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Issue No. 15, Article 1/July 6, 2007

Comparing Soil Insecticides and Bt Corn Rootworm Hybrids: Do Root Ratings Make Sense Anymore?

In the early 1970s, entomologists began to investigate the effectiveness of organophosphate and carbamate insecticides against corn rootworm larvae. This interest was fueled by resistance development and the eventual demise of the persistent chlorinated hydrocarbons that had been used as broadcast treatments against corn rootworms (adults and larvae in some instances) for many years. As organophosphate and carbamate soil insecticides were developed, there was considerable concern regarding their ability to protect corn roots throughout the growing season. Some entomologists believed that application of the newer, less residual insecticides during cultivation, closer to the time of rootworm larval feeding, was preferable to an application at planting time. Furthermore, some advocates believed that fields could be scouted, and if corn rootworm larvae were present at economic levels, "rescue" treatments could be applied during cultivation. Times certainly have changed.

In 1971, one of the most cited papers in the corn rootworm literature was published by two entomologists at Iowa State University:

Hills, T. M., and D. C. Peters. 1971. A method of evaluating postplanting insecticide treatments for control of western corn rootworm larvae. Journal of Economic Entomology 64: 764-765.

The authors described two root-injury rating scales. The first scale used a 1-to-6 rating system:

  1. No damage or only a few minor feeding scars.
  2. Feeding scars evident, but no roots eaten off to within 1-1/2 inch of the plant.
  3. Several roots eaten off to within 1-1/2 inch of the plant, but never the equivalent of an entire node of roots destroyed.
  4. One node of roots completely destroyed
  5. Two nodes of roots completely destroyed.
  6. Three or more nodes of roots destroyed.

This root-injury rating scale formed the basis for evaluating corn rootworm insecticide efficacy trials by university entomologists for decades. Often forgotten was the second scale (root-recovery scale) described by the authors of the 1971 paper

  1. No apparent recovery.
  2. Four to six roots on the top node showing regrowth.
  3. Top ring of roots all showing some regrowth.
  4. Considerable secondary roots and complete node of regrowth.
  5. Regrowth on more than one node and good secondary development.

In previous research projects, Hills and Peters had evaluated the efficacy of soil insecticides applied at cultivation by subtracting the recovery rating (0 to 4) from the damage rating (1 to 6). They referred to the difference as the "adjusted root damage rating."

Hills and Peters (1971) used data from an insecticide cultivation experiment conducted in 1969 in northwestern Iowa near Newell to examine the relationship between rootworm injury and yield. The investigators regressed yield on the root damage and adjusted root-damage ratings. A better statistical relationship was achieved with the adjusted root-damage rating scale than with the root-damage rating scale. The authors offered the following concluding statements: "The linear regression equation would indicate that there was a reduction of 5.83 bu/acre in yield for every unit increase in root damage rating. On the basis of other unpublished data at Iowa State, this is a reasonably conservative estimate of the yield loss and is quite representative of yield expectations in relation to rootworm damage."

In 1972, entomologists at Iowa State University published another article that sought to further quantify the relationship between corn rootworm damage and yield loss:

Turpin, F. T., L. C. Dumenil, and D. C. Peters. 1972. Edaphic and agronomic characters that affect potential for rootworm damage to corn in Iowa. Journal of Economic Entomology 65: 1615-1619.

Root-damage ratings were assessed in fields in 15 Iowa counties over a 6-year period. The authors regressed yield on root damage (526 corn-after-corn sites) using the Hills and Peters (1971) 1-to-6 root damage scale and provided the following remarks regarding root injury and yield: "Although it is impossible to define the economic-injury level precisely for rootworm damage from our study, it is possible to arrive at an injury level where control of the rootworm complex resulted in increased corn yields. Using yield data from our study, it was possible to estimate an economic-damage level of 2.5, based on the Iowa State scale for rating rootworm damage. The yield data showed little relationship with damage ratings below 2.5 and observed yield. Damage ratings greater than 2.5 were linearly related to decreasing yields where a damage rating increase of 1.0 was associated with a 10 bu/acre yield reduction."

These two journal articles published in the early 1970s led entomologists down a relatively restrictive path for more than three decades, during which the 1-to-6 root rating scale became the sole basis for comparing the level of root protection provided by soil insecticides. In a 1998 journal article, Gray and Steffey described how root damage, root volume, and root regrowth related to yield over four growing seasons in Illinois:

Gray, M. E., and K. L. Steffey. 1998. Corn rootworm (Coleoptera: Chrysomelidae) larval injury and root compensation of 12 maize hybrids: An assessment of the economic injury index. Journal of Economic Entomology 91: 723­740.

They made the following observations: "Root injury ratings, root volume measurements taken in July and August, and root regrowth parameters were evaluated for their usefulness in predicting yield. Root ratings were as useful as root volumes and root regrowth measurements in predicting yield. Large root systems in July and August generally were positive factors contributing to yield; however, compensatory root regrowth, particularly when soil moisture was adequate, negatively affected yield. Root regrowth after larval injury typically had a positive effect on yield when soil moisture was inadequate. Regression equations described the very dynamic nature of root injury, root volume, and root regrowth and their impact on yield in different growing seasons and at different locations." Relating yield to root injury continues to vex entomologists to this day.

