Establishment of Lolium species resistant to acetolactate synthase‐inhibiting herbicide in and around grain‐importation ports in Japan

Compared with natural seed dispersal, human‐mediated seed dispersal could spread herbicide resistance genes on a much larger scale. Herbicide‐resistant weed seeds have been reported as contaminants in commercial grain. We investigated the contamination of seeds of Lolium species with target‐site mutations conferring resistance to acetolactate synthase (ALS)‐inhibiting herbicides in wheat imported from the USA, Canada and Australia into Japan. We also investigated the establishment of ALS‐inhibiting herbicide‐resistant Lolium species in 12 seaports in Japan that are major entry points for international commodities. We found herbicide‐resistant Lolium spp. seeds from all classes of wheat samples. Resistant individuals became established at six of eight ports where more than 50 kt of imported wheat is unloaded every year. The establishment of resistant Lolium spp. individuals was common at major grain landing ports. Monitoring over 3 years at one port revealed that the frequency of resistant individuals did not fluctuate between years. Many resistant individuals were distributed in front of the entrance of a fodder company, but a few resistant individuals were found in areas 2 km away from the port. The results indicate that gene flow is rare through pollen or seed movement from resistant plants to peripheral populations. Further extensive and long‐term monitoring is necessary to perform a comprehensive risk assessment of herbicide‐resistant plants entering Japan through major commercial ports.