Effect of Bt corn expressing Cry34/35Ab1, Cry3Bb1, and Cry3Bb1 plus Cry34/35Ab1 on western corn rootworm (Diabrotica virgifera virgifera) field emergence

Western corn rootworm (Diabrotica virgifera virgifera LeConte) is a major pest of corn (Zea mays L.). This study evaluated the effect of corn hybrids containing the Bt proteins Cry3Bb1, Cry34/35Ab1, and Cry3Bb1 plus Cry34/35Ab1 compared with a control (isoline corn with no Bt proteins) on beetle emergence in field trials over three years (2010, 2011, and 2013). Each trial was infested artificially with 2500 viable western corn rootworm eggs per row meter and plots were covered with a screen tent. Beetles were collected twice weekly. The average reduction in total beetle emergence compared to the control was highest in Cry34/35Ab1 plus Cry3Bb1 (98.9%) followed by Cry3Bb1 (94.1%) and lowest in Cry34/35Ab1 (82.3%). More females were recovered than males from all Bt-corn treatments. The time to 50% emergence was delayed in all Bt-corn treatments when compared to the control. This delay was longest in the Cry34/35Ab1 plus Cry3Bb1 treatment, followed by Cry3Bb1 and then Cry34/35Ab1. In 2013, three egg infestation levels were tested and density-dependent mortality was observed at 1250 viable eggs per row meter - lower than has previously been reported. The effect of Bt proteins on the emergence timing and sex ratio of western corn rootworm has implications regarding the suitability of resistance management plans, specifically the effectiveness of the refuge strategy. Delays in emergence between Bt and non-Bt refuge hybrids may mean males from the refuge are not available to mate with late emerging females from Bt corn hybrids.