There are various ways of eradicating or limiting the proliferation of invasive exotic plants:
This aims to introduce specific predators of the invasive exotic plant, consumers (insects, gastropods, etc.) or parasites (fungus, bacteria). The technique can be very effective but requires lengthy preliminary research to identify predators and to ensure that, once they are introduced, they don’t attack indigenous plants and end up being an additional biological invasion! Significant financial aid is therefore needed to set this up, which means that it is used only for those species that have a heavy economic impact. It is still under-used in Europe.
In the overseas French territories, however, the procedure has been successfully used against the giant bramble (Rubus alceifolius) on Reunion Island and the bush currant (Miconia calvescens) in French Polynesia, for example.
This consists of pulling up, mowing, harvesting, clearing or cutting the invasive exotic plant, by mechanical means, choosing the suitable method for each plant. If the technique is used in the early stages on small populations, it may be highly effective and lead to the elimination of the species. However, once the invasion is more advanced, it may only temporarily limit plant proliferation and must be repeated regularly, which increases the cost.
It is also necessary to choose the correct technique for the particular species being tackled: cutting* a meadow of large-flowered primrose-willow, for example, will break the stalks and create large numbers of potentially viable cuttings*; careful uprooting is preferable.
Herbicides are sometimes used on both aquatic and land plants. Like the mechanical method, the results obtained are often partial and temporary, which means repeating the treatment. But the impact on indigenous plants, water quality and the water table poses problems. In the aquatic environment, only approved herbicides may be used and their use is strictly controlled.
The chemical method is only useful when combined with other methods, such as the mechanical method.
As the disruption and artificialization of ecosystems favour the arrival of large numbers of invasive exotic plants, environmental restoration can help to halt their proliferation. For example, the reconstitution of riverside vegetation and revitalization of alluvial environments can limit the invasion of Asian knotweeds (Reynoutria japonica, R. sachalinensis, …) in valleys, or an improvement in water quality in our rivers would stop the expansion of Nuttall’s waterweed (Elodea nuttallii). Similarly, stopping stubble burning in the Eastern Pyrenees would slow the expansion of Narrow-leaved ragwort (Senecio inaequidens) and the limitation of wasteland and monocultures such as sunflowers is also recommended in the Rhone Valley to fight against ragweed. However, as the results of this method can only be seen over the medium or long term, other control methods that have a more immediate effect will usually be needed in the first few years.
This other control method consists of limiting the expansion of a species outside a territory where it is accepted, either because the impact there is low or – more generally – because it is no longer possible to eliminate it completely from the area. It has the advantage of having a good “effectiveness / cost” ratio. For example, on Reunion Island, common gorse (Ulex europaeus) is an exotic plant that is invading high-altitude vegetation. It would be very difficult and expensive to eradicate it from an area in which it has become widely established, and the result would be a deteriorated environment. To prevent it from settling in surrounding areas where it is not yet prolific requires fewer means (eliminating a few plants) and preserves the “naturalness” of these areas.
The most effective and, without doubt, the least expensive solution is to prevent or limit the introduction of invasive exotic species.
While it is difficult to protect against exotic plants with unknown invasive potential, the greatest care should be taken with proven invasive exotic species, i.e. species that have already demonstrated this type of behaviour in other areas. But these plants are on sale without restraint, planted in public areas or around certain developments and are even disseminated by bee-keepers!
International conventions (Convention on Biological Diversity, known as the Rio Convention, at a worldwide level; Berne Convention at a European level) encourage signatory States to adopt measures to fight against biological invasion, but the regulations in this area are still in their early stages. In France, for example, article L 411-3 of the Environmental Regulations, based on the Law of February 1995 relating to increased protection of the environment, prohibits the introduction of non-indigenous and non-cultivated plant species into the natural environment. Sadly, to date, only the large-flowered primrose-willow (Ludwigia grandiflora and L. peploides) has been the subject of an order, under this article, forbidding the sale, use and introduction into the natural environment across the whole of Metropolitan France (Order of 2 May 2007, J.O. of 17 May 2007).
Information and raising public awareness of the dangers of biological invasion remains the best way of changing certain individual behaviour patterns and persuading the people responsible for disseminating of plant pests to think in advance about what they are doing: this includes landscape gardeners and local authority parks and gardens departments for ornamental species, aquarium lovers and garden centres for aquatic plants and beekeepers for honey-producing plants.
Finally, we should be encouraging the implementation of eradication or control operations on invasive exotic plant populations as soon as possible. This involves developing systems for monitoring these species in the natural environment.