Ludwigia species (synonym Jussiaea spp.) form a group of aquatic weeds native to South America. They grow rapidly and extensively and can double their biomass in 15-20 days in slow moving water. The stems and leaves of Ludwigia spp. float on the water surface and form dense mats that can quickly block waterways and interfere with navigation, fishing etc.
These vegetative mats shade deeper water plants, reducing their photosynthetic rate and therefore their growth. The lower photosynthetic rate also reduces the amount of dissolved oxygen in the water, which is not replaced because the surface is covered by Ludwigia spp., rather than submerged growth (Anon 2009). Ludwigia spp. can easily become invasive
outside their native range, especially as the main means of dispersal are seed spread and through stem fragmentation and viable seed production. Even a small piece of stem remaining in a water body can lead to new growth of the plant.
Ludwigia spp. are sold as garden plants in Great Britain and elsewhere, although their sale has been banned in France since 2007, due to the invasive nature of the species. The species have recently become established in England, and a Defra funded project has recorded the current distribution of Ludwigia spp. At the time of writing, 13 locations of L.
grandiflora have been recorded in England and one in Wales (Dr Jonathan Newman, pers.
comm.). None of the L. grandiflora sites are currently larger than one acre. The surveys of Ludwigia occurrence in the UK indicate that L. peploides currently does not occur in Great Britain, but there are some doubts about the identification and research is being conducted to clarify the species level identifications.
16.3.1 Curre nt Era dica tion Cos ts
The Defra project has tested two chemical methods to control the species: either a spray of glyphosate or a spray of a mixture of a non-oil soya sticking agent and glyphosate. The latter mixture resulted in near-complete removal of the plants. However the spraying treatments needed to be repeated to kill plants that were establishing from seed, even though seed set was prevented in the trials (Dr Jonathan Newman, pers. comm.). This research and work in southern France, has led to the conclusion that prevention or early intervention are the most effective option to control the species (Agence Méditerraneen de l’Environnement, 2002).
However, repeated intervention is necessary to keep the plants under control, firstly by mechanical clearance, followed by repeated hand removal (2-3 times per year over 2-3 years). Current costs, including the research to establish the extent of current outbreaks and eradicate them, amount to £10,000 (the cost of Defra funded project PH0422 (Defra 2007)), but Dr Newman estimates that he has spent £14,000 on research to date (pers. comm.).
Oreska (2009) estimated that the current control cost is £10,263 pa. In summer 2009, Ludwigia was found at Breamore Marsh SSSI and funding to spray this area twice in autumn 2009 has been secured. The annual cost for this site is £1,881.40 (Catherine Chatters, pers.
comm.). There are currently 13 sites in England and Wales. If it is assumed that the control costs are similar for all sites, then the estimated current annual control cost for Ludwigia spp.
is £24,457. This is about double that estimated by Oreska, but that figure was based on 6-7 populations (Oreska 2009, Table 3.6a) and therefore this figure is largely in accordance with that estimate.
Above is only the cost of the initial removal. In addition there is a need for regular follow-up treatment where re-growth has to be manually removed every three weeks (Bianca Veraart, Antwerp Province, pers. comm.). This can effectively eradicate the species, though there may be areas that are difficult to reach in which small plant parts remain and the infestation re-establishes itself. Intensive surveying for at least two years is necessary to ensure any new outbreaks are rapidly cleared. Assuming that the control is similar to that of Hydrocotyle, then the ratio between initial removal and follow-up control from Kelly (2006), who described the removal of Hydrocotyle from 1 km of ditch in the Gillingham Marshes, can be used. The cost of the initial removal was approximately half of the follow-up costs (Dan Hoare, pers. comm.). The cost of follow-up treatment at Breamore Marsh is also estimated at twice that of the initial treatment (Catherine Chatters, pers. comm.). Therefore, the cost of follow up treatments is estimated at £48,914 and the total cost of eradicating the current outbreaks of Ludwigia spp. can be estimated at £73,371.
