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Vigilance for European Corn Borer Escalates

June 28, 2002
During the week of June 17, the number of reports of infestations of first-generation European corn borers increased considerably. The heaviest infestations (most of them subeconomic) seem to be in western counties right now, with lighter infestations reported elsewhere. Following are a few reports from around the state, most of them from central Illinois:

· In Clark County, Kevin Wirth (Effingham Equity in Marshall) and Shannon Schultz (Monsanto) found third instars in the midribs in no more than 20% of the plants in fields they scouted on June 21.

· In Piatt County, Ben Reep found European corn borer larvae in every field of non-Bt corn, in which plants were taller than 18 inches, on June 17 and 18. The maximum infestation he observed was 7%, with only one to two larvae per plant. At that time the larvae were first and second instars.

· In Scott County, while visiting with the folks with Burrus Power Hybrids, we found second instars in less than 25% of the plants in one field.

· In Peoria County, Wayne Streitmatter observed infestations, ranging from less than 10% to 34%, on June 20. The field with 34% infestation had an average of 1.2 first instars per plant.

We sent a couple of our graduate students (Jered Schroeder and Nathan Wentworth) out to look for corn borers on June 24 and 25, and, in general, they found few worrisome levels of infestation. In three fields in Knox County, they found 4%, 8%, and 28% of the plants infested, with an average of 0.5 to 1 borer per whorl. In two fields in Henry County and Bureau County, they assessed 36% and 4% of the plants infested, with 1.5 and 0.5 borer per whorl, respectively.

Based on the observation by Kevin Wirth and Shannon Schultz, European corn borer larvae in the southern half of the state probably are beyond control with insecticides by now. Many of the larvae have tunneled into midribs and stalks where insecticides usually will not kill them. In central and north-central counties, many larvae still are feeding in whorls. If control is warranted, time remains to get results. In northern Illinois, people should be scouting for corn borers for the next week and a half.

A recurring theme in some of the messages we received this past week was confusion regarding the management worksheet for first-generation European corn borer that was printed in issue no. 11 (June 7, 2002) of the Bulletin. We discovered the source of the confusion quickly, and we apologize for what may have been an oversight. An explanation is in order.

About 3 years ago, we made a slight but significant change to the worksheet for first-generation European corn borers. In the past, we recommended the following procedure for scouting and using the information in the worksheet:

· Examine 100 plants (10 consecutive plants at 10 different locations in a field) for shot-hole feeding in the whorl leaves.

· At each location, unroll the whorl leaves of an infested plant (one with shot-hole feeding) and count the live borers per plant.

· Calculate the percentage of plants infested and the average number of live borers per infested plant.

The figures derived from the calculation were used in the worksheet to determine the average number of borers per plant (% of plants infested x average number of borers per infested plant = average number of borers per plant). The change in the worksheet occurred after some lengthy discussions with fellow entomologists.

Following is the current recommendation for scouting first-generation European corn borers and using the information in the worksheet:

· Examine a minimum of 50 plants (10 plants at each of five locations in a field) for the presence of larvae. The number of plants examined is not as important as the fact that you are supposed to examine every plant, not just infested plants.

* Pull whorls from plants and unroll leaves to count live borers.

* Record the total number of larvae found and the total number of plants sampled.

You are supposed to unroll the whorls from all plants examined, not just an infested plant at each location within the field. By doing this, you don't need to know the percentage of plants infested because the average number of borers per plant is determined simply by dividing the number of borers found by the number of plants sampled.

The primary concern about the "old" approach was that people might underestimate densities of corn borers. For example, first-instar corn borers might be present in some plants that are not exhibiting injury symptoms at the time of sampling. Although the newer approach probably provides a more precise estimate of the density of first-generation European corn borers, the older approach of unrolling the whorls of a subsample and then multiplying by the percentage of plants infested may be more time efficient for consultants.

Here's an example of how sampling the same field in two different ways can lead to different results. If you sample 100 plants, unroll all of the whorls, and find a total of 40 larvae, the average density is 0.4 larva per plant. If you sample the same 100 plants, 20% of which have evidence of infestation, and you happen to find only 1 larva in each of 10 infested plants sampled, the average density is only 0.2 larva per plant. Because you sampled only 10 infested plants and found only 10 larvae, you underestimated the density. You did not count the 30 other larvae in the plants that were not sampled.

We don't care if people use the older approach, especially if that's what they feel comfortable with. Obviously, consultants who have been scouting for European corn borers for a long time have some sense of how often mistakes are made. If the older approach resulted in few mistakes and is less time consuming, use the older approach. We simply wanted to clear up the confusion.

Please don't hesitate to let us know if you have any questions or concerns about our management guidelines. We invite all reasonable discussions about insect management.--Kevin Steffey and Mike Gray

Author: Kevin Steffey Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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