I think there will be at least three events that 2007 will be remembered for in eastern Iowa: 1) sudden death syndrome in soybeans, 2) all the planes flying on fungicides for corn and insecticides for aphids on soybeans, and 3) all of the corn rootworm problems on rotated corn.

I was trying to evaluate a fungicide trial on corn this week and had a difficult time walking through the field because so much of the corn had lodged due to rootworms. I have never seen so many corn fields on soybean ground with rootworm problems. Those that have scouted soybean fields this year know that the northern corn rootworm beetle (small green beetle) is the most abundant pest found in soybeans (other than aphids). Fortunately they seldom do enough feeding damage in soybeans to justify a treatment, but their numbers tell me that we definitely have a problem with rootworms in rotated corn. Although we do have more corn on corn this year, the vast majority of corn in eastern Iowa is still grown on soybean ground. I think many of the beetles we see in both corn and soybean fields are emerging from soybean ground from eggs that were laid 2 years earlier when the field was last in corn. Emergence traps placed in corn fields the last couple of years have confirmed that we do have northern corn rootworms emerging from the soil on rotated corn in many eastern Iowa corn fields.

Unfortunately, we don’t have a good way to predict which fields will have a problem with the northern corn rootworm in a corn-bean rotation. With the variant of the western rootworm beetle (small black striped beetle) that lays its eggs in soybeans, there is a method to make a management decision. Yellow sticky traps can be placed in the soybean fields, and a decision made on whether some type of rootworm treatment is needed the following year based on how many beetles are found on the traps. These beetles have been in Illinois for many years now and are becoming more common in far eastern Iowa. With the extended diapause population of northern beetles that are more common in Iowa, we have not found a good way to make a management decision. In studies done in the 90’s in NW Iowa where they have battled the extended diapause problem for decades, it was found that the number of beetles trapped in a corn field in a corn-bean rotation had no relationship with whether there would be a rootworm problem in that field 2 years later. It was also found that on the average it did not pay to use a corn rootworm insecticide on rotated corn even in areas where there were extensive problems with extended diapause. I’m not sure if we will change how decisions are made in the future considering the higher corn prices, as well as all of the additional rootworm management options we have today, including genetics and seed treatments.

SOURCE: Iowa State’s Jim Fawcett

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