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Issue No. 15, Article 2/July 2, 2004

Corn Rootworms Are Causing Serious Damage in Some Areas

As you have read in previous issues of the Bulletin, we have received a significant number of reports of severe damage caused by corn rootworm larvae feeding on corn roots. Many of these reports have come from fields in which a rootworm-control product (a soil insecticide or an insecticidal seed treatment) was used at planting time. Consequently, all sorts of explanations for lack of performance of a given product have been volunteered. Some explanations probably are legitimate; others sound fabricated. The truth of the matter is that performance of most rootworm-control products can be compromised by many factors, including, but not limited to, early planting, less-than-ideal environmental conditions (e.g., dry soil), and faulty application. If you are involved in assessing the performance of a rootworm-control product, please consider all possible reasons why a product may not have provided adequate protection of the roots. At the same time, please try to avoid extraordinary excuses that try one's sense of reason.

One of our research trials this year has been established to test the hypothesis that planting time has an impact on efficacy of rootworm-control products. In a split-plot, randomized, complete block design, we are evaluating four treatments (Aztec 2.1G, Poncho 1250, YieldGard Rootworm hybrid, and untreated check) at two different planting dates--April 15 and May 16. The trial is duplicated in two cropping sequences: corn planted after a trap crop (corn and pumpkins), and corn planted after soybeans.

The level of rootworm larval damage in some plots is severe. When we assess rootworm larval damage in mid-July, we will be able to compare root damage among the four treatments and between planting dates. Monitoring emergence of rootworm adults throughout the season in all plots also will help us determine whether the timing and pattern of emergence are affected by treatments, time of planting, and crop sequence. We hope to share some of the information with you in a future issue of the Bulletin.

Severe rootworm larval damage, time-of-planting trial, Urbana, Illinois, 2004.

As always, we also have established three "standard" trials to assess the efficacy of several rootworm-control products, including granular and liquid soil insecticides, insecticidal seed treatments, and transgenic hybrids. Root digs in these and other rootworm-focused research trials will begin in earnest in mid-July. Again, we will share the results with you at some time in the near future.

In the meantime, many people will continue to assess the performance of different rootworm-control products in many fields throughout Illinois, and some will be disappointed with the results, as we have learned already. The level of rootworm larval damage in several fields has been severe--roots completely destroyed, plants lodging and then goosenecking as they continue to grow. Ear development in these fields will be hampered, and yield undoubtedly reduced. Depending on the hybrid and the growing conditions during the next few weeks, some corn will be able to compensate for the root damage by growing new roots and/or producing reasonable ears. It is likely, however, that badly lodged fields will be difficult to harvest.

To help assess rootworm larval damage, you should be familiar with two rootworm larval injury rating scales. The 1-to-6 root-rating scale developed at Iowa State University many years ago is a relatively easy scale for assessing gross levels of injury. We still use this scale to evaluate the root systems we extract from many of our rootworm trials. The 0-to-3 node-injury scale, developed more recently by Jim Oleson and Jon Tollefson at Iowa State University, lets you quantify the amount of rootworm larval injury based on the amount of injury on each node of roots. Following are descriptions of the two scales.

1-to-6 root-rating scale:

  1. No damage, or only a few minor feeding scars
  2. Feeding scars evident, but no roots eaten off to within 1-1/2 inches of the plant
  3. Several roots eaten off to within 1-1/2 inches 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 completely destroyed

0-to-3 node-injury scale:

  1. No feeding damage (lowest rating that can be given)
  2. One node (circle of roots), or the equivalent of an entire node, eaten back to within approximately 2 inches of the stalk (soil line on the 7th node)
  3. Two complete nodes eaten
  4. Three or more nodes eaten (highest rating that can be given)

Damage in between complete nodes eaten is noted as the percentage of the node missing (1.50 = 1-1/2 nodes eaten, 0.25 = 1/4 of one node eaten, etc.).

Whatever rating scale you use, significant root pruning is an obvious indication of severe rootworm larval damage. For an excellent, interactive explanation of the node-injury scale, click here. To watch a video with a considerably younger Kevin Steffey explaining the 1-to-6 root-rating scale, go to the IPM web site videos page, then click on "Western Corn Rootworm Damage Ratings."

Another concern in fields with significant rootworm larval damage is the impact that adults will have on pollination. In severely damaged fields, many adults will emerge, so the potential for significant silk clipping exists. Whether such badly damaged fields would benefit from application of an insecticide to protect the pollination process depends on many interrelated factors. Making a rootworm management decision in such fields is challenging, and one management recommendation will not fit all situations.

Rootworm adults have been emerging in several areas of Illinois for at least 2 weeks, and in some areas, corn is tasseling and silking. The adults will be attracted to these flowering parts of corn plants, and their feeding on silks may interfere with pollination. As a rule of thumb, a treatment to protect against excessive silk clipping by rootworm adults may be warranted when there are five or more beetles per plant, pollination is not complete, and silk clipping is observed. Although silks will continue to grow after they have been clipped by insects, rootworm adults have a tendency to congregate on ear tips and continue feeding. Insecticides suggested for control of rootworm adults interfering with pollination are presented in Table 1. Please abide by all label directions and precautions.

All indications are that 2004 will be a "rootworm year" in Illinois, so take advantage of the opportunity, wanted or not, to assess the performance of whatever rootworm-control product has been used within given fields. The assessments always are more meaningful if there is at least one untreated strip within a field, to facilitate comparisons of roots from treated and untreated areas in a field. Regardless, learning more about the consistency of performance of different products over time and geography will enable better decision making about rootworm management in the future. --Kevin Steffey

Kevin Steffey

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