Using nature to curb nature
Invasive exotic plant species are one of the major threats to native plant diversity, they impact human health and cause economic damage. EU Regulation 1143/2014(opens in new window) on the prevention and management of the introduction and spread of invasive alien species states that management measures should avoid any adverse impact on the environment as well as on human health. Current methods of control are often indiscriminate: mechanical methods such as aggressive mowing impact other species, and chemical treatments involving herbicides have environmental consequences. The EU SOIL-4-CONTROL project focused on biocontrol methods. “If successfully implemented, this method is relatively cheap, and effective in the long term, but it is hardly being used in Europe,” explains Suzanne Lommen, who was supported by the Marie Skłodowska-Curie Actions(opens in new window) programme. “Interestingly, several European species that have become invasive on other continents have been controlled by the introduction of natural enemies from Europe. An example of this is common ragwort, one of our study species, that is being controlled by European herbivores in North America and New Zealand,” says Lommen, who conducted her research at Leiden University(opens in new window) in the Netherlands. Rather than taking an indiscriminate, unsustainable approach of traditional methods of control, using nature to curb nature involves a carefully targeted solution using natural antagonists from the native range of the target species.
Costly, unwanted guests
Not much data is available on the precise cost of invasive species in Europe. Estimates are often based on management costs and often do not consider the costs of the damage caused – hard to express in monetary value when it concerns biodiversity. Invasive alien species are the second most significant causes of biodiversity loss. The Institute for European Environmental Policy(opens in new window) (IEEP) has estimated(opens in new window) that they cost the EU at least EUR 12 billion and probably over EUR 20 billion a year. “Results from our study on invasive common ragweed in ‘Nature Communications’(opens in new window) indicate that this figure of 12-20 billion is highly underestimated, as we already estimate total costs of common ragweed at 7.4 billion per year. “We projected the cost of invasive common ragweed based on the distribution of the plant and its allergenic airborne pollen across Europe, human population densities and their sensitivity to becoming allergic. We considered the medical costs and sick leave of people suffering from reactions associated with this plant species,” adds Lommen.
Harnessing the power of nature
The project examined the damage inflicted by the leaf beetle Ophraella communa on hundreds of individual common ragweed plants. “We demonstrated that a beetle, O. communa, a natural enemy of common ragweed in its native range, reduced common ragweed pollen production in the field by around 80 %, corresponding to the reduction in aerial pollen loads found in the region,” says Lommen. The project has set out some of its findings(opens in new window) in the journal ‘NeoBiota’. “By integrating this data into spatial models of ragweed, aerial ragweed pollen densities, O. communa, the human population and ragweed-related health costs, we showed, for the first time in Europe, that costs of invasive ragweed can be reduced by EUR 1.1 billion a year, using a biological control agent. This highlights the huge potential of harnessing natural enemies for invasive weed control.” This is one of the first studies showing the impact of exotic weed biocontrol agents in Europe. It proves that above-ground herbivores can have huge impacts on invasive plants and can mitigate the negative impacts, highlighting the cost-efficacy of this method that is relatively cheap to implement but has long-term effects. The project also considered two Asian knotweed species, Reynoutria japonica, also known as Fallopia japonica or Japanese knotweed, and R. sachalinensis, giant knotweed, both of which have become invasive in Europe, where they are hybridised into R. x bohemica. The psyllid Aphalara itadori is an insect which lives specifically on these plants in its host range in Japan. A study confirmed the host specificity of A. itadori meant there were minimal potential risks for the Dutch flora resulting from a release into the wild. These results were used to submit a petition for permission to release A. itadori for biocontrol. This required an exemption from the law that prohibits the release of exotic organisms into the wild in the Netherlands. The petition, submitted in 2019, was approved in July 2020. Lommen explains that the project reared the psyllid in a greenhouse and released them on species of knotweed in the wild. “Together with stakeholders and the company Koppert Biological Systems(opens in new window) we are now looking for funding to expand the weed biocontrol programme in the Netherlands and the EU. We already have tens of supporting parties including EU policymakers, other authorities and nature organisations,” Lommen adds.
