Monitoring herbicide resistance gene flow in weed populations.

Although herbicide-resistant (HR) weeds can be regularly monitored in fields via surveys, areawide monitoring of both cropland and ruderal (non-crop disturbed) areas is required for species with high propagule mobility. With increasing occurrence of HR weed populations in many agro-ecoregions, the relative contribution of independent evolution through herbicide selection and movement of HR alleles via pollen or seed needs to be elucidated to inform management and help preserve the remaining public good and common resource of herbicide susceptibility. Molecular markers available for many weed species can be utilized to assess regional gene flow accurately. In this chapter, we outline recommended principles and protocols for areawide monitoring of herbicide resistance gene flow in weed populations, exemplified by a case study of glyphosate resistance in kochia (Bassia scoparia A.J. Scott syn. Kochia scoparia (L.) Schrad.) in western Canada. Since being introduced from Eurasia to the Americas over a century ago, both seed- and pollen-mediated gene flow in the species have aided rapid range expansion and the spread of herbicide resistance.