a problem, in 1981, the UK made it illegal to “plant or otherwise cause the plant to grow in the wild”. In 1990, it became an offence to move even the smallest piece of knotweed or contaminated soil, which gave rise to a lucrative industry in knotweed removal.
“Suddenly knotweed became a national talking point,” says Colin Hawke, chair of the Cornwall Knotweed Forum. “One result of the growing awareness is that people panic when they see knotweed. They think it will engulf them overnight.” And not just gardeners and homeowners – mortgage lenders panicked too, refusing mortgages on properties they considered at risk from the plant.
Despite its reputation, it is possible to kill knotweed with common herbicides, although stubborn infestations may need up to five years of repeat treatment. Developers needing faster results are forced to adopt more drastic and costly solutions. The conventional method of clearing sites is to remove both the plants and the top few metres of soil and take them to special landfill sites. For large developments, the bill can run into millions of pounds. The cost to the UK economy is estimated at more than ÂŁ165 million a year.
The piecemeal attacks on knotweed infestations have done little to check its advance. “The invasion continues fairly relentlessly,” Shaw tells me back at his lab in Egham, at the UK base of the intergovernmental research organisation CABI. To make matters worse, there’s a growing risk from hybrids between Japanese knotweed and other knotweeds introduced in Victorian times. One such hybrid is Bohemian knotweed, a cross between Japanese knotweed and the even larger giant knotweed.
“The hybrid is even more invasive than its parents,” says knotweed expert Lois Child of Loughborough University, UK. “It grows larger and spreads faster and it does produce seeds, although in the UK they don’t germinate in the field – at least they haven’t so far.” The seeds are windborne and can reach places that rhizomes cannot, so if they start sprouting, matters would become even worse.
The best hope of forcing Japanese knotweed into retreat, says Shaw, is to introduce one of the predators or diseases that help keep it in check in Japan. “When an invasive plant has become physically and financially impossible to control using other methods, then it’s the only tool left.” Efforts to cut the use of herbicides – already banned in sensitive habitats or near water – makes biocontrol even more appealing, he says.
I must have pulled a face, but Shaw is used to horrified looks when he mentions biocontrol. It has a bad reputation, thanks to high-profile disasters in which control agents have done more harm than the aliens they were meant to control. “Everyone’s heard about the cane toad that’s eating Australia’s wildlife. And most people have heard about the harlequin ladybirds that are eating the UK’s native ladybirds.”
These voracious Asian ladybirds were introduced to Europe to protect greenhouse crops from aphids. “It was inevitable they would get out,” says Shaw. The list of attempts at biocontrol that went wrong is long – but in almost every case the target was an animal. And in none of the cases was there a proper risk analysis or adequate safety testing.
Biocontrol against weeds is a very different story, says Shaw. “It has an incredible safety record.” Since the late 1800s there have been 1400 releases of biocontrol agents against weeds in more than 70 countries. These have been , with more than 75 per cent of target weeds brought under control. Some agents have done the job with astonishing speed and efficiency (see “Biocontrol triumphs“). So far, just 11 of the agents have been seen feeding on non-target plants. “All but two of those were predicted by scientists and the authorities sanctioned the releases knowing the risks,” says Shaw.
In one much-publicised case, a weevil was introduced to Canada and the US in the late 1960s to control European thistles. By the 1990s, it had spread widely and was attacking the flower heads of seven native thistles. It is . Yet the weevil’s varied diet was no surprise: in Europe its hosts include a broad range of thistles. The case highlighted the dangers of ignoring one of the key tenets of biocontrol: don’t release an agent unless you know that it feeds exclusively on the target.
Ideal target
The risks of using biocontrols must also be weighed against the risks of the continued spread of invasive alien plants. Those advocating biocontrol against weeds are winning the debate: new EU legislation requiring countries to tackle invasive species explicitly recognises the need for it.
Japanese knotweed is the ideal target for Europe’s first trial, not just because several countries consider it to be their worst weed, but because the plants are genetically identical. “The fact that they are clones is a huge advantage,” says Shaw. The lack of variation means knotweed is highly unlikely to evolve defences against the psyllid.
The hunt for a suitable control agent began in earnest in 2003. Surveys around Nagasaki, the source of von Siebold’s specimen, identified 186 arthropods and more than 40 fungi that attack knotweed. Shaw’s team spent three years testing which of these were faithful to knotweed and which might attack other plants, by introducing them to up to 90 other plants, mostly British natives but also important crop and garden species. Of the handful , the most promising was the psyllid Aphalara itadori.
Adult psyllids lay their eggs on knotweed, but it is the emerging nymphs that do the damage. “Once the nymphs start feeding, the plants won’t get any taller,” says Shaw. The rhizomes of psyllid-infested plants start to shrink within a year.
In March 2010, the trials were given the go-ahead, subject to a contingency plan should anything go wrong. The first batches of psyllids were released at three sites chosen for their isolation, to reduce the risk of the insects spreading. Rapid-response teams were on call to eradicate the psyllids at the first sign of any harm to non-target species. The locations were also kept secret to prevent theft. “In Australia, farmers couldn’t wait to get their hands on the rubber vine rust fungus, so they helped themselves,” says Shaw. “People ask all the time if they can buy some of our psyllids. That’s why our sites are secret.”
Four times a year, local teams check on the psyllids and assess their effects. With a year left of the five-year trial, progress has not been as fast as Shaw had hoped. The psyllids have survived, but the populations are not growing. “We think that could be because these insects have been cultured in the lab for more than 100 generations and are not best fitted for life outside. New stock from Japan should solve that problem and we are looking at that option,” he says.
The hope is that the psyllids will eventually become permanently established and spread out across the country from one patch of knotweed to the next. They will never eradicate knotweed altogether, Shaw says. But if they stunt the growth of the plants, knotweed will lose its competitive edge over native species. “That might stop it spreading, and native plants might even start to fight back and begin to push it out.”
With the weed in retreat, it won’t matter any more if you find some in your garden. “It will no longer be the monster of myth but just another weed,” says Shaw.
Biocontrol triumphs
The use of biocontrol against invasive weeds goes back more than a century. Outside Europe, 70 countries have introduced agents against more than 130 problem weeds and many have been spectacularly successful.
Rubbervine
Madagascan rubber vine (Cryptostegia grandiflora) was introduced to Australia in the 19th century as a garden plant. By the 1980s it infested more than 40,000 square kilometres, smothering trees and threatening tropical ecosystems. The leaf-destroying rust fungus Maravalia cryptostegiae was released in 1995, with dramatic and rapid results. Rubber vine is now almost completely under control – years sooner than predicted.
Water hyacinth
South America’s water hyacinth (Eichhornia crassipes) is one of the world’s worst aquatic weeds, clogging waterways on several continents. In Africa, one of the worst infestations was in Lake Victoria. Two species of Neochetina weevils (pictured below), released in the 1990s, cleared 80 per cent of weed in less than three years, .
Purple loosestrife
European purple loosestrife (Lythrum salicaria) is another ornamental plant gone bad. In the US and Canada, it ousts native plants and animals from wetland habitats. After assessing more than 100 loosestrife-eating insects, two leaf-eating beetles (Galerucella calmariensis and Galerucella pusilla) were released in the west and midwest US. They destroyed 95 per cent of the weed within five years.
This article appeared in print under the headline “Let them eat weeds”
