Corn Rootworm Silk Feeding

• 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 Silk Clipping Economic Thresholds)

  

  Figure 1. Silk clipping by adult corn rootworms reduces kernel number; in seed production fields this increases round kernel fractions.

• 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

  

  Figure 2. Northern corn rootworms feeding on silks and kernels.

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
  • 5 beetles per plant feeding on green silks

Crop Rotation

Rootworm populations can be greatly reduced with rotation except in the southern U.S. where the southern corn rootworm occurs at significant levels. Rotating out of corn for multiple years may be necessary in areas experiencing the Eastern variant of the Western corn rootworm or extended diapause in the Northern corn rootworm.

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 may 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.