Summary1. This account presents information on all aspects of the biology of Ambrosia artemisiifolia L. (Common ragweed) that are relevant to understanding its ecology. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, and history, conservation, impacts and management. *Nomenclature of vascular plants follows Stace (2010) and, for non-British species, Flora Europaea.

Aim  The problems in public health and field management in France caused by Ambrosia artemisiifolia L. require a better knowledge of the introduction and naturalization of this species in both the past and present. Location  France. Methods  The pattern of spread of A. artemisiifolia was investigated through the study of herbarium specimens. More than 1200 specimens were found in 58 herbaria and virtual herbaria in France and in bordering countries. The spread was analysed by mapping the localities for each 30‐year period since 1863. Specific indications as ‘new plant’ were used to determine the timing of the introduction of the species into a new area. Results  It seems that the spread of A. artemisiifolia is not linked to its presence in botanical gardens. The study of specimen labels indicates that this species has been introduced in France in seed crops at various independent geographical points and at various times since its introduction in natural habitats. Commercial trade and American troops have contributed to its spread. Main conclusions  The spread of the species in area and in time over France showed no clear front: new localities separated by large distances were colonized simultaneously. Cumulative numbers of localities show a continuous increase during the twentieth century. Herbarium specimens can be used to follow the spread of A. artemisiifolia.

Summary 1.Pleiotropic effects associated with genes endowing resistance to herbicides are generally predicted to reduce plant fitness. Quantifying these effects is necessary to develop management strategies against herbicide-resistant weeds. We assessed the pleiotropic effects associated with three mutant alleles of the herbicide target enzyme acetyl-coenzyme A carboxylase (ACCase) on plant growth and seed production in black-grass Alopecurus myosuroides . 2. In each of two field experiments, black-grass populations segregating for Leu-1781 (five populations), Asn-2041 (three populations) or Gly-2078 (two populations) ACCase were produced to obtain several distinct, homogenized genetic backgrounds and to permit reliable comparisons among wild-type, heterozygous and homozygous mutant ACCase plants grown in competition with a wheat crop. 3. No significant differences from wild-type plants in vegetative biomass, height and seed production were observed in Leu-1781 or Asn-2041 ACCase plants. 4. Over both experiments, homozygous Gly-2078 ACCase plants displayed a significant reduction in biomass (42%), height (6%) and seed production (36%). Reduction varied with the segregating population and between field experiments. 5. Synthesis and applications . Our work illustrates the variation in fitness cost depending on the resistance gene, the plant genetic background and the environment. This underlines the necessity to identify the resistance gene(s) present in a weed population before designing resistance-management strategies. Competitive crops should be effective against Gly-2078 ACCase plants. The effect of resistant ACCase alleles on seed survival and germination needs to be studied in order to develop cultural practices creating or maximizing fitness costs in resistant plants. However, the variation of fitness cost with the environment and the weed population, as well as the likely absence of fitness cost associated with resistance genes such as Leu-1781 ACCase, renders the success of specific cultural control practices uncertain. A solution could be a weed control programme maximizing the diversity of cultural practices and including anti-resistance cultural practices to avoid or reduce selection for resistant ACCase alleles.

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