In 2005, a new rootworm-injury scale was introduced, once again by entomologists at Iowa State University:

Oleson, J. D., Y-L. Park, T. M. Nowatzki, and J. J. Tollefson. 2005. Node-injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). Journal of Economic Entomology 98: 1-8.

The authors of this paper accurately pointed out that the old 1-to-6 root damage scale is not a linear scale and does not accurately reflect the actual degree of root injury. So, they developed the now widely used 0-to-3 node-injury scale:

  1. No feeding damage.
  2. One node (circle of roots), or the equivalent of an entire node, pruned back to within approximately 3.8 cm (1.5 inch) of the stalk (or soil line if roots originate from above ground nodes).
  3. Two complete nodes pruned.
  4. Three or more complete nodes pruned (highest rating that can be given).

The authors hoped to achieve the following goal: "We developed the node-injury scale to more accurately quantify corn rootworm larval injury based on the proportion of nodal roots that contain feeding injury. With the node-injury scale, the relationship between the numerical scale and the amount of root injury is linear and intuitive."

In our corn rootworm product efficacy trial conducted last year (2006) in Urbana, almost every soil insecticide we tested prevented significant rootworm larval injury, despite very heavy corn rootworm pressure in the untreated check plots (average node-injury rating = 2.95). Yet, nonpublished data revealed that the YieldGard RW hybrid (node-injury rating = 0.96) had the highest yield (by a large margin) in the trial, significantly greater than the yields of all plots treated with soil insecticidesabout 56 bushels per acre greater than the average yield of four plots treated with granular soil insecticides. This lopsided yield edge in favor of the Bt treatment occurred even though the soil-insecticide treatments were applied to the isoline of the Bt corn rootworm hybrid. (This yield disparity between Bt corn and plots treated with soil insecticides was not apparent at our DeKalb site in 2006.)

Results such as our results from the Urbana experiment in 2006 have been reported by other investigators as well, raising questions about the utility of root ratings as the sole determinant of rootworm injury and their relationship with yield. Consider the following questions:

  • Why have large yield differences between Bt corn and plots treated with soil insecticides occurred when the rootworm-injury ratings were essentially equivalent? In fact, why would Bt corn out-yield isolines treated with soil insecticides even when the Bt corn hybrid had more rootworm injury?
  • Are we comparing apples with oranges when we use the same rootworm-injury scale for roots treated with soil insecticides and Bt corn hybrid roots?
  • Is the current 0-to-3 node-injury scale the most appropriate scale to compare rootworm injury to corn treated with soil insecticides and rootworm injury to transgenic corn rootworm hybrids?
  • Does it make sense to use any rootworm-injury scale to compare the performance of soil insecticides and transgenic corn rootworm hybrids?
  • Should we develop a new rootworm injury rating scale for Bt corn rootworm hybrids?
  • Are root injury rating scales relevant for Bt corn rootworm hybrids?

Over the past 2 weeks, we have received considerable precipitation across much of east-central Illinois, and plants in our plots have responded favorably. On July 2, 2007, we observed a considerable height advantage for one of the Bt corn rootworm hybrids in our Urbana experiment. Even though these Bt corn plants revealed no aboveground signs of stress, examination of the roots revealed considerable pruning. The level of pruning already is more than one node of roots destroyed, although the roots affected are primarily brace roots. In previous years, we have not encountered this degree of pruning on a Bt corn rootworm hybrid until late July or early August. An examination of root systems dug from soil-insecticide treatments revealed much less pruning, particularly on the brace roots. The roots of plants in the untreated check plots in this experiment are well on their way to having all major nodes destroyed. We intend to begin evaluations of our rootworm-control trials at four locations next week. We intend to take these experiments to yield.

Comparison of heights of corn plants in a rootworm-control trial, July 2, 2007, Urbana, Illinois (University of Illinois at Urbana-Champaign).

Root pruning of a rootworm Bt corn hybrid in a rootworm-control trial, July 2, 2007, Urbana, Illinois (University of Illinois at Urbana-Champaign).

Rootworm injury to four corn root systems, each treated with a different soil insecticide, in a rootworm-control trial, July 2, 2007, Urbana, Illinois (University of Illinois at Urbana-Champaign).

Rootworm damage to a root system from an untreated check plot in a rootworm-control trial, July 2, 2007, Urbana, Illinois (University of Illinois at Urbana-Champaign).

We look forward to sharing the results of these trials with you. Over the next several weeks, it may be worthwhile to grab a shovel and take a look at the roots in fields in your area. Don't assume that just because you planted a Bt corn rootworm hybrid or used a soil insecticide that no rootworm injury occurred. You may be surprised. In the meantime, we will continue to contemplate the implications of rootworm-injury ratings and their relationship to yield, and we invite your insights.--Mike Gray and Kevin Steffey

Kevin Steffey
Mike Gray

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