Submitted to: Journal of Economic Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/13/2009
Publication Date: 2/4/2010
Citation: Hibbard, B.E., Meihls, L.N., Ellersieck, M.R., Onstad, D.W. 2010. Density-Dependent and Density-Independent Mortality of the Western Corn Rootworm: Impact on Dose Calculations of Rootworm-Resistant Bt Corn. Journal of Economic Entomology. 103:77-84. Interpretive Summary: The registration of transgenic corn with resistance to corn rootworm larval feeding offers a viable alternative to insecticides for managing the most economically important insect pests of corn. Maintaining susceptibility to transgenic crops (resistance management) is in the interest of growers, the Environmental Protection Agency, and industry, but requires an understanding of corn rootworm biology that does not currently exist. Among research needs concerning rootworm biology and rootworm/Bt corn interactions considered particularly important for resistance management is a calculation of dose, which requires an understanding of density-dependent survival/mortality. The percent of eggs of the western corn rootworm which survived was evaluated for the effect of egg density in 2005 and 2007 in Central Missouri. Seven egg densities were evaluated in each year. At the highest egg density evaluated 231 beetles emerged per meter while 562 beetles would have emerged if it were not for density-dependent mortality. At all egg densities, more than 90% of the viable eggs did not establish on a plant (density-independent mortality). Little or no density-dependent mortality occurred at infestation levels of 850 viable eggs per 30.5 cm, while more than half of the larvae that did establish at the highest egg density of 2,400 viable eggs per 30.5 cm died due to density-dependent mortality. This information is required for dose calculations of Bt corn targeted toward corn rootworms and is important to seed companies, the Environmental Protection Agency, and modelers in their attempts to finalize resistance management plans for transgenic corn.
Technical Abstract: The percent of viable eggs of the western corn rootworm, Diabrotica virgifera virgifera LeConte, which survived to the adult stage was evaluated for the effect of egg density in 2005 and 2007 in Central Missouri. In 2005, each plot was 2.44 m × 3.05 m and contained 64 maize, Zea mays L., plants. In 2007, plots were 3.05 m × 3.05 m and again contained 64 maize plants. Seven egg densities (25, 50, 100, 300, 600, 1200, and 2400 viable eggs per 30.5 cm) were evaluated with 6, 5, 5, 4, 4, 4, and 4 replications, respectively, in each year in a completely randomized design. In 2007 only, an additional row was infested near each plot to evaluate plant damage. In both years, there was no correlation of infestation level and percent emergence between infestation levels of 25 to 600 viable eggs per 30.5 cm. In 2005, 8.6% of the viable eggs established on a corn plant and produced an adult at these lower infestation rates. In 2007, this value was 2.9%. Regardless of egg density, at least 91 to 97% failed to establish and produce adults (density-independent mortality). In 2005 and in the combined analysis, as viable egg densities increased from 600 to 2400 per 30.5 cm there was a significant decrease in percent emergence. In a broken line analysis, the point where density-dependent mortality began in the combined analysis was 881 eggs per 30.5 cm with a 95% confidence interval from 675 to 1088. In 2005, density-dependent mortality was important at high infestations and killed 58% of those larvae that successfully established on a plant at the highest egg density. However, little or no density-dependent mortality occurred at infestation levels less than 850 viable eggs per 30.5 cm in either year of the study. These data are discussed in terms of dose calculations for products targeting the western corn rootworm.