No. 3 Article 6/April 22, 2011

Herbicide-Resistant Weeds in Illinois: A Cause for Concern

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. Table 2 lists the weed species in Illinois that include herbicide-resistant populations and the herbicide families to which they are resistant.

Table 2. Weed species in Illinois that include herbicide-resistant populations and the herbicide families to which they are resistant.

Weed species

Herbicide family/families to which species is resistant

Common name

Scientific name

Common lambsquarters

Chenopodium album


Smooth pigweed

Amaranthus hybridus

Triazine, ALS inhibitors


Kochia scoparia

Triazine, ALS inhibitors

Common waterhemp

Amaranthus tuberculatus

Triazine, ALS inhibitors, PPO inhibitors, glyphosate, HPPD inhibitors

Eastern black nightshade

Solanum ptycanthum

ALS inhibitors

Giant ragweed

Ambrosia trifida

ALS inhibitors

Common ragweed

Ambrosia artemisiifolia

ALS inhibitors

Common cocklebur

Xanthium strumarium

ALS inhibitors


Sorghum halepense

ALS inhibitors

Giant foxtail

Setaria faberi

ALS inhibitors, ACCase inhibitors


Conyza canadensis


Palmer amaranth

Amaranthus palmeri


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 website maintained by Dr. Ian Heap.

Most weed species listed in Table 2 are resistant to only one herbicide site-of-action family. Resistance to one herbicide family can be challenging to manage, but even more significant challenges arise when a single weed species contains more than one type of herbicide resistance. Resistance to multiple herbicides, which can occur in an individual plant or in a particular field, has become commonplace in Illinois waterhemp populations.

The development of herbicide resistance in Illinois waterhemp populations (Table 3) began in the 1990s, when widespread use of ALS-inhibiting herbicides led to intense selection for waterhemp populations resistant to these products. While not as predominant as waterhemp with ALS resistance, waterhemp resistant to triazine herbicides populates many Illinois fields (moreover, at least two mechanisms of triazine resistance are present in the Illinois waterhemp population). When ALS-inhibiting herbicides failed, soybean farmers relied on PPO-inhibiting herbicides for post┬Čemergence control of waterhemp before the commercialization of glyphosate-resistant soybean varieties. Pursuit plus Cobra or Status were popular tank-mixes during the mid-1990s, when Pursuit alone no longer controlled this "strange pigweed." Soon enough, waterhemp resistant to PPO-inhibiting herbicides was discovered.

Table 3. Chronology of herbicide resistance in Illinois waterhemp populations.


Resistance to ALS inhibitors
Resistance to triazines


Multiple resistance in one biotype: resistance to both ALS inhibitors and triazines


Resistance to PPO inhibitors: the first report of 3-way resistance in a single plant; resistant to PPO inhibitors, ALS inhibitors, and triazines


Resistance to glyphosate: biotype resistant to glyphosate and ALS inhibitors


The first report of 4-way resistance in a single plant: resistant to glyphosate, PPO inhibitors, ALS inhibitors, and triazines


Resistance to HPPD inhibitors: biotype resistant to HPPD inhibitors, ALS inhibitors, and triazines

As farmers began to rely heavily/exclusively on glyphosate in soybean and corn, selection for glyphosate resistance became acute, and resistance to glyphosate was discovered in 2006. Finally, last July we reported the discovery of a waterhemp population resistant to HPPD-inhibiting herbicides. (In January 2011 we published a more detailed account of this novel herbicide resistance in the journal Pest Management Science, Vol. 67, pp. 258-261.) In case you lost count, Illinois waterhemp populations now have evolved resistance to five different herbicide families. We have no evidence that the list of herbicide-resistant weeds will stop growing.

For the past two years weed scientists have surveyed Illinois waterhemp populations to determine the prevalence of individual plants and fields with multiple herbicide resistance. Verification of resistance/sensitivity was conducted using molecular biology laboratory assays to detect the presence of mutations known to confer (or, for glyphosate, to likely confer) resistance to ALS inhibitors, PPO inhibitors, and glyphosate. While the details of the laboratory assays are not germane to this article, the survey results are quite sobering (especially if you think herbicide resistance is predominantly an academic concern).

