No. 15 Article 1/July 1, 2005

Rootworms South of I-70, and Assessing Rootworm Larval Injury

In one of last week's articles about corn rootworms ("We Probably Are In for Tough Times with Corn Rootworms"), I mentioned that Dan Beckman (Effingham Equity) and Randy McElroy (Monsanto Company) suspected that the variant western corn rootworm was becoming established in some areas south of I-70. On June 23, in response to Dan's invitation, I visited four fields of corn planted after soybeans in Crawford County with Dan and Randy and several other individuals from both companies. The cornfields were in the Annapolis area, in the north part of the county. I rated at least 10 root systems in each of the four fields, and we discovered a range of rootworm larval injury, from virtually no feeding on some roots to complete destruction of the root system. The average node-injury ratings for the four fields were 0.28, 0.44, 0.63, and 1.83. Depending on environmental conditions, the injury in three of the four fields could be economic (more on this later in the article). One field (average node-injury rating = 1.83) was severely damaged, with many of the root systems in the end rows completely destroyed. At the time of our visit, the field had virtually no aboveground signs that anything was wrong. That will change dramatically after a strong wind, heavy rain, or continued drought conditions.


Severe rootworm larval feeding injury on corn roots from one field in Crawford County, June 23, 2005 (University of Illinois).

During that visit, I learned from some of the individuals present that rootworm larval injury in corn planted after soybeans may have been observed as far south as southern Lawrence County. I should learn soon whether those reports have been confirmed. Regardless, it is clear that the distribution of the variant western corn rootworm has extended into southern Illinois, so corn growers in at least the couple of tiers of counties south of I-70 should examine their cornfields for rootworm larval injury. As I have mentioned previously, we intend to conduct a survey of corn planted after soybeans in several southern counties this summer: Madison, St. Clair, Clinton, Washington, Monroe, Jefferson, Clay, Wayne, Richland, Edwards, Lawrence, and Wabash. We may adjust our survey slightly in July, but our objective will remain. We do not anticipate finding a great deal of rootworm larval injury, but we hope to be able to delineate the southernmost extent of the distribution of the variant western corn rootworm in Illinois. We also intend to survey counties along the Mississippi River, from JoDaviess in the north to St. Clair in the south. As always, we will share the results from this survey in the Bulletin, as well as during meetings this winter.

The exercise of examining roots for rootworm injury in Crawford County once again made me realize how important it is for accurate assessment of the extent of rootworm larval injury. Whether one wishes to assess performance of rootworm control product or to get a feeling for potential yield loss, accurate assessment of rootworm larval injury is crucial. In last week's article "We Probably Are In for Tough Times with Corn Rootworms," I provided a limited discussion of the 0-to-3 node-injury scale developed by entomologists at Iowa State University. A scientific article describing the node-injury scale and its relationship to corn yield was published in the Entomological Society of America's Journal of Economic Entomology (JEE). The article, "Node-Injury Scale to Evaluate Root Injury by Corn Rootworms (Coleoptera: Chrysomelidae)," was published in JEE vol. 95 (1), pp. 1­8. The article includes some useful information about the association of rootworm larval injury with corn yield.

The authors of the article (James D. Oleson, Yong-Lak Park, Timothy M. Nowatzki, and Jon J. Tollefson) discussed the relationship between rootworm larval injury and grain yield from nine insecticide efficacy trials, from 2000 to 2003. However, to examine the relationship under different levels of environmental stress, they selected three fields that they characterized as growing in conditions of low, medium, and high environmental stress. They used stress degree-days (SDDs) to represent the accumulation of degree-days above a threshold temperature of 30EC (86°F); temperatures greater than 30EC can inhibit corn productivity. In the field with low environmental stress (11.7 SDDs), which also received adequate rainfall, yield reduction was very low, even with a high level of rootworm larval injury. However, yield reduction was more apparent in the fields that were characterized by moderate and high environmental stress. I have excerpted information from two tables in the article to demonstrate the relationship between rootworm larval injury and corn yield. Tables 1 and 2 show the minimum node-injury ratings that cause economic damage at various combinations of control cost and value of the corn. The data in Table 1 are based on corn grown under moderate environmental stress conditions (21.7 SDDs); the data in Table 2 are based on corn grown under high environmental stress conditions (31.7 SDDs).

The information in Tables 1 and 2 shows clearly that as market value for corn increases, less rootworm larval injury is required for economic damage (i.e., cost of the damage greater than the cost of control). As control costs increase, more rootworm larval injury is required for economic damage. These are general principles for assessing economic damage for many insects that attack crops. However, what is most revealing is that as environmental stress increases, the amount of rootworm larval injury necessary to cause economic damage decreases significantly.

For example, economic damage when corn sells for $2.25 per bushel and control costs $18 per acre occurs when slightly more than one node of roots is destroyed (1.05 node-injury rating) in corn growing in moderate stress conditions (Table 1). For corn growing in high stress conditions, the amount of rootworm larval injury required to cause economic damage is about 1/3 of one node pruned (0.36 node-injury rating) when corn sells for $2.25 per bushel and control costs $18 per acre. Other authors have found a similar relationship. Some research that Mike Gray and I conducted in the mid-1990s revealed that economic loss could result from rootworm larval injury of about 2.00 on the Iowa State University 1-to-6 root-rating scale (Gray and Steffey [1998], JEE 91: 723-740) when corn was growing in high environmental stress conditions. A root rating of 2.0 on the 1-to-6 scale indicates scarring but very little root pruning.

Although the information in Tables 1 and 2 is based on only two fields, the data clearly demonstrate that rootworm larval injury does not have the same effect on corn yield each year. The relationship between corn rootworm larval injury and corn yield is affected by many factors, not the least of which are environmental conditions and corn hybrid. Consequently, it may be tough to determine whether the rootworm-control product used "worked" or "failed" if there is no comparative untreated check strip in the same field.

On June 27, at a field day at the Northern Illinois Agronomy Research Center near DeKalb, I asked those in attendance whether they had observed rootworm adults. They all indicated they had not. So with rootworm adults yet to emerge in northern Illinois, rootworm larvae are still feeding. The rootworm larval injury in our untreated check rows in our efficacy experiment ranged from about 0.25 to 1.0; however, the injury should get worse as the larvae continue to feed. We also noted that the root systems in many fields suffering from the heat and lack of moisture are puny, even when rootworm larvae are not adding insult to injury. When you inspect cornfields for rootworm larval injury, don't link rootworms with the puny root systems unless evidence of larval injury is clear (scarring, pruning).

We continue to appreciate all of your reports about the rootworm situation in your area, so please keep them coming. As always, let's hope for some much-needed rain.--Kevin Steffey

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