Fields of first-year corn are occasionally damaged by corn rootworm larvae in northwestern and north central Iowa. Because rootworms typically have a one-year life cycle, rotating corn with another crop has been a very successful strategy for controlling this insect. Unfortunately, some populations of the northern corn rootworm have adapted to the corn-soybean rotation and now have a two-year life cycle.

This two-year life cycle is called extended diapause because the eggs remain dormant in the soil for almost two years before hatching. There is a possibility of lodging, and even a smaller chance that yield loss may occur, from these extended diapausing populations of northern corn rootworms in first-year corn.

What is the probability of yield loss from northern corn rootworms in first-year corn? Paul Kassel and Joel DeJong, extension field crop specialists at Spencer and Sioux City, respectively, and I collected yield data from 60 first-year corn fields that had northern corn rootworms. Fields had strips of corn both with and without a soil insecticide at planting. Machine harvests were collected at the end of the season. The results showed that only 11 percent of the fields had a yield increase that exceeded the cost of the insecticide. Based on our findings, the odds are only 1 out of 9 in favor of your getting an economic return by using an insecticide in first-year corn. Those are very poor odds and strongly suggest that a soil insecticide may not be a good choice.

If you will be planting a first-year corn field in 1994, and you know that there were northern corn rootworms in that field in 1992, what are your management options? Basically, there are three. I will discuss the most practical option first.

Option 1: Don't use a soil insecticide on first-year corn. This should always be the approach when there is no information available regarding the insect population size or there has not been a problem of corn lodging. As I stated above, the probability of getting a yield return that exceeds the cost of the insecticide is not very good.

Option 2: Use a soil insecticide in first-year corn. This is recommended only if extensive lodging occurred in the field during 1992 and beetle counts exceeded five per plant during that year. Beetle count information is probably not available for most fields, so the amount of lodging that occurred may be the best indicator of a potential problem.

Option 3: Rotate out of corn for two years. This is a biological solution that will eliminate most of the northern corn rootworms from a field. But it is probably the least desirable of the three options from an economic perspective. Most farmers will not want to keep corn out of a field for more than one year.

Fields that have not had an extended diapause problem are at low risk from northern corn root-worms. Development of a significant problem may take many years and is influenced by rotation schemes, environmental factors, and genetics of the rootworm. If large areas in a field have not lodged, using a soil insecticide does not make good economic sense.