Over the past few years, we (entomologists and weed scientists) have received reports from some farmers who have experienced poor control of giant ragweed after application of a translocated herbicide. In many instances, the weeds that were not killed were infested with a stalk-boring insect that had disrupted the translocation of the herbicide. Although the species of the insect involved was not verified in all fields, stalk borers were responsible for this in at least some of the fields. As you know, giant ragweed is a primary host for stalk borers.|
Recently I received a report that some marestail (horseweed) plants had not been killed after an application of Roundup to a field that had not yet been planted. Two Monsanto Company representatives, Ron Lloyd (agronomic systems manager) and Craig Riley (retail sales manager), dug up some of the surviving marestail plants and found extensive tunneling in the stems. Instead of stalk borers, they found small beetle grubs feeding in the stems. I inspected the samples and have determined that the larvae are some type of beetle; however, they have not been identified to species.
Marestail plants with evidence of insect tunneling. (Photograph courtesy of Ron Lloyd, Monsanto Company.)
I will try to have the specimens identified to species by a Coleoptera expert with the Illinois Natural History Survey. We should have additional information about this situation shortly.
If you become aware that insects of any type seem to be interfering with translocation of any herbicide, please contact me or one of the Extension weed scientists, Aaron Hager or Christy Sprague. We are interested in learning more about this phenomenon.--Kevin Steffey