Mid-to-late July is the best time to examine corn rootworm injury to corn roots and to evaluate the performance of soil insecticides, seed treatments, or YieldGard Rootworm¨ corn. The rootworm larvae have done most of their injury by this time. Also, the corn roots have not yet had sufficient time to regrow new roots, which sometimes obscures the injury and makes it difficult to assess.
Evaluating rootworm injury is not difficult once you understand the rating scale. Jim Oleson, a research specialist in the Department of Entomology, has developed a 0-3 rating scale that is straight forward and easy to use. This scale is intuitive and a major improvement over the old Iowa 1-6 scale, which had been used for decades.
To evaluate corn rootworm injury, use the Iowa State node-injury scale (0-3): 0.00, no feeding injury (lowest rating possible); 1.00, one node (circle of roots), or the equivalent of one node (partially eaten nodes can be combined) eaten back to within approximately 2 inches of the stalk; 2.00, two nodes eaten; and 3.00, three or more nodes eaten (highest rating possible). Injury in-between complete nodes eaten is noted as the percentage of the node missing, e.g., 0.25, 1/4 of one node eaten or 1.50 = 1 1/2 nodes eaten.
An interactive root injury movie can be viewed here. At this site, you can examine the progressive injury to corn roots that would be caused by corn rootworm larvae.
These plants rate 0.10 (left) and 3.00 (right) on the Iowa State node-injury scale.
This article originally appeared on pages 134-135 of the IC-490(18) -- July 28, 2003 issue.