Distribution and abundance of an allergenic weed, common ragweed (Ambrosia artemisiifolia L.), in rural settings of southern Quebec, Canada

Simard, M.-J. and Benoit, D. L. 2010. Distribution and abundance of an allergenic weed, common ragweed (Ambrosia artemisiifolia L.), in rural settings of southern Quebec, Canada. Can. J. Plant Set. 90: 549-557. Common ragweed (Ambrosia artemisiifolia L.) is an important weed of urban and rural settings in eastern Canada. Where the species is abundant, its wind-dispersed pollen is responsible for most cases of allergic rhinitis or hayfever in August and September. Despite its adverse health effects, there is little information on the actual abundance or distribution of ragweed plants in rural settings. Ragweed surveys were therefore done in July and August (after herbicide application) in corn and soybean fields, field borders and along rural roadsides surrounding two cities in southern Quebec. Based on zero-inflated Poisson regression models, ragweed density averaged 4.1 plants m(-2) (Saint-Jean-sur-Richelieu area) and 16.1 plants m(-2) (Salaberry-de-Valleyfield area) along roadsides. Ragweed density in field borders (1.3 plants m(-2)) and fields was lower than on roadsides. Conventionally tilled fields and fields where tillage was reduced had equivalent densities of ragweed. Ragweed abundance in fields was likely related to the efficacy of herbicides used in transgenic vs. conventional crops. Transgenic herbicide-resistant corn fields had higher ragweed densities than conventional fields (0.44 vs. 0.07 plants m(-2)), while herbicide-resistant soybean fields had lower densities than conventional fields (0.02 vs. 1.33 plants m(-2)). Field borders located closer to roadsides had slightly higher ragweed counts, while roadside densities did not depend on the proximity of a field entrance. This suggests that roadsides are currently potential sources of spread into fields more than the opposite. Further research on the pollen production and dispersal of these rural populations is needed.