Issue No. 17, Article 2/July 27, 2012
New Insights on the Western Corn Rootworm's Adaptation to Crop Rotation
In 1995, widespread and severe root injury to rotated cornfields was common across the east-central Illinois landscape. Seventeen growing seasons later, corn growers in the eastern Corn Belt commonly accept the possibility that first-year corn is susceptible to corn rootworm damage. Prior to 1995, soil insecticides delivered at planting were used predominantly in continuous corn production systems. Following the remarkable adaptation by western corn rootworms to a cultural management practice, use of soil insecticides increased significantly.
In 2003, Bt hybrids were commercialized that offered protection against corn rootworm larval injury. These hybrids became the dominant management approach for corn rootworms on both rotated and nonrotated corn acres. Fueled by growing concerns over lackluster Bt hybrid performance in some areas of the Corn Belt, it is becoming more common for growers to use Bt corn rootworm hybrids and to treat those hybrids at planting with a soil insecticide. Many producers also are inclined to broadcast insecticide applications on their fields tank-mixed with fungicides during pollination. The argument for these treatments includes protecting silks against clipping by western corn rootworm adults and Japanese beetles. Many producers freely admit, though, that they also want to "take the edge off" of the egg-laying population of western corn rootworm adults to minimize the pressure placed on soil insecticides and Bt hybrids the following growing season.
To a large extent, these practices represent a new form of IPM--insurance pest management. Producers have significant investments in every acre of corn, and when commodity prices are very high, the strong tendency is to protect those investments with escalating inputs. A key point to remember, however: western corn rootworms have adapted to virtually every management approach that is used repetitively and not integrated, including several classes of insecticides, the Cry3Bb1 protein used in some Bt hybrids, and crop rotation.
Let's return to the crop rotation story that unfolded in the mid-1990s. Just recently, some University of Illinois researchers published a paper that provides some insights into the adaptation by western corn rootworms to crop rotation:
Curzi, M.J., J.A. Zavala, J.L. Spencer, and M.J. Seufferheld. 2012. Abnormally high digestive enzyme activity and gene expression explain the contemporary evolution of a Diabrotica biotype able to feed on soybeans. Ecology and Evolution: DOI 10.1002/ece3.331.
The investigators were able to prove that variant western corn rootworms could better withstand soybean foliage than non-variant western corn rootworm adults. In essence, rotation-resistant western corn rootworm adults had more tolerance to soybean foliage in their diets, the result of their having 3 to 4 times the level of a key enzyme (cathepsin L-like protease) compared with non-variant rootworm adults. Because of the elevated enzyme levels, variant western corn rootworm females could remain in soybean fields for longer periods, feed on soybean foliage, and ultimately lay eggs in the soil, setting the stage for potential damage in rotated corn the following growing season. Even these variant western corn rootworm females returned to cornfields following the more dietary stressful conditions of soybean foliage and fed on pollen and other corn tissue to ensure their survival.
Since 1995, we have learned a great deal about the evolution of this adaptation to crop rotation by western corn rootworms. Many may recall that the initial explanation of egg-laying in soybean fields by western corn rootworms was met with surprise and skepticism. Through the years, entomologists have been able to demonstrate behavioral differences between variant and non-variant western corn rootworms, develop sampling procedures and economic thresholds for the variant pest in soybean fields, develop models predicting its spread, quantify egg-laying levels in soybeans and other crops, and now more carefully describe the fundamental mechanism behind this evolutionary development.--Mike Gray