Illinois Natural History Survey - University of Illinois

Rootworm Problems in First-year Corn: an Update

Western and northern corn rootworms are the most serious insect pests of corn grown after corn in the Midwest. Both species have a single generation per year. The adult beetles are present in cornfields from July through frost. They feed on corn pollen, silks, immature kernels, and foliage. From late July through September, egg laying occurs primarily in cornfields; few eggs are normally laid in other crops. The eggs remain in the soil until the following spring, when egg hatching begins in late May and early June. The larvae can survive only on the roots of corn and on the roots of a limited number of grasses. This larval feeding may reduce the amount of water and nutrients supplied to the developing corn plants. Extensive root damage makes plants more susceptible to lodging (plant lean- over or elbow). Larval feeding may also facilitate infection by root and stalk rot fungi, resulting in further damage. Yield losses also result from difficulty in harvesting lodged corn.

For many years, the practice of growing corn in rotation with soybeans, wheat, oats, or alfalfa provided excellent control of corn rootworms because their eggs are laid almost exclusively in cornfields, and larvae must feed on corn roots the following season to complete their life cycle. Unfortunately, western corn rootworm injury to first-year corn following soybeans has been increasing in severity for several years in east-central Illinois. Use of pyrethroid insecticides, primarily permethrin for corn earworm control in seed corn production fields, was initially suspected in the early outbreaks of 1987-1992. It was thought that these insecticides repelled adult western corn rootworm beetles from treated seed cornfields to nearby soybean fields where they laid their eggs. In 1993, 1994, and especially in 1995, the problems became increasingly more frequent and severe and included many fields of commercial corn that were not near cornfields treated with pyrethroid insecticides the preceding year.

Rootworm larvae collected in four east-central Illinois damaged cornfields in July 1995 and reared to the adult stage were all the western species, as were all beetles captured with emergence cages placed in two affected fields. Eggs from female western corn rootworm beetles captured in problem areas have not shown evidence of the prolonged diapause trait, that is, eggs hatch in a normal fashion after a single winter. Soil samples taken in October 1995 from three soybean fields that were adjacent to injured cornfields revealed that significant western corn rootworm egg laying was taking place. No viable eggs were present in earlier soil samples taken in mid-July. This confirms our belief that the problem is being caused by eggs that are laid in soybean fields rather than eggs that prolong their diapause for more than one winter.

Large cage studies with western corn rootworm beetles from problem fields in Saunemin, Illinois, and beetles from nonproblem areas in Mead, Nebraska, were conducted in a greenhouse during the summer of 1995. The beetles from Mead were used because of the heavy concentration of continuous corn in that area. If differences in egg-laying behavior were to be evident, we reasoned that these two population extremes would be good choices for this test. When given a choice between mature corn and soybean plants, western corn rootworm beetles from Illinois laid a significantly greater percentage of their eggs in the soil of the soybean plants than beetles from Nebraska.

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Greenhouse cage containing western corn rootworm beetles and mature corn and soybean plants.
(Photo by Eli Levine.)

We believe that because of the intense crop rotation in east-central Illinois, corn producers may have selected inadvertently for a strain of western corn rootworm that lays eggs in soybean fields. To date, all evidence suggests that some western corn rootworm adults are laying eggs in certain soybean fields. The eggs overwinter in the soil for a single season and after hatch occurs, the larvae begin to feed on corn roots in early June. If this hypothesis is proven true, corn production throughout the Corn Belt could be affected significantly. Why this egg laying in soybean fields is occurring is not known at this time.

Rapid dispersal of the western corn rootworm has occurred previously. This species was of little concern to U.S. corn production prior to the 1950s because this insect remained along the western edge (Colorado, Nebraska, and Kansas) of the major U.S. corn production area. As irrigation increased, western corn rootworms began to lay eggs in irrigated soils, many of which were treated with chlorinated hydrocarbon insecticides. In a few years, western corn rootworms developed a 1,000-fold resistance to these insecticides (aldrin and heptachlor). The resistant strain spread rapidly from a single location in southeastern Nebraska in 1961: by 1973, the entire Corn Belt was enveloped.

Corn producers in neighboring states are watching with apprehension as the situation in Illinois unfolds. Research is under way to keep crop rotation a viable option for corn rootworm management. If crop rotation fails as a management tool for corn rootworms, the economic impact could exceed $100 million for Illinois corn producers.

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Western corn rootworm beetle feeding on soybean flowers.
(Photo by Eli Levine.)

The use of crop rotation has, and continues to be, the main pest management strategy for corn rootworms in Illinois and across the Corn Belt. Since 1993, the incidence and severity of corn rootworm larval injury in first-year cornfields throughout much of east-central Illinois have increased. Producers in the following counties have been significantly affected most often: Champaign, Ford, Grundy, Iroquois, Kankakee, Livingston, McLean, Vermilion, and Will. Only very isolated cases of first-year corn larval injury have been reported outside this cluster of counties. In addition to east-central Illinois, some producers in northwestern Indiana have reported similar rootworm problems in corn following soybeans. Thus far, no other states have indicated that they are experiencing western corn rootworm larval injury in first-year cornfields, suggesting the problem is very isolated.

If producers in east-central Illinois experienced first-year corn larval injury and found western corn rootworm adults in adjacent soybean fields in 1995, they should consider the use of a soil insecticide in corn following soybeans in 1996. This recommendation will remain in effect until more complete explanations and economic thresholds can be determined. Growers outside of east-central Illinois are strongly encouraged not to use a soil insecticide on first-year corn for rootworm control as a standard practice.

We gratefully acknowledge funding from the Illinois Council on Food and Agricultural Research (C-FAR) that supported this research.

Eli Levine and Michael Gray, Center for Economic Entomology



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Subject: INHSPUB-00415
Last Modified 4/24/96



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