MN - Vegetable IPM Newsletter

In this issue

Feature Article: Regional Analysis of Pyrethroid Efficacy Against ECB and CEW

Tissue Analysis as a Nutrient Management Tool for Potatoes

Pest Alert Updates

False Chinch Bug Update in Sweet Corn

**No newsletter will be published next week - look for the August 3rd issue **

Vol. 3 No. 9   July 20, 2001

Regional Analysis of Pyrethroid Efficacy Against European Corn Borer and Corn Earworm - 2001 Update

Bill Hutchison, Eric Burkness, and P.K. O'Rourke, Dept. of Entomology, University of Minnesota, St. Paul, Minn.

During the past year, and with additional data for European corn borer (ECB) and corn earworm (CEW) control in sweet corn, we have updated our regional analysis of pyrethroid efficacy against the pest complex. In last year's report (July 21, 2000, MN Veg. IPM Newsletter; reference below), I also limited the review to ECB data. Our 2001 report includes data for CEW.

Rationale: We are often asked which pyrethroid insecticide performs best against ECB and CEW in sweet corn. Based on a number of Minnesota trials over the past 12 years, as well as data from colleagues throughout the Midwest, I am aware of the overall efficacy of most of the insecticides currently labeled for each pest. However, relying on 1-2 years of recent data, or the results from 1-2 locations, does not always provide the most complete answer. Although insecticide efficacy is just one of many issues to consider in developing an IPM program for a given crop, it is nevertheless a key element that deserves more attention. In particular, I believe that with many pest management tactics (e.g., resistant varieties, biological control and chemical control) it is useful to look at the risks (variability) with specific pest control tactics as well as the expected mean (average) level of control.

With this analysis, we provide one approach to summarizing multiple data sets to assess the efficacy of the four primary pyrethroid insecticides currently labeled for ECB control in sweet corn. Although this is not a true meta-analysis, based on the strict definition of looking for a significant treatment effect, it is similar to a meta-analysis in that we have conducted a careful evaluation of multiple trials (multiple researchers, locations and years), where similar protocols were used.

Protocol: Data sets for analysis were collected from published reports in Arthropod Management Tests, Entomol. Society of America, and unpublished annual reports from colleagues throughout the Midwest. States currently included in the data set include: MN, WI, IN, IL, OH and PA. All data for this analysis met the following criteria: late-season sweet corn trials (typically conducted between July-Sept.) for Midwest U.S. latitudes, standard randomized complete block designs with 4 replications, a minimum of 25 ears/replication examined (100 total/treatment), minimum of 30% ear infestation in the untreated check plots, and the timing of the first spray ranging from early tassel to 20% silk. Based on typical Midwest summer temperatures, the time period ranges from 28 days before harvest (dbh) at row tassel to 21 dbh for first silk. A total of 28 trials were found to meet all of the criteria. However, each trial did not always include all treatments. The number of trials available, that provided useful data, are shown in parentheses, following each treatment/rate combination (Figs. 1-4). The mean number of sprays ranged from 2 to 6/trial (mean of 3.6 sprays), and the ear infestation in the untreated check plots ranged from 0.28 to 3.9 ECB larvae/ear (mean of 1.2/ear).

Results: Figs. 1-2 provide a summary of the mean % control results for ECB and CEW respectively. An estimate of the variability (standard deviation) is shown on the x-axis. Products and rates that perform the best are those with the highest mean % control, and the lowest variability (left-hand side of the graph). Also, for ECB (Fig. 1), we have included data from multiple Bt sweet corn trials in MN, WI, and IL (Burkness et al. 2001, Crop Protection, in press). The Bt sweet corn results are based on sweet corn hybrids developed by Syngenta/Rogers Seed Co.

Key points: For ECB, nearly all products and rates provide >90 or 95% control of ECB. Performance of the Bt sweet corn is very high for both MN and WI, and slightly less for IL (due to higher CEW and/or fall armyworm pressure). Also, note: for Bt sweet corn, despite NO foliar insecticides being used, a high level of control was obtained. For CEW, there is a greater spread in % control. This pest is inherently more difficult to control; in sweet corn, CEW becomes a unique challenge because eggs are laid directly on the silks, and following hatch (4-6 days), quickly move into the developing ear. Thus, residual activity on the silk tissue is critical for continued control.

Figs. 3-4 summarize the Probability of Control for ECB and CEW, respectively, for any given level of % Control. Again, for ECB there is considerable overlap among all 4 pyrethroids and both "low and high" rates for each. For CEW, however, there is more of a spread in the Probability of % control.

Summary: In summary, this analysis builds upon the 2000 article, in that new data sets, based on efficacy trials in 2000, as well as additional unpublished data from Dr. Tom Rabaey (Green Giant Agric. Research Group, Le Sueur, MN). We will continue to add to this data set, following new results from the 2001 season. As we indicated previously, % control is a function of many variables. Other factors that are equally critical to insecticide choice, include tip cover and silk channel length of the sweet corn hybrid, timing of the FIRST spray (if this is too late, 20+ additional sprays after larvae are within the ear, will not help control).

Additional information regarding the methods used for this analysis will be provided in upcoming reports; readers may also refer to Burkness et al. 2001 (below).

References cited:
Burkness, E.C., W.D. Hutchison, R.A. Weinzierl, J.L. Wedberg, S.J. Wold & J.T. Shaw. 2001. Efficacy and risk efficiency of sweet corn hybrids expressing a Bacillus thuringiensis toxin for Lepidopteran pest management in the Midwestern U.S. Crop Protection. (in press).

Hutchison, W.D. 2000. Regional analysis of pyrethroid efficacy. MN Veg. IPM Newsletter, (7/21)

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota,
Jeanne Ciborowski, IPM Program, Minnesota Department of Agriculture,
Cindy Tong, Department of Horticulture, University of Minnesota,
Production Editor: Suzanne Wold, Research Specialist, University of Minnesota,

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Last Revised July 19, 2001.
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