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Ambrosia artemisiifolia (common ragweed) is an invasive species native to North America and was accidentally introduced to Europe in the 19th century. Widespread in disturbed habitats, it is a major weed in spring-sown crops and it causes serious allergic rhinitis and asthma due to its allergenic pollen. The aim of this research was to analyse the effects of both competitive vegetation and herbivory by Ophraella communa to control A. artemisiifolia in an agricultural area of north-western Italy. Hayseed mixtures, both over-seeded over the resident plant community or after ploughing, when seeded before the winter season, were able to suppress the establishment of A. artemisiifolia as well as to reduce its growth in terms of plant height and inflorescence size. Defoliation of A. artemisiifolia by O. communa at the end of the growing season was conspicuous but most of the plants still produced flowers and seeds. However, significant O. communa attack was recorded for reproductive structures. As for non-target species, O. communa was mainly recorded on Asteraceae, with low density and low degree of damage. Reduction of inflorescence size due to competitive vegetation and damage to male flowers by O. communa may diminish the amount of available pollen. The results of this study may be useful for the implementation of management measures to control A. artemisiifolia in agricultural areas using mixtures of native species.
Ambrosia artemisiifolia is an invasive annual herb infamous for the high allergenicity of its pollen, which is related to increasing medical costs. Additionally, it can cause serious yield losses as agricultural weed. Common ragweed seeds accumulate in the soil and can remain therein viable for decades, which poses a problem for the sustainable management of these populations. A long term management should thus target a reduction of the soil seed bank. We observed the influence of four different mowing regimes on the ragweed soil seed bank at six roadside populations in eastern Austria. The mowing regimes were based on methods from common roadside management practice and specifically adapted to reduce seed production. After three years of application, the soil seed bank was indeed reduced by 45 to 80 percent through three of the four mowing regimes tested. Therefore, we suggest that the best mowing regime for the most effective reduction of the size of the soil seed bank is the one consisting of one cut just after the beginning of female flowering (around the 3rd week of August in Eastern Central Europe), followed by a second cut 2–3 weeks later.
The knowledge of phenotypic variation in the European range of the highly allergenic Ambrosia artemisiifolia L. (common ragweed) is not entirely complete, even though it is an invasive species of utmost concern. We hypothesized the prevalence of phenotypic differentiations between common ragweed populations in the introduced range, and we assumed that those differentiations were related to environmental conditions at the points of origin. Using a common garden experiment, we investigated biomass allocation, growth rates, and flowering phenology of 38 European common ragweed populations originating from a major geographical gradient. We observed considerable phenotypic variation in growth parameters and flowering phenology, e.g. mean aboveground biomass varied from 23.3 to 47.3 g between the populations. We were able to relate most measured traits with environmental parameters prevailing at the points of origin. For example, early growth of ruderal populations was highly correlated with temperature and precipitation at the point of origin. Late growth and flowering phenology were highly correlated with latitude, i.e. individuals from northern populations grew smaller and flowered and dispersed their pollen and seeds up to 5 weeks earlier than individuals from southern populations. We also found a longitudinal gradient in flowering phenology which has not yet been described. The existence of such a high variability in the introduced range may facilitate further range expansion. We suggest that the correlation with environmental variables rests upon genetic variation possibly due to adaptations to the respective environment. To clarify if such adaptation results from multiple events of introduction or as evolutionary response after introduction, genetic investigations are needed.