The biology and host specificity of the beetle,Gastrophysa atrocyaneaMotch were studied in the laboratory in Nanchang, China. This univoltine insect completed its immature stage in 32–39 days with adult overwintering in soil. A total of 35 plant species from 21 families were tested. Both larvae and adults attackedRumex japonicusandRumex acetosawithR. japonicusthe main host. Larvae also attacked twoPolygonumspecies,P. hydropiperandP. perfoliatum.But the adults are not concurrent with the twoPolygonumspecies. Though oviposition occurred on five other plant species, none of them supported larval development. Field observations showed thatG. atrocyanaecaused a progressive decline in naturalR. japonicuspopulations. Thus it is recommended as a valuable biological agent for this weed.