16.3.2 Wide s pre a d Era dica tion Cos ts
The cost of Ludwigia spp will increase if the species spreads, as is exemplified by the situation in continental Europe and the US. Ludwigia species are a widely distributed pest in France (L. grandiflora and L. peploides) and California (L. hexapetala). In southern France (Languedoc-Roussillon) and California, various control methods have been tested with mixed results. The most effective methods rely on a combination of herbicides and labour-intensive manual removal of plants (Trocme and Pipet 2005). Manual/mechanical removal has been successful in reducing the abundance of plants the following year, but only if the size of the Ludwigia patch was smaller than 20m2. In both countries, positive results were achieved during the first 1-2 years after the initial single clearance, but the re-growth occurred at pre-treatment levels, indicating that continuous control is necessary (Agence Mediterranée de l'Environnement 2002, CAL-IPC News 2009). In the Pays de la Loire, 269,000m2 (26.9 ha) of waterways were cleared of Ludwigia in 2003 and 2004, costing
€350,000 (€11.8 per ha, £8.07 per ha) (Dubos 2005). The French Department of Maine and Loire spent €100,000 per year (£68,367 per year) on the control of Ludwigia species (Genillon 2005). In California, the cost of removal from two wetlands varied between US$14.67 and US$39.95 per km2 (£8.06 - £21.94) in 2005 (McNabb and Meisler 2006) demonstrating that the cost of control is very variable, depending on habitat (i.e.
accessibility, abundance, etc.).
Ludwigia spp. are likely to spread in the UK if the current populations are not eradicated and continued sale is not banned, therefore increasing the associated costs. The use of herbicide on a vast area would be very damaging for the environment and it may be that that future control would be mechanical, as in other countries where the species is currently causing problems (Mike Sutton-Croft, pers. comm.). Assuming that mechanical removal will be used, that about 10% of all 68,310 km of waterways (rivers and canals) in England and Wales will be invaded and that the cost of removal per km is similar to other species and countries (e.g. £1800-2000/km for Hydrocotyle spp., Jonathan Newman, pers. comm.), this would amount to £13,662,000 per year. There are 24,404 km of rivers in Scotland89 and 220 km of canals (British Waterways Scotland website). Therefore, using the same cost of clearance per kilometre, but assuming only 5% of rivers are affected, then the cost of removal of Ludwigia from Scottish waterways would be approximately £2,462,400 per year.
The lochs of Scotland have a combined surface area of 1527.9 km2, England 322.5 km2 and Wales 73.9 km2 (FAO). Assuming that 0.1% of the surface of the lakes close to tourist destinations and on main transport corridors becomes invaded and that the cost of management is similar to that in other countries (€129,727.5/km2 ~ £103,782/km2), the annual costs for eradicating Ludwigia from lakes in England, Scotland and Wales would be
£33,470, £1,585,685 and £7,670 per year, respectively. The total area of wetlands in England is 9,322 km2 (Marina Flamank, EA, pers. comm.). No similar figure could be obtained for Wales, so it was assumed that the area of wetlands in Wales is 20% of that in England, and that 5% of the wetland area could become infested with Ludwigia (466 and 93 km2 in England and Wales, respectively). Again, no figure for the wetland area in Scotland could be obtained, but it was assumed that the area where Ludwigia would become established in Scotland if widespread would be 10% of the wetland area in England (47 km2). Based, as above, on the assumption that the cost of management would be similar to that in other countries, the cost of eradicating Ludwigia from wetlands would be
£48,372,790, £9,674,558 and £4,837,279 in England, Wales and Scotland. This gives a total annual cost for the three countries of £80,635,852.
As discussed above, removal of Ludwigia on a single occasion is unlikely to result in eradication of the species. Eradication would require repeated treatments and would cost double the amount needed for the original treatment. Hence, repeated removal of widely occurring Ludwigia could cost £161,271,704. The total cost of Ludwigia eradication if it became widespread in Great Britain is estimated to be £241,907,556. However, as Bianca
Veraart indicated, though the above treatment will lead to reductions in the abundance of Ludwigia, it is unlikely to lead to complete eradication (pers. comm.). One may expect that complete eradication of Ludwigia would require a continuous effort over a longer period of time and the cost of complete eradication would consequently be higher. The figure presented above should therefore be seen as a conservative estimate of the eradication cost.