Monitoring the effectiveness of three Bt corn hybrids against corn rootworm in South Dakota

The western corn rootworm, Diabrotica virgifera virgifera Leconte, is one of the most economically important corn pests in the United States.  High corn injury is inflicted by the larval stage of the insect, which consumes corn root tissue, contributing to root pruning, poor nutrient uptake, and lodging of the corn plant.  Transgenic corn hybrids expressing toxins from the bacteria Bacillus thuringiensis (Bt) have been employed to combat the pest.  However, corn rootworm resistance to plants expressing Cry3Bb1 has recently been reported in Iowa, which has spurred research on the effectiveness of these and other Bt toxins against the corn rootworm.  The objective of this research is to determine the effect that three Bt corn hybrids have on the larval feeding of western corn rootworm in South Dakota. Three Bt hybrids were tested at two locations with previous corn rootworm infestations or fields that were in continuous corn for three or more consecutive years.  Root injury caused by western corn rootworm was evaluated using the 0-3 Node Injury Scale. We found damage to corn expressing Cry3Bb1 and mCry3A toxins, indicating that localized populations in South Dakota may not be sufficiently suppressed by these Bt hybrids.  We also found high numbers of western corn rootworm compared to northern corn rootworm in the sticky traps and emergence traps placed in the fields.  This research has implications for corn production and sustainable corn rootworm management in South Dakota and the Northern Plains, and will advance our knowledge of the incidence of resistance to Bt toxins among the western corn rootworm populations.