Issue No. 24, Article 3/November 9, 2007
Is the European Corn Borer an Endangered Species?
Our annual survey for second-generation European corn borers was completed relatively recently by University of Illinois Extension educators, county-based Extension personnel, and Department of Crop Sciences staff and graduate students. We are not certain how much longer we will conduct this historic survey (primarily because of constraints on time and human resources), but through 2007 we were able to include it on our annual list of things to do. We thank everyone who volunteered for this annual effort, which continues to add data to our ever-growing database.
After making something of a comeback in some regions of Illinois during the fall of 2006, second-generation European corn borers apparently encountered a hostile environment during the late summer and fall of 2007. Widespread planting of transgenic Bt corn with traits for control of corn borers was probably the primary component of the hostile environment in 2007. According to estimates published by the USDA Economic Research Service, acres of Bt corn in the United States increased from 19% in 2001 to 49% in 2007. Corn with stacked genes (Bt plus herbicide resistance) increased significantly in Illinois from 2000 to 2007. It is possible that the corn borer-resistant Bt corn hybrids are suppressing corn borer populations over large areas. The trend for lower population densities of European corn borers has been observed in several other Midwestern states.
Results from the 2007 survey of second-generation European corn borer larvae in Illinois are presented in Table 1. We will prepare a PowerPoint slide set with these data in the near future and post it to the University of Illinois IPM Web site on the European corn borer "home page."
Population densities (numbers of borers per 100 plants) declined noticeably in most counties and Crop Reporting Districts from 2006 to 2007. Average densities of corn borer larvae in the nine Crop Reporting Districts ranged from 2.8 larvae per 100 plants in the East to 57 in the West Southwest. The largest average of 120.6 borers per 100 plants was observed in Madison County (West Southwest). No European corn borer larvae were found in any of the 10 fields surveyed in each of six countiesMacon, McLean, Iroquois, Clark, Lawrence, and Jackson. On the other extreme, one field in Calhoun County had an average of 300 corn borers per 100 plants, one in Madison County had an average of 484, and one in Montgomery County had an average of 450. So apparently the environment in the West Southwest Crop Reporting District was suitable for survival of European corn borers . . . as long as the corn was not Bt corn.
The mean percentages of corn plants infested with European corn borer larvae ranged from 1.9% in the East to 28.6% in the West Southwest. The highest average percentage infestation (50.4%) occurred in Greene County. The data in many counties revealed an all-or-nothing fluctuation in corn borer densities. In other words, there were no corn borers in most of the fields within a given county, with one, two, or, at most, five fields with relatively significant infestations of larvae (60% to an occasional 100% infestation).
The statewide average density of second-generation European corn borers in Illinois in 2007 was 13.4 larvae per 100 plants, the lowest density we have ever determined from these annual surveys, which began in 1943 (Figure 1). The next-lowest density (about 16 borers per 100 plants) determined from these surveys occurred in 2004. In the 63 years that the survey has been conducted (it was not conducted in 1997 and 1998), the average density of corn borers has been less than 0.35 larva per plant in 9 years . . . six times during the past 9 years, including the past 4 four years. These data suggest that Bt corn has become a major mortality factor in populations of European corn borers.
Data from the annual survey of second-generation European corn borers usually cannot be used to predict infestations of European corn borers throughout the following year. However, we know that if the population in the fall is relatively small, the potential population of first-generation European corn borers the following spring usually is small. Unfortunately, we have not been able to determine the impact of two disease organisms that infect European corn borer larvaeBeauveria bassiana (a fungus) and Nosema pyrausta (a microsporidian)over the past few years.
As we increase the acres of corn in Illinois to meet the demands for food, feed, and fuel, the habitat available for European corn borers will increase. However, if the potential habitat is populated primarily with Bt corn with borer-control traits, the mortality of European corn borers should continue to be significant. This latter point actually underscores the need for corn producers to comply with the insect resistance management strategies that require 20% non-Bt corn. Without these refuges, the population selection pressure will be intense, increasing the potential for European corn borer populations to develop resistance to Bt proteins.
The European corn borer is not an endangered species, but its populations certainly have taken some serious hits over the past several years.--Kevin Steffey and Mike Gray