INVASIVE WEEDS JAPANESE KNOTWEED ndash; the worldrsquo;s largest female! Hamish Kidd describes efforts to control this invasive weed in the UK especially using biological control l Reduction of biodiversity through out-competing native vegetation l Restriction to riverbanks for anglers bank inspection and amenity use l Reduction in land values l Increased risk if flooding through dead stems washed into river and stream channels l Increased risk of soil erosion and bank instability following removal of established stands in riparian areas l Accumulation of litter in well-established stands l Aesthetically displeasing l Expensive to treat In the UK it is an offence to cause Japanese knotweed to grow in the wild. The plant forms a dense canopy and decomposes very slowly preventing native seeds around it from germinating.This has serious consequences for the biodiversity of our native habitats. The weed is also extremely difficult to control since herbicide use is restricted along watercourses where it likes to grow. In many places it disrupts flood defenses and hinders access to recreational sites. Developers may have to add around 10 to the costs of developing a site if the weed is present to cover the removal and disposal of contaminated soil at specialist landfill sites. Interestingly the plant is only a serious problem in its introduced range where the natural enemies that would normally inhibit its growth are absent. In Japan there are at least 28 species of insects and 6 species of fungi that complete their life cycle on the plant.Of these two primary pathogens and a chrysomelid beetle are specially adapted to live on Japanese knotweed and so may have potential as biocontrol agents for the UK and North America. Introduction This giant Japanese lsquo;superweedrsquo; has invaded almost every corner of the UK tearing up tarmac and concrete and displacing native plants as it goes. Currently the only effective way to treat Japanese knotweed is through repeated applications of costly chemical herbicides over a number of years. However its days could be numbered if a biological control programme supported by the recently formed Japanese Knotweed Alliance takes off. This would be the first comprehensive biocontrol programme against a weed ever carried out in the UK.Introduced to Europe and North America in the early 19th century as an ornamental garden Japanese knotweed quickly escaped into the wild where its rate of spread has been exponential. It has bamboo-like stems arching branches and clusters of creamy-white flowers which appear late in the season. The orange-to-brown coloured woody dead stems persist erect throughout the winter and new shoots produced from the extensive rhizome system grow up amongst these the following spring to form dense thickets. Its roots can extend to a depth of 2 metres and it can grow 4 cm a day to reach over 3 metres in height. A small fragment of the stem or root weighing just 0.7 g can regenerate into a new plant. All of the Japanese knotweed plants found in Europe and North America are female.Not only is it single sex it is also a single clone as work at the University of Leicester has shown. Given that it must occupy many thousands of hectares in Britain alone (the same clone is also known to occur in continental Europe and North America) in total biomass terms it is probably the biggest female in the world! Japanese knotweed now occupies more than half of the 10 km squares used to map plant distribution in the British Isles and is absent only from the Orkney Islands. The problem is particularly severe in Wales and Cornwall with Swansea the worst affected part of the country. The infestation in Swansea alone is estimated to weigh in excess of 650,000 tonnes Overall the UK currently spends millions of pounds every year controlling the weed through physical and chemical treatments.Problems caused Specific problems caused by Japanese knotweed are l Damage to paving and tarmac areas l Damage to flood defence structures l Damage to archaeological sites Control methods Japanese knotweed is not an easy plant to control. The extensive underground rhizome system sustains the plant even when top growth is removed. Therefore the aim of any control programme should be to target the rhizomes. Chemical control Translocated herbicides such as glyphosate and 2,4-D amine may be used in the UK in riparian areas providing consent is obtained from the Environment Agency. In areas where there is no risk of run-off to watercourses and where no sensitive vegetation (including trees and shrubs) will be affected the herbicides triclopyr picloram and imazapyr Pes tic ide Outlook ndash; June 2000 This journal is copy; The Royal Society of Chemistry 2000 9 9 Japanese knotweed (photo courtesy of CABI).INVASIVE WEEDS may be used. The latter three herbicides are persistent in the soil and may delay planting of replacement species. Herbicides are usually applied as a foliar spray but can also be applied directly to target plants using a weed-wiper or herbicide glove. Good results have been achieved by injecting glyphosate into hollow stems immediately after cutting. Biological control Amazingly no full weed biocontrol programme has ever been undertaken anywhere in Europe despite there being plenty of invasive alien weeds.In fact in the UK such natural weed control techniques are still considered lsquo;novel and controversialrsquo;. And yet there is nothing new about biological control. To date there have been around 1000 releases of over 350 biocontrol agents against 133 target weeds in 70 countries worldwide many of them notable successes. In the USA Canada Australia and New Zealand biocontrol is now regarded as the prime weapon against alien weeds. The Japanese Knotweed Alliance was formed recently at a workshop hosted by CAB International a non-profitmaking scientific organisation that has been involved in weed biological control for more than 70 years and has been developing this biological control initiative for over a decade.Other partners in the Alliance include the Environment Agency the Welsh Development Agency the National Botanic Garden of Wales and Cornell University in the USA. A proposal for a UK-wide biocontrol programme is currently under consideration by potential UK and US sponsors. CAB International scientist Dick Shaw who is leading the programme said ldquo;Local authorities and developers are having to spend millions of pounds every year on chemical herbicides to control or eradicate this weed. Through this research programme we believe we can identify a knotweed-specific insect and/or fungus capable of bringing the weed under permanent control in a low-cost and environmentally-friendly wayrdquo;. More detailed information about the Japanese Knotweed Alliance can be obtained from Dick Shaw Biocontrol Scientist CABI Bioscience Silwood Park Ascot Berkshire. SL5 7TA (E-mail r.shaw@cabi.org; Tel +44 (0)1491 829 168; Fax +44 (0)1491 829 123 or visit the Japanese Knotweed Alliance website at http://www.cabi.org/ BIOSCIENCE/japanese_knotweed_alliance.htm. 100 Pest ic ide Outlook ndash; June 2000
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