Understanding unexpected rootworm injury to corn expressing Cry3Bb1 in agro-ecosystems of southwestern Nebraska

Greater than expected injury by western corn rootworm (WCR) (Diabrotica virgifera virgifera) to Cry3Bb1 expressing hybrids has been reported in southwestern Nebraska. Affected areas of fields are often characterized by high pH calcareous soils, where corn growth is poor and Palmer amaranth weeds bloom attracting adult WCR later in the season. Studies were conducted in 2013 and 2014 to ascertain whether poor soil and associated poor corn growth leads to reduced expression of Cry3Bb1. Quantitative determination of Cry3Bb1 protein expression levels in root tissues using the enzyme-linked immunosorbent assay was carried out on plants at V5- V6 growth stage. Furthermore, emergence studies were conducted in these fields and laboratory bioassays conducted with WCR populations collected from four locations. The expression of Cry3Bb1 protein was not significantly impacted by soil type as the Cry3Bb1 expression level was similar in plants from poor and good soils (18.9- 21.2 ug/g). Also, WCR survival from plants in each soil type was not significantly different. There was no correlation between soil pH and either the expression level of Cry3Bb1 protein or emergence of WCR. In contrast, there was great variability in susceptibility to Cry3Bb1 proteins in the on-plant bioassays. The corrected survival ranged from 16.25 - 85.58%, across the locations. Results suggest that Cry3Bb1 expression levels are sufficient to provide adequate root protection against WCR regardless of soil type and a mosaic of Cry3Bb1 resistance exists within this agro-ecosystem.