Effects of Bt Corn and Egg Density on Western Corn Rootworm (Coleoptera: Chrysomelidae) Adult Emergence and Estimation of Effective Bt Dose.

Since 2003, rootworm-protected transgenic corn has been commercially deployed in the United States as a principal method of control of western corn rootworm, Diabrotica virgifera virgifera LeConte. Durability of this technology depends partly on larval mortality ("dose") exerted by the traits, but density-dependent mortality can confound calculations of dose. Research reported here examined the effects of density-dependent mortality on adult emergence and estimates of trait dose. At sites in Illinois and Indiana, western corn rootworm eggs were infested at four densities on non-Bt corn and at a single density on corn hybrids with transgenic events MON 88017 (VT Triple PRO), DAS-59122-7 (Herculex Insect Protection), and MON 88017 × DAS-59122-7 (SmartStax corn). Beetles were collected weekly in large emergence cages. Density-dependent mortality and the effect of Bt traits were examined using percent survival from egg to adult, sex ratio, and beetle mass. Beetle emergence from Bt treatments was very low, and percent survival from non-Bt treatments was greatest at the lowest egg density (410 eggs per row-meter). Therefore, emergence from the lowest infestation density on non-Bt corn was used to estimate the effective dose of the Bt treatments. Sex ratio and beetle mass were unaffected by density-dependent effects and were not consistently affected by Bt traits. Dose was estimated at 97.4-99.3% for MON 88017, 98.8-99.9% for DAS-59122-7, and 99.7-100.0% for MON 88017 × DAS-59122-7. This study confirms the need to account for density-dependent mortality when estimating dose of corn rootworm protection events even at relatively low egg infestation densities.