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Check for Corn Rootworm Injury

Assessing damage is fairly simple by following these helpful tips.

With the 4th of July just past, Kansas State University's Phil Sloderbeck advises that now is a good time for growers to assess corn rootworm injury.

"This practice is especially useful in comparing rootworm treatment options such as Bt rootworm corn with soil insecticides," says Sloderbeck, who is the entomology state leader for K-State Research and Extension.

"Assessing damage is really a fairly simple process," says Sloderbeck, who offered these tips:

  • Dig 10 to 20 roots at random from the management systems you want to compare (ideally a treated area versus an untreated area, but one can also compare one treatment option with another, soil insecticide vs. rootworm resistant corn, for example).
  • Take the roots to an area where they can be thoroughly washed to remove all soil and then rate the roots using one of the two damaging rating scales commonly employed for rating rootworm damage. The scales are based on the number of nodes of roots having heavy rootworm injury. One is based on a 3-point scale and the other is based on a 6-point scale. Both focus on the appearance of the three functional nodes (or whorls) of roots on a normal corn plant.
  • On the 3-point system, no damage is rated a zero, and one node (circle of roots), or the equivalent of an entire node, eaten back to within approximately 2 inches of the stalk is rated a 1, two nodes destroyed gets a rating of 2 and three nodes lost is assigned a 3.

"While one could debate which scale is better, either one is useful to determine if there appear to be differences in the amount of damage observed among treatments," Sloderbeck says. "Minor differences are probably not too meaningful, but no damage versus an entire node or two of roots missing will probably be meaningful."

Making notes on root damage, combined with yield estimates could be helpful in fine-tuning future management strategies, he says.

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