Elson Shields reported a corn field in Cayuga County showing symptoms of a failed corn rootworm (Cry3Bb1) Bt hybrid. The field planted to a Monsanto VT3 single event Bt hybrid has extensive goose necking, lodging and severely pruned roots characteristic of corn rootworm (CRW) injury.

Aerial view of CRW resistant Bt corn hybrid field damaged by corn rootworm, Cayuga County, NY. Photos by Elson Shields, Cornell Entomology.

Performance problems with Cry3Bb1 were first reported in the Minnesota and Iowa in 2009 (and Nebraska since 2007). Since then similar reports have emerged from locations in Colorado, Illinois, Kansas, Missouri, Nebraska, South Dakota, and Wisconsin. This recent sighting in New York is among our first in the northeast. The following information is drawn from the Cornell and Penn State articles cited below.

Problem fields share the following characteristics:

  • Planting corn continuously for multiple years (typically more than four years)
  • Using the same (Cry3Bb1) Bt trait year after year as a single event.
  • Refuge compliance was (often) problematic (no/not enough refuge, or refuge too far from the Bt field) on farms with unexpected damage.

Warning signs of potential corn rootworm resistance:

  • Large number of CRW beetles in the field, possibly leaf feeding, clipped silks resulting in poor pollination.(this is a very weak indicator since the beetles are strong fliers)
  • Unexpected lodging and root damage that cannot be explained by an agronomic or environmental problem, or presence of another root-feeding pest.

Recommendations to reduce the selection pressure on corn rootworm to develop resistance to CRW-BT corn:

  • ROTATE-ROTATE–ROTATE corn with non-host crops such as alfalfa, soybeans, small grains.
  • Fields with low levels of CRW pressure should be planted to a non-CRW-BT variety. These fields are typically first or second year fields. First year fields do not need any rootworm insecticide, but second year fields may need a high rate of seed treatment or a reduced rate of soil insecticide.
  • Only plant CRW-BT corn varieties in fields with high rootworm pressure. These fields are typically continuous corn fields in 3 or longer years of continuous corn.

For more information see:

Corn Rootworm Resistance to BT-Corn Reported

 Managing Western Corn Rootworm Resistance to Bt on the Fringe