Fields of first-year corn are occasionally damaged by corn rootworm larvae in northwestern, western and north central Iowa. Because rootworms typically have a one-year life cycle, rotating corn with another crop has been a very successful management strategy. Unfortunately, some populations of the northern corn rootworm have successfully adapted to a 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 59 first-year corn fields that had northern corn rootworms during 1988-1993. Fields had strips of corn both with and without a soil insecticide at planting. Machine harvests were collected at the end of the season. I calculated the value of the corn using the market-year average price and the cost of insecticide at $12 per acre. Only 20 percent of the fields had a yield increase in the treated strips that exceeded the cost of the insecticide. Based on our findings, the odds that you will get an economic return by using an insecticide against northern corn rootworms in first-year corn are only 1 out of 5. Those poor odds strongly suggest that a soil insecticide is not necessary in many fields.
Can the problem be predicted during 1995? Not with a high degree of reliability. We examined northern corn rootworm beetle populations during our study and found that there was not a good relationship between beetle counts one summer and the amount of corn root injury two years later. In some fields, beetle counts of 3-4 per plant resulted in root injury ratings of 3.5 or more two years later. In other fields, beetle populations of 14-17 per plant did not translate into significant root injury two years later.
If you will be planting first-year corn in northwestern, western or north central Iowa, and you know that there were northern corn rootworms in that field in 1993, what are your management options?
Option 1: Dont use a soil insecticide on first-year corn. If there was nolodging of corn during the 1993 harvest, an insecticide may be an unnecessary expense. As stated above, the probability of getting a yield return that exceeds the cost of the insecticide is only about 1 in 5.
Option 2: Use a soil insecticide in first-year corn. This is recommended only if extensive lodging occurred in the field during 1993 or if beetle counts exceeded 3-4 per plant during that year. Beetle count information is probably not available for most fields, so the amount of lodging that you noticed during harvest may be the best indicator of a potential problem in 1995. You may also want to consider using a reduced rate of the soil insecticide.
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 in a particular field are at low risk from northern corn rootworms. Development of a significant problem in any field may take many years and is influenced by rotation schemes, environmental factors, and genetics of the corn rootworm. If large areas in a field were not lodged in 1993, using a soil insecticide in 1995 for corn rootworms does not make good economic sense.