Dig and Rate Roots to Evaluate Corn Rootworm Control

Dig and Rate Roots to Evaluate Corn Rootworm Control

July 11, 2008

Photo of a western corn rootworm beetle
Western corn rootworm beetle

Western corn rootworm beetles began emerging this week in south central Nebraska, indicating that rootworm larval feeding is ending. Mid to late July would be a good time to dig roots to evaluate the efficacy of your rootworm management program.

The presence of adult beetles or rootworms in a field is not necessarily an indication of product failure. Soil insecticides are applied in a narrow band or in furrow to the soil, or as a seed treatment, and corn roots grow beyond the treated zone where rootworm larvae may survive. Some rootworm beetles will emerge from Bt corn hybrids labeled for corn rootworm control. Also, plant lodging may occur without significant rootworm feeding. Dig and wash some roots to check for rootworm injury before assuming that rootworm damage is responsible for lodging. Rootworm efficacy can only be evaluated reliably if replicated, untreated check strips are left in the same field as the treatment. Without check strips, you won't know whether the absence of injury is due to product efficacy or the absence of rootworms.

Before corn plants can be rated for injury, they need to be at a growth stage where at least three nodes of roots are clearly visible. Dig at least 10 randomly selected plants from several areas of a field. Leave a 9-inch cube of soil surrounding the root system, wash the roots to remove soil and rate each plant for injury using the rating scale. If several weeks have passed between the end of rootworm injury and the time of root rating, new root growth may hide the injury. Examine roots carefully to accurately rate them.

Photo of corn roots showing various degrees of damage from corn rootworm.
Varying degrees of corn rootworm injury. Determining damage using a standardized rating system can help determine a threshold for treatment.

The most widely used method to evaluate root injury has been developed at Iowa State University. It is based on a 0-3 scale. This system was developed to avoid some of the perceived problems with the traditional 1-6 scale, including that the 1-6 scale is not linear (e.g., a rating of 4 does not represent twice as much injury as a rating of 2), and that the 1-6 scale is difficult to explain. The 0-3 scale is linear and the meaning of the injury values are easy to understand. Another potential advantage to the 0-3 scale is that it is more sensitive in detecting differences at low levels of injury compared to the 1-6 scale. This is particularly important in some research applications.

In this scale

0 = no damage,
1 = one complete node of roots is pruned (as defined above),
2 = two complete nodes of roots are pruned, and
3 = 3 nodes of roots are pruned.

Fractional ratings are possible, e.g. 1.5 = equivalent of 1.5 nodes of roots pruned. A description of the 0-3 rating system is available on an Iowa State University Web site. The relationship between root injury rating and yield loss is complex, but usually a root injury rating of 0.25 or more on the 0-3 scale is needed to cause economic yield loss. The corn plant has the capacity to regrow roots and compensate for some early season injury, especially if soil moisture and fertility are adequate during the regrowth period.

Bob Wright
Extension Entomologist