Extension Ag Update
November/December 2003
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What’s in Your Weeds?

Kelly Cook, Extension Entomologist, Urbana, 217-333-6652, kcook8@uiuc.edu and Dawn Nordby, Extension Weed Scientist, Urbana, 217-244-7497, dnordby@uiuc.edu

In 2002 and 2003, many growers were faced with poor herbicide performance. While performance failures occur every year, many of these control failures appeared to be different than those associated with weather, herbicide resistance, and other common factors known to reduce herbicide efficacy. Affected plants initially appeared to succumb to the herbicide with the death of the upper portion of the weed. However, new growth appeared from the lower stem allowing the weed to survive. The failure of the herbicide to work effectively was, in at least some instances, due to the presence of weed-boring insects. These insects bore into the stems of the plants, injuring the vascular system. The insect feeding appears to prevent the herbicide from translocating and working effectively.

This past summer, a statewide survey was conducted to determine the distribution of stem-boring insects in Illinois. Results from the survey identified regions with high occurrences of reduced herbicide efficacy due to stem boring insects. Two areas with high occurrences of this problem were western and northeastern Illinois. Weeds identified during the survey with insects included giant ragweed, Pennsylvania smartweed, curly dock, common lambsquarters, and waterhemp. Several of the insects discovered tunneling in the stems of the weeds including stalk borer, European corn borer, Epiblema species (Family Tortricidae), and several unidentified Lepidopteran larvae. Also found in the stems of weeds were the Dectes stem borer and the Hippopsis stem borer (Ragweed borer). Some weevil larvae were found tunneling in the roots of common waterhemp.

A herbicide application timing study was conducted at the Northern Illinois Agronomy Research Farm in DeKalb to investigate the role of herbicide application timing in insect-weed interactions. Conclusions from Dekalb indicate that herbicide application timing plays a large role in insect infestation. The time of herbicide application affected the number of weeds with tunneling and the number of insects in the weeds. An early post treatment was the most effective in reducing the occurrence of insect-weed interactions. Insects had the highest frequency in the giant ragweed treated with preemergence and late postemergence herbicides.

We have just started to understand the effects of insect tunneling in weeds. Little is known about the habits and life cycles of many of the insects as well as the role of weeds in this triangle. Additional research is being done to examine the effect of insect tunneling and injury on herbicide rate and uptake effects. We will once again be conducting a survey to determine the distribution of weed-boring insects in Illinois. If you experience this problem in your fields, feel free to contact Kelly Cook (Extension Entomologist, 217-333-6652, kcook8@uiuc.edu) or Dawn Nordby (Extension Weed Scientist, 217-244-7497, dnordby@uiuc.edu).