University of Illinois weed scientists have confirmed resistance to the herbicide dicamba in a Champaign County waterhemp population. In the study, dicamba controlled 65 percent of the waterhemp in the field when applied at the labeled rate. And in the greenhouse, plants showed a 5-to-10-fold reduction in dicamba efficacy compared with sensitive plants. It’s not a huge level of resistance, but the population had never been sprayed with dicamba or its relative 2,4-D, to which it is also resistant. The team tested dicamba resistance in the same field back in 2014 and 2015, showing 80 percent efficacy. Dicamba resistance was recently documented in a Tennessee waterhemp population, and in a Tennessee population of Palmer amaranth. Dicamba has been used in Illinois for at least 50 years. But with more acreage being planted in dicamba tolerant soybeans, use of the chemical is on the rise. And with increasing selective pressure, dicamba resistance could spread quickly.