Nitrogen application effects on Bt gene expression in corn roots and trait performance against corn rootworms (Diabrotica spp.)

Nitrogen is a necessary input for optimal commercial corn production.  Recommended application rates have been suggested throughout the years for maximum economic yield, but not for optimal expression of Bt (Bacillus thuringiensis Berliner) Cry protein(s) and their impacts on the target insect.  This study examines the impact of nitrogen rates on the expression of corn rootworm (Diabrotica spp.) resistant traits in transgenic corn, the resulting root protection, and its effects on corn rootworm populations.  Specifically, we hoped to learn if N rates need to shift to optimize Bt-RW trait expression and corn rootworm protection provided by different traits and whether the pyramiding of traits require higher nitrogen application rates.  The experiment contained a factorial treatment arrangement in a split-plot randomized complete block design with six nitrogen rates as the main plots and three hybrids differing in corn rootworm traits as the sub-plots.  Corn roots were sampled at the beginning and just after peak larval feeding and run through an Enzyme-linked Immunosorbent Assay (ELISA) to determine gene expression levels.  Leaf tissue was also analyzed for nitrogen content.  Resulting root injury and adult emergence were measured to assess impacts on larval survival and damage. Root injury generally decreased with nitrogen rate with all hybrids even though adult emergence generally increased, except at higher N rates. Implications of these results for growers in terms of adjusting nitrogen application rates in fields to obtain better gene expression, optimizing hybrid protection from corn rootworm traits, and reducing the risk of resistance will be discussed.