In 2010 we surveyed 122 plants from 24 different fields. (All samples were submitted in response to an invitation last year in issue 13 of the Bulletin.) Screening for resistance to three different herbicide families can result in eight possible outcomes, which are easily presented in Venn diagrams. The diagrams illustrate all possible outcomes or combinations of multiple factors (or here, all possible combinations of resistance to one, two, or three herbicide families). The results from last year's survey are presented for individual plants in Figure 1 and on a field basis in Figure 2.

Figure 1. Herbicide resistance profile of 122 waterhemp samples submitted for herbicide resistance screening in 2010.

Figure 2. Herbicide resistance profile of 24 fields from which waterhemp samples were submitted for herbicide resistance screening in 2010.

Of the 122 plants submitted and screened (Figure 1), only 13% were sensitive to all three herbicides; 87% of the plants were resistant to at least one herbicide family. The percentages of plants resistant only to glyphosate, to ALS inhibitors, or to PPO inhibitors were 28, 20, and 1, respectively. (In the Venn diagram, these are the numbers in the areas of the circles that do not overlap.) To determine the total percentage of plants resistant to each herbicide, you simply add the four numbers in the shaded area of each herbicide family; doing that arithmetic, we find that 66% (28 + 29 + 7 + 2) of the plants were resistant to glyphosate, 59% (20 + 3 + 7 + 29) were resistant to ALS inhibitors, and 13% (1 + 2 + 7 + 3) were resistant to PPO inhibitors.

How would you determine the percentage of individual plants that were multiply resistant, say to both glyphosate and PPO inhibitors? We can find this percentage (9%) by adding the two numbers (7 + 2) in the intersection of the glyphosate and PPO circles. Similarly, we see from the diagram that 10% (3 + 7) of the plants were resistant to both ALS and PPO inhibitors, 36% (29 + 7) were resistant to both ALS inhibitors and glyphosate, and 7% were three-way resistant-to ALS inhibitors, glyphosate, and PPO inhibitors (7 is the number in the intersection of all three circles).On a field basis (Figure 2), none of the 24 fields from which plants were sampled were without herbicide-resistant waterhemp. A third of the fields contained waterhemp resistant to PPO inhibitors, three-quarters of them contained waterhemp resistant to ALS inhibitors, and 85% contained glyphosate-resistant waterhemp.

Keep in mind that these fields were not randomly selected, but rather were sampled based on suspected resistance to glyphosate. Nevertheless, and perhaps most disconcerting, almost a third of the fields (29%) contained three different types of herbicide resistance, which greatly increases the difficulty of managing these populations using only postemergence herbicides labeled for use in conventional or glyphosate-resistant soybean varieties. Furthermore, the counties from which these samples originated were not confined to a particular area in Illinois; Table 4 lists the counties from which glyphosate-resistant waterhemp populations were confirmed in 2010 using this molecular biology laboratory assay. This list is not exhaustive; in fact, results from this and previous surveys suggest that glyphosate-resistant waterhemp occurs in many fields across the southern half to two-thirds of Illinois.

Table 4. Illinois counties with glyphosate-resistant waterhemp populations confirmed by a molecular biology laboratory assay in 2010.













We hope that these survey results will help erode the perception that herbicide-resistant weeds are a problem that "I don't need to worry about until they occur on my farm." Based on the prevalence of multiple resistance in Illinois waterhemp populations, there might not be a viable postemergence herbicide alternative to control glyphosate-resistant waterhemp the season it becomes well established on your farm. The data also suggest that the occurrence of multiply resistant waterhemp will continue to increase; that evidence, coupled with the lack of development of herbicides with novel sites of action, translates into additional challenges for several seasons to come.

This survey was only one of the projects in the ongoing research on herbicide-resistant weeds in Illinois. Weed scientists at several Illinois universities are working diligently to better understand this phenomenon and find viable solutions for Illinois farmers. Much of the laboratory research at the University of Illinois is under the direction of Dr. Patrick Tranel and Dr. Dean Riechers, while Dr. Adam Davis directs most of the modeling and field dissemination research. Weed science colleagues at Southern Illinois University (Dr. Bryan Young) and Western Illinois University (Dr. Gordon Roskamp) continue to make valuable contributions to the overall research efforts on herbicide-resistant weeds, as do many current and former graduate students and postdoctoral scholars. Much of this research is made possible by many public and private funding sources, including the Illinois Soybean Association.--Aaron Hager

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