It is generally thought that the seeds of Ambrosia artemisiifolia L. were introduced in large numbers into Europe from the 19th century onwards as contaminants of agricultural products, such as cereals, that were imported from the USA and Canada where this species is native. This has recently been documented by a comprehensive study of many herbarium specimens deposited in France since 1863 (Chauvel et al., 2006). Based on herbarium data, it was possible to depict the historical spread of this noxious and highly allergenic weed in France, following multiple introductions from North America. Some of the conclusions of this work were supported by a recent molecular analysis (Genton et al., 2005a). The herbarium materials analysed by Chauvel et al. (2006) indicated that the key factors in the introduction of A. artemisiifolia in France were anthropogenic: the increase of agricultural trade between America and Europe in the 19th century and the First World War could have been the major historical events that facilitated its introduction and spread in France. This is not surprising, as A. artemisiifolia is an agricultural weed and also a primary colonizer of disturbed and abandoned lands (Bassett & Crompton, 1975), so it is regularly found in agricultural products and can become established in areas disturbed by humans.
Parallel to its spread in France, ragweed has also appeared in southern and eastern parts of Europe (Déchamp & Méon, 2002; Béres, 2003). The analysis of its European invasion outside France was not addressed by Chauvel et al. (2006). We would like to draw attention to a few characteristics of this process, with emphasis on the most recent population expansion that started in the early 1990s in Eastern Europe, as this might complement the story of the European ragweed invasion and could highlight the role of anthropogenic factors in its spread.
Ragweed has in the last two decades become the best-recognized weed species in Eastern Europe. This has happened because so many people developed allergies to the airborne pollen of this weed that the national governments had to initiate programs to bring attention to this noxious weed. The last boom in its spread could be linked to the political transitions that led to the formation of young democracies in Eastern Europe. During that process, many socialist-type agricultural co-operatives were closed and their lands were subdivided and re-distributed to their former owners or descendants, who, in many cases, did not continue to cultivate them for years. Thus, large, formerly well-kept agricultural fields were abandoned and quickly colonized by ragweed (Makra et al., 2005; Kiss, in press). In addition, new roads, motorways, shopping centres, etc. were built, but little effort was put into landscape management. This created large disturbed areas where ragweed readily became established. In less than a decade, ragweed became the most widespread weed species in both agricultural and urban areas in Hungary, as well as in many neighbouring countries, with the notable exception of Austria, where landscape management standards remained unchanged.
Until the 1970s, common ragweed was merely one of many weed species present especially in agricultural fields in some parts of Eastern Europe, although its expansion during previous decades has been well documented (Béres, 2003). In Hungary, for example, the national weed surveys in wheat and corn fields showed that A. artemisiifolia occupied the 21st position on the list of the most widespread weeds in 1950, then became 8th on the list in 1970, 4th in 1988, and since the early 1990s it has been the most widespread weed in the country (Béres, 2003).
Pollen counts identified airborne ragweed pollen in Hungary for the first time in the late 1960s. Since then, its concentration, measured during the main pollination period of A. artemisiifolia, has increased dramatically: since the early 1990s it has regularly reached values as high as 1000–2000 pollen grains m−3 air on peak days, especially in southern Hungary (Makra et al., 2004). From the 1990s onwards, the values of the average daily counts, the total counts per year, and the counts on peak days have all indicated that the southern part of Hungary and the northern part of Serbia-Montenegro are the areas of Europe most heavily polluted with ragweed pollen.
It is interesting to note that the Eastern European spread of ragweed is associated not only with the collapse of communism, but also with its inception. Soon after 1945, when private agricultural fields were seized from their owners to create socialist-type co-operatives, ragweed populations started to spread in many fields (Béres, 2003). As the appearance of these plants, not seen before by most people, was clearly associated with the new political situation, the popular name ‘Stalin weed’ was used for ragweed in some regions of Hungary.
The massive spread of ragweed in different parts of the world is always correlated with major socio-economic transitions that increased the area of disturbed lands. For example, analysis of pollen in Canadian soils showed that in the 18th and 19th centuries the increased agricultural activity as a result of settlement by Europeans coincided with an increase in ragweed pollen in these regions (Bassett & Crompton, 1975). The data provided by Chauvel et al. (2006) showed that the intense agricultural trade between America and Europe and the First World War have both contributed to the spread of ragweed. The most recent explosion of its populations in Eastern Europe further highlights the role of socio-economic factors in its expansion.
Despite its importance in both agriculture and human health, little is known about ecological and other aspects of the European ragweed invasion. It is somewhat surprising that no reliable maps of the European distribution of A. artemisiifolia are available to date (Déchamp & Méon, 2002). A recent intercontinental transplant assay showed that experimental ragweed plants suffered much less herbivore damage in France than in Canada (Genton et al., 2005b). However, our knowledge of natural enemies of A. artemisiifolia in Europe is scarce (Kiss, 2006). Molecular analyses have revealed, at least in part, the North American origin of some French invasive ragweed populations (Genton et al., 2005a), but no such information is available for the Eastern European populations, which have invaded far larger territories than the western European ones. Clearly, many more studies are needed to obtain a comprehensive picture of ragweed invasions in Europe.