The establishment and spread of Euphorbia graminea Jacq. in sugarcane fields of Miyako islands Okinawa prefecture, Japan

Euphorbia graminea Jacq. (Euphorbiaceae), which is native to Mexico, was first recorded at Okinawa-honto Island, Japan in 2004. The authors recognized E. graminea growing in a wide range of Miyako-jima Island and invading sugarcane fields. To confirm the E. graminea distribution in the Miyako-jima region and to elucidate its expansion process, we conducted a herbarium survey, re-examination of earlier works related to weeds in sugarcane fields, and field surveys of 2017–2018 and 2019. The first record of the species in the Miyako-jima region was in 2007. Subsequently, within about five years, it expanded its distribution quickly and invaded sugarcane fields. Within about five years, it spread to almost all areas, except for a few islands. It was found in 20–30% of all the sugarcane fields in 2019. Our results suggest that E. graminea has become a major weed in sugarcane fields of the Miyako-jima region. Considering the rapid expansion of its distribution in the Miyako-jima area, it is expected to be necessary to monitor the alien species carefully in sugarcane fields of other regions of Okinawa Prefecture.