Fitness costs related to selection for resistance to the Cry3Bb1 protein in a genetically diverse population of non-diapausing western corn rootworm

The western corn rootworm (Diabrotica virgifera virgifera LeConte) is an important pest of maize in North America. Since approved for commercial use in 2003, the acreage planted to maize (Zea mays L.) expressing the Bt derived Cry3Bb1 protein has significantly increased each year in the United States. The Cry3Bb1 protein is toxic to pest Diabrotica beetles when ingested. However, given the previous adaptability of Diabrotica beetles to management tactics, evolution of resistance to the toxin is a concern. From a genetically diverse population of non-diapausing D. v. virgifera, we selected for resistance to the Cry3Bb1 protein by exposing neonate larvae from three lines to maize sprouts expressing the toxic protein. Exposure time increased 24 hours each generation through completion of the second instar. Two control lines were treated identically to the experimental lines except for the Cry3Bb1 exposure. Each generation we measured head capsule widths on all instars from all lines. Beginning with the fourth generation and alternating every other generation we compared adult emergence for each line reared entirely on Bt maize and non-Bt maize to indicate the relative magnitude of resistance. Also, from the two control lines we selected for complete resistance by exposing the beetles to the Bt maize from neonate through adult emergence. At the tenth generation of selection we compared beetle fecundity from each line reared on Bt maize and non-Bt maize. Understanding how relative fitness changes with resistance evolution to a transgenic crop could provide valuable information for resistant management and population modeling.