By Adam Varenhorst

One of the easiest ways to scout for corn rootworm adults is the use of yellow sticky cards. As their name implies, these cards are covered in a gluelike substance that captures insects that are attracted to the yellow-colored paper. For corn rootworms, the presence of two adult rootworms captured within a single week is an indicator that management strategies for that field need to be altered. The yellow sticky cards should be placed in several locations of the field and replaced on a weekly basis. The traps should be attached to the stalk above ear height. This monitoring should continue through August to determine the corn rootworm populations present in the field.

Another method for determining if rootworms are the culprits of lodged corn is to evaluate the roots for injury. Roots should be dug and washed to remove excess soil. Nodes 4, 5, and 6 should be evaluated for the number of roots on the node that are pruned to within 1.5 inches of the stalk. This method of evaluating uses a 0 to 3 injury scale, with 0 indicating that no injury was present. A rating of 3 indicates that all of the roots on the three nodes are pruned. The ratings vary based on the number of roots that are pruned, and a more-comprehensive explanation can be found in our corn rootworm fact sheet. A root rating of 1 indicates average yield loss at approximately 15%, a root rating of 2 indicates approximately 30% yield loss, and a rating of 3 indicates approximately 45% yield loss.

Identification

Adult corn rootworms vary slightly in size and greatly in color between the two species that are pests of corn in South Dakota. The northern corn rootworm (abbreviated as NCR) is typically a solid green color that can vary from light to dark green (Figure 1). Shortly after their emergence, the NCR adults may appear yellow, but their color will change after feeding. The NCR adults are slightly smaller than the western corn rootworm (abbreviated as WCR) adults. The WCR adults are yellow in color and have black markings on their hardened forewings (Figure 2). These markings may appear as three distinct lines to a single dark patch.