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

The corn rootworm is one of the most economically important corn pests in the United States.  The larval stage consumes corn root tissue, contributing to poor nutrient uptake and lodging of the plants.  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 and mCry3a have recently been reported in Iowa, which has spurred research on the effectiveness of Bt toxins against the corn rootworm.  The objective of this research was to determine the efficacy of three Bt corn hybrids against larvae of western and northern 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 the corn rootworms 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 significantly higher numbers of western corn rootworm than northern corn rootworm in the sticky traps and emergence traps placed in the fields, suggesting that western corn rootworm are the dominant species in the region. 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.