Illinois Natural History Survey - University of Illinois

Western Corn Rootworm Flight Activity in Soybeans

Imagine you are a farmer standing in an east-central Illinois cornfield. It is July 1995, and for as long as you can recall, crop rotation has controlled the western corn rootworm (WCR) in your first-year corn (corn planted after another crop, usually soybeans) -- that is, until now. Instead of row after row of erect green corn plants, you're facing a patchy field of wilting stalks leaning this way and that. You pull up a plant and find that the roots are being nibbled away by the maggotlike larvae of the WCR beetle. You are not alone, because the same story is playing out all over the region: first-year corn is devastated by a pest that should not be there.

 

The WCR is one of the most serious insect pests of corn in the Midwest. Adult WCR beetles, active from July through frost, have one generation per year. Adults feed on corn pollen, silks, kernels, and foliage. Eggs are laid in cornfields from July to September; normally, few eggs are laid in other crops. Eggs remain in the soil until spring and they hatch in late May and early June. The larvae can survive only on the roots of corn and a few grasses. Root feeding reduces the water and nutrient supply to developing plants. Extensive injury makes plants susceptible to lodging (plants lean over or elbow and are difficult to harvest) and may lead to root and stalk rot infections, resulting in further damage.

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Corn lodging caused by larval rootowrm feeding.

Until recently, crop rotation, a method of planting corn and soybeans (or another crop whose roots don't support WCR larval development) in alternate years, controlled the WCR. Rotation disrupts the WCR life cycle because soybeans are grown where corn was planted the previous year. Since WCR lay eggs almost exclusively in cornfields and the larvae need corn roots to develop, planting soybeans where WCR eggs were laid starves the young larvae. Rotation allows first-year corn to be produced without WCR-targeted soil insecticide treatments. Indeed, crop rotation has been one of the great "success stories" for controlling the WCR.

Unfortunately, a behavioral change in some WCR populations is threatening crop rotation. Beginning in 1993, serious WCR larval injury to first-year corn in east-central Illinois and northwest Indiana began to increase. Growers who had successfully controlled WCR with crop rotation in the past suffered serious crop losses. In the early Illinois outbreaks of 1987-1992, it was suspected that pyrethroid insecticides used in seed corn had forced adult WCR beetles into nearby soybeans at egg-laying time; however, by 1993-1995, the problem region included commercial cornfields that were far from any pyrethroid-treated areas.

Studies ruled out pyrethroid repellency or a multiyear egg diapause as explanations, leaving the possibility that rotation itself had selected for a WCR strain that circumvented control by laying eggs in soybean fields (where larvae will emerge into corn the following spring and survive). Egg sampling and field collection of larvae confirm that WCR in east-central Illinois now lay eggs in both corn and soybean fields.

Since the initial damage reports, WCR injury to first-year corn following soybeans has increased in east-central Illinois and northwestern Indiana. Unlike 1995, favorable weather during the summer of 1996 enabled corn plants to tolerate WCR larval root damage; few damage reports were received. During 1996, our monitoring revealed that beetles were moving out of corn and into soybean fields (and other crops) soon after their first detection in corn; thereafter, WCR populations remained higher in soybeans than in corn.

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Western corn rootworm female on corn leaf.

During the 1996 growing season, we were amazed to see that WCR from our problem area were feeding on soybean foliage in the vicinity of nutritionally suitable corn plant parts. Curiously, laboratory studies demonstrated that the soybean plant is a poor food for adult WCR. WCR that were fed only soybean plants (for two weeks) produced no eggs, weighed less, and died before their siblings, which were fed an artificial diet or one of corn silks, tassels, foliage, and immature ears. We also noted that beetles restricted to a soybean diet were much more active than WCR that were fed better diets. Increased activity is a frequent insect response to impending starvation; by moving more actively, the chance of finding food is improved. The lost vigor of soybean-feeding insects was restored by feeding on corn plant parts. Perhaps moving to corn after feeding on soybeans is a consequence of a malaise associated with the inadequate soybean foliage diet. Stomach content analysis of field-collected WCR revealed that 15% of females in cornfields had fed on both corn and soybeans within the last hour. Because WCR that are eating soybean foliage in the field carry significantly fewer eggs than those not eating soybeans, we believe WCR likely eat soybeans only after egg-laying. Given the selective advantage of laying eggs outside of corn, the cost of feeding on an inadequate food source, like soybeans, following egg-laying is small.

WCR egg-laying in soybeans is a problem of movement; crop rotation selects for individuals that move outside of corn to lay eggs. Using a variety of trapping and monitoring methods, we've made exploration of WCR movement a major focus. We have found a variable pattern of WCR movement in the field. Days with high WCR movement frequently were those with moderate to low wind and warm temperatures, while those with less movement had higher average wind speeds. Beetle abundance also changes in periodic fashion during the day; WCR abundance peaks in early to mid-morning samples and just prior to sunset. Significantly fewer WCR are present in the soybean field during late morning and afternoon. Direct observation of insect immigration to and emigration from soybean fields corroborate sweep-sample abundance data.

Our behavioral data reveal a population of insects that may be found in a number of different crops and weeds at times during the growing season when corn is nutritionally at its best. Perhaps close proximity to corn and a capacity to recover from poor food choices by eating corn is behind the WCR willingness to consume a plant (like soybeans) whose tissues do not support adult maintenance or reproductive development.

Egg-laying by WCR beetles in soybean fields continues to be a serious threat to the continued efficacy of a corn-soybean crop rotation for rootworm management. Because crop rotation rewards female WCR who deposit all or some of their eggs outside of cornfields, evolution of the new behavior by the WCR can hardly be unexpected.

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We gratefully acknowledge funding from the Illinois Council on Food and Agricultural Research and the Illinois Soybean Program Operating Board that supported this research.
Entomologist Joe Spencer taking aerial sample of WCR.

Joseph L. Spencer and Eli Levine, Center for Economic Entomology; Scott A. Isard, Department of Geography, University of Illinois



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