Managing Insect Pests of Corn in a Transgenic Landscape
Professor & Interim Assistant Dean
ANR Extension Programs
Department of Crop Sciences
Professor & Extension Entomologist
Department of Crop Sciences
“If your farm is located anywhere in Illinois except in the extreme southern part, and weather conditions are favorable to the borer, you will run the risk of serious commercial damage in 1943 unless you take steps to prevent it. Remember the country needs all the corn Illinois farmers can produce” (University of Illinois Extension circular 539, October 1942). Although the agricultural landscape has changed dramatically in 65 years, the second sentence of the quote rings true more than six decades later. Our growing season did not begin well, with record precipitation and severe flooding in many areas. Planting of corn and soybeans in Illinois, Indiana, and Iowa was significantly delayed, and replanting was common. Poor yields are anticipated at a time of escalating international and domestic demand for grain used in production of food and biofuel. Grain shortages and increased commodity prices are expected. Against this bleak backdrop, at least the economic threat of European corn borers has been drastically reduced over the past decade, due in large measure to the widespread use of Bt corn.
Since the commercialization of Bt corn in the mid-1990s, densities of European corn borers have declined sharply. In 2007, the average density of second-generation European corn borers in Illinois was 0.13 larva per plant, the lowest density ever determined from these annual surveys since 1943. In the 63 years of the survey (not conducted in 1997 and 1998), the average density has been less than 0.35 larva per plant in nine years–six of the past nine years, including the past four years. These data suggest that Bt corn has become a major mortality factor in populations of European corn borers, a tremendous success story for agriculture.
Rootworm Bt corn hybrids were commercialized in 2003, promising a transgenic solution for the most economically significant insect pests of corn. However, unlike Bt hybrids for control of European corn borers (high-dose events), rootworm Bt hybrids express low to moderate doses of Bt toxin. The efficacy of rootworm Bt hybrids has been less consistent than the efficacy of corn borer Bt hybrids, especially in areas where variant western corn rootworms (rotation resistant) are entrenched.
The use of Bt hybrids that offer control of both European corn borers and corn rootworms–“stacked” hybrids–has increased significantly, from 19% to 40% of all planted corn acres in Illinois in 2006 and 2007, respectively (USDA ERS). Acreage planted to stacked hybrids in Illinois in 2008 could exceed 60%. We are in the midst of an agricultural revolution that is having significant effects on the principles and practices of integrated pest management (IPM).
We have witnessed an erosion in commitment to the principles of IPM. The use of prophylactic inputs has largely supplanted decision making based upon scouting information and consideration of economic thresholds. Of particular concern is the lack of integration of crop protection inputs because of our reliance on Bt hybrids. Do producers need to plant stacked Bt hybrids on more than half of the corn acres in Illinois with European corn borers at such low levels? The use of stacked hybrids is popular and convenient, but the consequences of reliance on one tactic could be serious. Are rootworms and corn borers both present at economic levels in most fields? Increasing the selection pressure on these key insect pests of corn increases the potential for development of populations resistant to Bt toxins. Field-evolved resistance of cotton bollworms (a.k.a. corn earworms) to Bt cotton (Cry1Ac) was documented in February 2008 in Nature Biotechnology, 26(2): 199–202. The authors indicated that global monitoring data confirmed an increase in resistance genes in some field populations of cotton bollworms. Is it merely a matter of time before resistance to Bt is discovered in field populations of corn rootworms and European corn borers?
We believe that diligent implementation of insect resistance management strategies (e.g., the non-Bt refuge) to date has held the resistance concern at bay. However, researchers continue to explore alternative resistance management strategies, such as pyramiding multiple genes with diverse modes of action against single pests within a given hybrid. In the meantime, let’s not abandon IPM principles simply because we have new pest management tools.