Illinois Natural History Survey - University of Illinois

Western Corn Rootworm Problems

Western and northern corn rootworms are the most serious insect pests of nonrotated corn in the Midwest. In June 1987, severe corn rootworm larval injury to corn grown for seed production (inbred corn) was reported within a 1-square mile area near Piper City in Ford County, Illinois. The injury to corn roots occurred in six fields that had been planted to soybeans grown for seed production the previous year. All fields were free of volunteer corn or heavy weed infestations in 1986. Since that time, my laboratory has been trying to find the cause for this damage. The severe corn rootworm problem reoccurred in the same area in 1988 and in the years since.

We quickly determined that the damage was caused by the western corn rootworm, not the northern corn rootworm. This was unexpected because prolonged diapause or dormancy is well known in the northern corn rootworm and only recently has it been reported in the western corn rootworm, but at very low levels (less than 0.2% of any eggs observed). Prolonged diapause allows eggs to pass through two or more winters before hatching rather than the normal one winter pattern. Egg hatch studies with eggs from the Piper City population of western corn rootworms, however, did not show any evidence of prolonged diapause.

Although egg laying by Piper City western corn rootworms was indeed taking place in soybean fields, a large field study with different plantings of soybeans in Urbana, less than 60 miles away, confirmed earlier published studies that neither western nor northern corn rootworms lay enough eggs in weed-free soybean fields to cause economic damage to a subsequent crop of corn. Because western corn rootworm adults are quite mobile and considerable genetic mixing is thought to occur, we expected that Urbana and Piper City populations would show similar egg-laying behavior.

We also investigated the possibility that western corn rootworms may have laid eggs in the Piper City soybean fields because pyrethroid insecticides used on neighboring seed corn may have repelled rootworm females into the nearby soybean fields. Pyrethroid insecticides are routinely used for corn earworm control in seed corn and are typically applied during the first two weeks of August, the period of initial corn rootworm egg-laying. In several laboratory bioassays, we demonstrated that permethrin, a pyrethroid insecticide, could repel western corn rootworms from treated corn and cause them to lay eggs in untreated soybeans. We concluded that the situation at Piper City could very well have been caused by pyrethroid insecticide use.

Western corn rootworm beetle on corn leaf. (photo by Eli Levine)

During the summer of 1993 we received a few reports of rootworm larval injury to first-year seed or commercial (hybrid) corn following soybeans outside the Piper City area (but still in east-central Illinois). One of the fields was a seed cornfield in Flatville where pyrethroid insecticides were routinely used for corn earworm control. The remaining fields were in the Homer area and involved commercial corn with no history of pyrethroid use in the immediate area. Corn rootworm larval injury was severe in the Flatville field and moderate in the Homer fields. The western corn rootworm was overwhelmingly the predominant species in both areas. Western corn rootworm eggs from females collected in the Homer area were subjected to natural overwintering conditions in the laboratory. Eighty-three percent of the eggs hatched and 11% remained unhatched, but appeared to be in good condition, by the end of June 1994. These eggs are being subjected to another overwintering cycle. If they hatch in June 1995, we will know that they have the prolonged diapause trait. As mentioned earlier, the percentage of western corn rootworm eggs found with the trait has been less than 0.2%. If a large portion of the Homer eggs hatch in 1995, this would certainly be cause for concern.

During the summer of 1994, a number of new reports of rootworm larval injury to first-year commercial corn following soybeans were received, again all in east-central Illinois. One field near Dewey, several fields near Crescent City, and a couple of fields near Sibley sustained severe rootworm injury. The predominant species was the western corn rootworm in the fields near Dewey and Crescent City. The fields near Sibley also contained sizable populations of northern corn rootworm adults, so prolonged diapause in the northern corn rootworm cannot be ruled out. Pyrethroid use in the vicinity of all these fields was minimal. Eggs were obtained from western corn rootworm females collected at Dewey, Crescent City, and Sibley and from northern corn rootworm females collected at Sibley to check for the prolonged diapause trait. Preliminary results will not be available until June 1995 and a final determination will have to wait until June 1996.

Rootworm beetles are found frequently in soybean and alfalfa crops during the growing season. However, that does not necessarily mean that they are depositing their eggs in these locations. Although we did not find significant western corn rootworm egg-laying in our earlier soybean planting date study at Urbana, we decided nonetheless to sample western corn rootworm beetle population densities in soybean fields adjacent to problem cornfields.

For comparison, soybean fields in the Champaign-Urbana area, where no reports of problems have been received, were also sampled.

Western corn rootworm beetle on corn tassel. (photo by Eli Levine)

Western corn rootworm beetle counts averaged 8.4 beetles per 100 sweeps in Champaign-Urbana soybean fields. In contrast, beetle counts in soybean fields near problem cornfields averaged 61.4 beetles per 100 sweeps. Although this does not prove that the greater abundance of rootworm adults in soybean fields near problem cornfields leads to greater egg-laying in these soybean fields, the results are intriguing nonetheless. The assumption that the egg-laying behavior of different populations of western corn rootworms in east-central Illinois is the same may have been incorrect.

Although problems with western corn rootworms at Piper City can be explained by pyrethroid use, other problem fields in east-central Illinois do not fit that pattern. It is possible that intense crop rotation in this part of the state may have selected for western corn rootworms that lay eggs in soybean fields. Whether pyrethroid use played a role in this process is open to question.

Eli Levine, Center for Economic Entomology 

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Subject: INHSPUB-2161
Last Modified 3/19/96

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