New weed threats: extent, origins, and proper management.

Abstract

Abstract Weed problems continue to be an obstacle in plant protection and in natural areas, causing high costs of control and asking for adequate prevention measures. New weed problems in any given area (risk area) may arise from three basic sources: by introduction and naturalization of new weedy plant species, by increasing spread of weedy taxa that are already present in the risk area, and by the evolution of new weedy taxa in the risk area. Introduction and subsequent naturalization of new plant species is taking place worldwide and the number of alien plant species being introduced is steadily increasing. Newly recorded alien plant species prevail in disturbed sites and semi-natural habitats, increasing the risk for new plant invasions into areas of high conservation value. New weed problems in agroecosystems are associated with recently naturalized species, with adaptation and evolution of plant species, but also due to land use change, global warming, and changes in the environmental conditions. These may affect the dynamics of weed populations, triggering sudden range expansion and abundance increase. Evolution of new weedy taxa involves crop/weed hybridization, hybridization between native and alien plants in the risk area, and hybridization between formerly separated taxa that co-occur in the risk area. Evolution of herbicide resistance is another source of new weed taxa. A literature survey revealed more than 40 new weed species of agroecosystems within the last 20 years and about 35 new plant species invading natural and semi-natural habitats during the last ten years. Measures towards mitigating problems associated with new weeds includes regular monitoring and risk assessments, aiming at preventing entry of new weeds and prioritizing weed species for control.