Bidens spp., known as beggarticks are important weeds and reduce grain yield in several summer crops in Brazil. In the states of Rio Grande do Sul and Paraná, the control of this weed with ALS-inhibiting herbicides is becoming inefficient, suggesting an increasing weed resistance problem. The objectives of this research were to evaluate the geographic distribution of ALS-resistant Bidens spp. in the states of Rio Grande do Sul and Paraná and to determine the main agronomic causes of resistance selection. Seeds of beggarticks biotypes were sampled in 35 counties in the states of Rio Grande do Sul and Paraná, in places with suspected resistance to ALS-inhibitors. Farmers were applied questionnaires on methods of weed control, tillage system, crop rotation and harvesting operations during current and previous years. Bidens spp. resistant to ALS-inhibiting herbicides were found in all counties assessed. No crop rotation and intense use of no-tillage are predominant on the farms. The most probable cause of selection of herbicide-resistant Bidens spp. is the intensive use of ALS-inhibitors.
chlorimuron; beggarticks; resistance
