Corn Rootworms

Bryan Jensen
UW Extension

The summer is going fast. July is here, rootworm larvae are feeding and adults will soon start emerging if they haven’t already. July is a great time to gain field information that will be essential for future rootworm management decisions.

Digging and evaluating for feeding will tell you several things which include:

  1. How well your rootworm control practice(s) worked
  2. IF you have problems with rotation resistance western corn rootworms
  3. IF you have problems with Bt resistance

There is no perfect time of the year to dig roots. However, late July/early August will be hard to beat. Earlier and larvae may not have finished feeding and later you may find that root regeneration has mask some of the damage. Dig about a 6-7 inch ball of roots and pressure wash all the soil off to expose damage.

To quantify root damage use the Nodal Injury Scale developed by J. Oleson, Y. Park, T. Nowatzki and J. Tollefson at Iowa State University. This is an excellent rating system and more information is available at Essentially, the injury scale uses a decimal system. The number to the left of the decimal indicates the number of complete nodes (or equivalent number of nodes) of roots pruned back to within 1 ½ of the stalk. The number to the right of the decimal indicates the % of the next node of roots pruned. A root rating of 1.2 indicates the equivalent on one complete nodes of roots is pruned and 20% of the next.

Information gain from continuous corn fields will of course tell you how well your rootworm management practice worked. Relating injury to yield loss can be difficult because of several variables which include, weather, hybrid, etc. Typically, a field rating of greater than 0.75 indicates economic yields loss. Ratings less than 0.25 will probably not have economic loss. Injury between 0.25 and 0.75 is a gray area. Economic loss will be dependent on the factors mentioned above as well as compaction, general plant health and future environmental conditions.

Surveying roots on first year corn will give you information regarding the prevalence and/or severity of damage from the rotation resistant western corn rootworm. Although damage to first year corn was originally diagnosed in Wisconsin during the 2002 growing season, its incidence seems to have diminished (for now?). Also, there have been no reports of first year corn injury outside of southern Wisconsin. As we stress the need to revive IPM practices for corn rootworm, this information can give corn growers and crop consultants information needed to make an informed decision in rotated corn.

One of the first tools needed for resistance management is identification of the problem. Making a practice of evaluating Bt CRW hybrid performance by assessing roots will give you the information needed to make appropriate management decisions that will help delay resistance. It is unlikely we can “turn the clock back” on resistance to individual Bt CRW proteins so early detection will be important. Resistance could be expected if you have a field average NIS of 1.0 and you have been using a single Bt toxin for two consecutive years or more. Or, if a field average, NIS rating of 0.5 or higher is noticed in a field that has used a pyramid Bt CRW toxin for at least two consecutive years. If resistance is expected, please contact your county extension agent as well as your seed dealer.

A frequently asked question I get is how reliable is lodging as a predictor of larval feeding?

The short answer is that it is a very poor indicator of rootworm damage. Corn can lodge because of several causes. Rootworm feeding can be a reason but you still have to dig/wash roots to verify. Incidentally, you may have corn that is standing straight yet have significant rootworm feeding.

Larval damage: notice root pruning on several nodes
Larval damage: notice root pruning on several nodes