February 12, 2013

The invasion of the allergenic ragweed in Europe

The common ragweed is one of the most pollen-allergenic plants and as such represents a serious health risk for humans. Its pollen is a potent trigger of hay fever, rhinoconjunctivitis, and may often cause severe asthma-like symptoms. In Europe the incidence of ragweed allergy ranges widely from 2-50% of the allergic population (roughly ¼ of the European population shows general allergic rhinitis). The impact of common ragweed on human health – affecting mostly children and urban populations (but also horses, dogs and cats) - is not restricted to areas invaded by the plant. In fact, due to transport of ragweed pollen by air masses, allergy reactions are recorded in distances of 100s of km from the site where the plant is situated. Besides, the common ragweed also contains volatile oils that may cause skin irritation and hypersensitivity dermatitis. The associated economic costs are estimated to be around 4.5 billions of euro per year. In addition to this, the ragweed can also have an harmful impact on other sectors, such as agriculture (depending on infestation levels and success of control, yield losses of over 50% are reported). 

A synthetic and systematic review of all available information on the current extent of ragweed infestation in Europe has just been published by the EU within the new comprehensive report “Assessing and controlling the spread and the effects of common ragweed in Europe”.  The study, carried out by an international team of experts led by the Natural Environment Research Council (UK), includes an economic, social and environmental quantification of direct and indirect harmful effects in all sectors, as well as an assessment of measures to control ragweed spread and introduction (now and in future climates). 

Common ragweed (male flowers) ©  Photo: Daniela Bouvet

The common ragweed is actually a native to North America, which has colonised several countries around the world – e.g. Europe, as well as parts of Australia, China, Japan, South America and Taiwan – mostly as a contaminant of agricultural products (including the grain mixtures used as food for birds), machinery or construction materials. In Europe, where it was first introduced in France and Germany in the 1860s, started its spread in the entire region some 20-25 years ago. The result is that large populations of ragweed are now present in the old continent, particularly in Croatia, France, Hungary and Italy, but the distribution range is still expanding as a consequence of changing climate and perhaps adaptation to local climate. Moreover, changes in agricultural land use with large-scale set-aside and abandonment practices, along with an increase of the construction sites and wasteland, are expected to provide new suitable habitats for ragweed. 

The key successful features of this annual herbaceous plant - which can reach a height of over 2 m - are its great adaptability to hostile habitats, its strong ability for re-growth after mowing, and the capability of seeds to remain viable for up to 35 years in soil seed banks. It is most frequently associated with agriculture and is found in cultivated fields (mainly maize, sunflower, leguminous plants) and along irrigation canals. This typical pioneer species is also associated with frequent and extensive disturbance regimes resulting from other human activities (e.g. riverbanks, roadsides, railways, gravel pits, construction sites, waste sites, urban areas, building yards, private gardens and parks).

Common ragweed (leaves) ©  Photo: Daniela Bouvet

The common ragweed is now so widespread in Europe that eradication at this stage of the invasion is no longer feasible. However, it is possible to keep the ragweed under control by every year eliminating emerging plants as far as possible, and preventing or reducing the spread of seeds from infested to non-infested sites. So far, no successful biological control methods have been developed, thus the most effective management measures to control the propagation of this plant are clipping/mowing, uprooting, ploughing, mulching and chemical treatment. Preventive measures include initiatives to limit unintentional spread of ragweed seeds by developing and implementing best practices. In this context, a welltargeted intensive awareness raising campaign is a key to success of both prevention and control strategies (e.g. by reporting observations and making early detection possible). In fact, the EU study has shown some promising future scenario: while in 20 years time the overall impact is expected to slightly increase (about 3%), the implementation of sound management strategies could help reducing such impact by approximately 50%.