[BBC News: Opinion - Mathew Cock, 9 Mar, 2010] For the first time in Europe, the UK government has granted approval for an insect from Japan to be released in order to control the invasive plant, Japanese knotweed. It is the first time a "biological control" approach has been used in Europe against a plant. But the concept is far from new, and Europe has been lagging behind other regions that have had more challenging experiences with invasive non-native species.
It has been estimated that to manage Japanese knotweed in the UK, without successful biological control, would leave farmers, gardeners and local councils facing a bill of at least £13.5bn
Japanese knotweed is a notorious plant, famed for its ability to burst through tarmac and crack concrete. Just finding it on a development site can result in huge additional costs or force developers to find alternative sites.
There have even been reports of mortgage companies refusing to lend to house buyers whose dream homes have Japanese knotweed in the garden.
In contrast, in its native range in Japan, it is just another member of the Japanese flora and it is fed upon by over 200 natural enemies, mostly plant-eating insects and plant diseases.
It is amongst these natural enemies that scientists believe the answer lies.
Counting the cost
Alien plants in the UK cost a vast amount of money and this expenditure, like the plants themselves, tends to expand at an ever-increasing rate.
The Aphalara itadori is a natural predator of Japanese knotweed
It has been estimated that to manage Japanese knotweed in the UK, without successful biological control, would leave farmers, gardeners and local councils facing a bill of at least £13.5bn ($20.4bn; 15bn euros) by the end of the century.
This does not include the less obvious costs to the environment and those species that are displaced by these invaders.
The rate at which new plant invasions are happening is accelerating, as a result of factors such as increasingly mobile human populations, larger volumes of traded goods and climate change, all of which make it easier for plants to reach new areas.
Europe needs to find new ways to combat the most damaging species that become established.
The majority of these newcomers arrive without their natural enemies that can keep them in check in their native range, and this may give the exotic plants a great advantage over their new neighbours.
Biological control - the use of living organisms to control pest populations - can be thought of as a means of levelling the playing field by introducing some of the specialist natural enemies that exert control on the pests in their native range.
However, this can only be done with extreme care.
Safety first
I am an environmental scientist. The first question I ask before proposing to import a new organism is: "Is it safe?"
The introduction of cane toads in Australia show the risks of biocontrol
Worldwide, there have been more than 7,000 introductions of biological control agents to date, about 1,300 of which were for weed biological control. The remainder was for control of invertebrate pests, mostly insects.
Of the 1,300 releases against weeds, more than 400 different agents have been released against more than 150 different target weeds over the last 110 years. Of these, only nine produced any collateral damage such as feeding on native species.
I imagine that some readers will be asking: "What about the cane toad?"
The cane toad was introduced into Australia from South America by the sugar cane industry in 1935 because it was known to eat some of the most important sugar cane pests.
There was no consideration of what the potential food range would be, or what impact the cane toads might have on the native fauna.
Not surprisingly, the toads - which grow to 20cm (8in) - turned their carnivorous attention to anything that moved. They will eat anything that they can get into their mouths.
The toads went on to become a significant problem, having a detrimental effect on native fauna, including amphibians and reptiles, as well as poisoning domestic and wild animals that tried to eat them.
Today's practitioners of biological control consider this release an ecological disaster, but one which demonstrates very clearly what can happen when a generalist predator is introduced without considering the risks that they present to species other than the target pests.
Those who regulate the introduction of biological control agents have learnt this lesson.
The methodology of science and predictability of introducing an exotic insect or plant disease for biological control has improved greatly, so that tests carried out by scientists allow good predictions to be made of what will happen in terms of safety when a weed biological control agent is introduced.
No introduction of an exotic organism can be entirely risk free, but the risk of a highly studied, specialist natural enemy feeding outside its experimentally evaluated host plant range is extremely low.
Overall, the safety record of weed biological control has been exemplary and the beneficial impact has been enormous.
Making up lost time
In the last 50 years, five countries have led the world in implementing successful weed biological control: Australia, Canada, New Zealand, South Africa and the US. In countries like these, research into biological control is always considered and often prioritised when a new and problematic invasive plant species get out of hand. Between them, they have tackled more than 100 weeds and achieved partial or substantial control of more than 50, with at least 30 programmes active at the moment and too early to judge.
To give just one example, the alien invasive rubber vine weed was successfully brought under control in Australia.
Introduced from Madagascar in the 19th Century, this aggressive climber was considered the single biggest threat to natural ecosystems in tropical Australia.
By the late 1980s, infestations were vast, covering 40,000 square kilometres. A rust fungus, Maravalia cryptostegiae was identified as a highly promising biological control agent and after extensive trials to assess it safety, aerial releases were made in Australia in 1995.
It is promising to be one of the most successful biological control programmes ever carried out, with heavy damage to the weed allowing regeneration of native forests. It was estimated to have provided a net value of £140m by 2005 - a saving that is growing year on year.
The UK's Department for Environment, Food and Rural Affairs (Defra) decision to grant a licence to release a biological control agent for Japanese knotweed is a milestone decision for Europe that will open the way to assess the scope for biological control of other alien weeds such as floating pennywort and Himalayan balsam, themselves very obvious and damaging invaders.
Europe can now start to catch up with the rest of the world and gain the benefits of sustainable, environmentally safe biological control of some of its most environmentally damaging alien invasive weeds.
Dr Matthew Cock is chief scientist for Cabi, a not-for-profit science-based development and information organisation
