Update on Corn Rootworm Development

June 9, 2000
Corn rootworm larval hatch is well under way in central and northern Illinois counties. Figure 2 indicates that by May 29 as many as 50% of corn rootworm larvae were hatched across a broad band of central Illinois. When 380 to 426 heat units (base 52°F) have accumulated (since January 1), as many as 50% of corn rootworm larvae should have hatched. Larval survival should be excellent in fields unless they are saturated with water for several days. With the early planting that took place this spring, corn rootworm larvae should have little difficulty in securing a root to begin feeding on. Scouting for rootworm larvae at this point in the season is very difficult. However, by mid-June, corn rootworm larvae will be "easier" to locate if soil surrounding a root system is shaken over a sheet of black plastic. Placing root systems and attached soil (7-inch cubes) into 5-gallon buckets of water also helps locate corn rootworm larvae. After soaking for an hour or so, corn rootworm larvae will begin floating to the surface of the water.

If you find three or more larvae per plant (7-inch cube of soil and roots from base of plant) and root injury is evident, a rescue cultivation treatment may be warranted. Don't forget, an insecticide applied during cultivation does not generally perform as well as a planting-time treatment. We'll provide more updates on corn rootworms as the season progresses.--Mike Gray

Author: Mike Gray