Four species of adult corn rootworms can affect kernel fertilization
Adult corn rootworms feed on green silks and can prevent silks from capturing pollen
In commercial corn fields, large beetle populations (≥ 20 per plant) are necessary to cause economic damage and to justify an insecticide application
In seed production fields, as few as 1 to 5 beetles per plant feeding on green silks and kernels can reduce seed quality and grain yield (see economic thresholds section below)
Northern corn rootworms feeding on silks and kernels
Injury and Pest Symptoms
Adult corn rootworms feeding on green silks and immature kernels:
reduce number of kernels per pound
reduce number of kernels per acre
increase round kernel fractions
decrease flat kernel fractions
Interesting Fact: Adult western corn rootworms live an average of 52 days (maximum 86) in the field
Silk clipping by adult corn rootworms reduces kernel number; in seed production fields this increases round kernel fractions
Adult Corn Rootworms
Corn Rootworm Economic Thresholds*
Hybrids (commercial production fields)
To justify the cost of chemical control and application:
Average of 20 beetles per plant, or
Silks clipped back to within ½ inch of husk prior to 50% pollen shed
Inbreds (seed production fields)
Plants drought or heat stressed
1 to 3 beetles per plant feeding on green silks
Plants with optimum moisture and moderate temperatures
Rootworm populations can be greatly reduced with rotation (except western corn rootworms in the “Red Zone” of northern Illinois and the southern corn rootworm in the southern U.S.)
Planting Time
Plant early; late-planted and late-silking fields, (even first-year corn fields), can attract large numbers of beetles from neighboring fields
Bt Corn
Hybrids and inbreds expressing a corn rootworm Bt trait will greatly reduce the number of larvae that survive to the silk-feeding adult stage; Bt corn does not affect the adult corn rootworms
Insecticides
Numerous broad-spectrum and long-residual insecticides are labeled for beetle control
Photos¹ and Text
Marlin E. Rice, Pioneer,¹ corn ear by Mark Jeschke Reviewed by Tim Nowatzki, Mike Rupert and Jeff Schussler
