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.

Achieving sustainable crop production while feeding an increasing world population is one of the most ambitious challenges of this century. Meeting this challenge will necessarily imply a drastic reduction of adverse environmental effects arising from agricultural activities. The reduction of pesticide use is one of the critical drivers to preserve the environment and human health. Pesticide use could be reduced through the adoption of new production strategies; however, whether substantial reductions of pesticide use are possible without impacting crop productivity and profitability is debatable. Here, we demonstrated that low pesticide use rarely decreases productivity and profitability in arable farms. We analysed the potential conflicts between pesticide use and productivity or profitability with data from 946 non-organic arable commercial farms showing contrasting levels of pesticide use and covering a wide range of production situations in France. We failed to detect any conflict between low pesticide use and both high productivity and high profitability in 77% of the farms. We estimated that total pesticide use could be reduced by 42% without any negative effects on both productivity and profitability in 59% of farms from our national network. This corresponded to an average reduction of 37, 47 and 60% of herbicide, fungicide and insecticide use, respectively. The potential for reducing pesticide use appeared higher in farms with currently high pesticide use than in farms with low pesticide use. Our results demonstrate that pesticide reduction is already accessible to farmers in most production situations. This would imply profound changes in market organization and trade balance.

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Summary• The geographical structure of resistance to herbicides inhibiting acetyl-coenzyme A carboxylase (ACCase) was investigated in the weed Alopecurus myosuroides (black-grass) across its geographical range to gain insight into the process of plant adaptation in response to anthropogenic selective pressures occurring in agricultural ecosystems.• We analysed 297 populations distributed across six countries in A. myosuroides' main area of occupancy. The frequencies of plants resistant to two broadly used ACCase inhibitors and of seven mutant, resistant ACCase alleles were assessed using bioassays and genotyping, respectively.• Most of the resistance was not endowed by mutant ACCase alleles. Resistance and ACCase allele distribution patterns were characterized by mosaicism. The prevalence of resistance and of ACCase alleles differed among countries.• Resistance clearly evolved by redundant evolution of a set of resistance alleles or genes, most of which remain unidentified. Resistance in A. myosuroides was shaped by variation in the herbicide selective pressure at both the individual field level and the national level.