Source: University of Illinois

The occurrence of weed biotypes and populations in Illinois that are resistant to one or more herbicide site-of-action families continues to increase in frequency, acres infested, and number of species. The most recent evidence indicates that Illinois weeds have evolved resistance to six sites of herbicide action and that 12 different species have populations resistant to one or more of these herbicide families. A list shows the weed species in Illinois that include herbicide-resistant populations and the herbicide families to which they are resistant.

Some instances of resistance have been known for many years, while others have been documented relatively recently. Triazine-resistant common lambsquarters, for example, was identified in Illinois during the 1980s, whereas glyphosate-resistant horseweed was identified just a few years ago. Dr. Bryan Young, weed scientist at Southern Illinois University Carbondale, recently identified at least one population of palmer amaranth in far southern Illinois that is resistant to glyphosate. He has also expressed concerns about poor control of some southern Illinois giant ragweed populations with glyphosate. Those who are interested in learning more about the prevalence and distribution of herbicide-resistant weed populations in Illinois and around the world can find a great deal of information at the International Survey of Herbicide-Resistant Weeds, a Web site maintained by Dr. Ian Heap. 

Original news release