Comparative performance of Bt-rootworm traits in Cry3Bb1 performance problem fields

Rapid development of resistance to Cry3Bb1 has left many farmers wondering what management strategies will work in fields experiencing performance issues with this trait.  To answer that question, we investigated the performance of all available single and pyramided Bt-rootworm traits, with and without an insecticide overlay. Multiple in-field studies were initiated in four MN/SD fields with a history of Cry3Bb1 performance problems and in one MN field where all traits work.   Hybrids expressing single traits (Cry3Bb1, Cry34/35 Ab1, or mCry3A) or pyramids (Cry3Bb1 + Cry34/35 Ab1) were planted in plots measuring 8 rows X 15.2m that were replicated 4X in a RCB design.  Data were collected on root injury, lodging, yield and adult emergence. Root injury data verify resistance issues (ongoing performance problems) with Cry3Bb1 in all four problem fields and suspected cross-resistance with mCry3A in two fields.  This pattern agrees with laboratory bioassay results, where available.  Drought amplified corn rootworm impacts on plant growth and yield.  Hybrid yield performance reflected Bt-RW resistance.  A soil insecticide overlay did not result in significant yield benefits where traits worked, except with seed blends under heavy pressure and drought conditions.  Yield data were analyzed to compare potential management strategies.  Implications of these strategies from IRM and IPM perspectives are discussed.