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Weeds to Watch for in 2003

May 1, 2003

Over the past two winters, at Extension meetings throughout the state, we conducted a survey titled "The Illinois Invasive Weeds Survey." This survey was designed to determine what weeds are the most prevalent throughout the state and to give us a head start on what may be some of the emerging weed problems in the future. Results from the 2002 survey can be found in the article "Weeds on the Horizon" in issue no. 6 of the 2002 Pest Management and Crop Development Bulletin. Prior to last year's survey, it had been several years since a survey of this nature had been conducted in Illinois, and it is very interesting to look back and compare the results from years past.

The first question of the survey asked the participants to rank the top 6 weed species that they encountered most frequently in their cornfields and soybean fields. This question generated a list of more than 50 different species of weeds that grew in corn and soybeans over the past 2 years. Not surprising, waterhemp and giant ragweed were the top two weeds common in both crops. In 2002, giant foxtail was among the top three, and this year velvetleaf was added to the top-three list. These results are very different from the results obtained from a survey conducted in 1995, in which velvetleaf, common cocklebur, and giant ragweed were ranked as the most common weeds in corn, and velvetleaf, common lambsquarters, and common cocklebur as the most common weeds in soybeans.

Table 6 lists the weeds that growers, retailers, consultants, and Extension educators ranked as the top six broadleaf weeds in corn and soybeans for 2003. This table also provides the percentage of times each weed appeared as the number one most common weed on the survey. In this survey, the annual grasses that appeared most frequently in corn were giant foxtail, fall panicum, and woolly cupgrass; and in soybean giant foxtail was the predominate grass species. In comparing this year's results to the 2002 survey, the only change that shows up is that horseweed (marestail) is added to the list of the most common weeds in soybean. Horseweed is a species that has shown up in more soybean acres across the state over the past few years. Stay tuned to next week's newsletter on why this weed has possibly become more predominate throughout the state.

The next question asked the participants to rank the top 3 weed escapes that they most frequently encounter in their cornfields and soybean fields, and whether they were escapes due to"late emergence"or "hard to control." The top three escapes in corn were waterhemp, giant ragweed, and woolly cupgrass; in soybeans, waterhemp again topped the list for 2 years in a row, followed by giant ragweed and velvetleaf (Table 7). Woolly cupgrass replaced giant foxtail as one of the escapes in corn this year compared with last year's results.

The final question asked what weed species are becoming more frequent or invasive in fields, ditches, and wooded areas. The top three weeds that are becoming more invasive in fields are waterhemp, giant ragweed, and common pokeweed. Even though waterhemp and giant ragweed ranked extremely high in the previous questions, many growers are just beginning to see these weeds move into their fields. Giant ragweed, common pokeweed, and poison hemlock were ranked the top 3 weeds in ditches, and in wooded areas common pokeweed, poison hemlock, and poison ivy were the top three. Poison hemlock and poison ivy replaced multiflora rose and giant ragweed from the 2002 survey.

Knowing which weeds are becoming more prevalent in these areas should give us an idea of what will be the problem weeds of the future. We have seen some very consistent results from our surveys over the past 2 years and hope this information will help keep you aware of what weeds you should be looking for in your fields this coming season.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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