Cutworms, armyworms, wet weather, … and your corn? Regional analysis of insecticide efficacy.

Bill Hutchison, Patrick O’Rourke and Ken Ostlie, Department of Entomology, University of Minnesota, St. Paul, Minn.

As many of you know, we have had several cutworm “hot spots” develop this spring in southern Minnesota and other Midwestern states. Last week Bruce Potter and Ken Ostlie reported infestations reaching over 50% of the plants infested in selected south-central (e.g., Waseca) and southwestern Minn. fields. Most of the damage in Minnesota this year, has been due to black cutworm, Agrotis ipsilon, a species that does not overwinter here, but must migrate into the upper Midwest each spring. The MDA Pest Monitoring program recently provided a map of cutworm infestation for much of Minnesota; this can be viewed (PDF file) at: http://www.mda.state.mn.us/pestsurvey/PestReports/2001jun01.pdf

The critical step in responding to a cutworm infestation is proper identification, then follow-up assessment of the extent of current damage, AND prognosis for future damage if nothing is done for control. For example, for two of our common species, the black and dingy cutworms, the black is clearly the most damaging as it will cut plants at the growing point, or just below the soil surface. In contrast, damage by the dingy cutworm will typically be limited to leaf feeding, which rarely has a detrimental impact on stand, or uniform maturity of the stand.

As indicated by most MN reports, the dominant species this year is the black cutworm. Fortunately, even with this species, growers have some lead time for making a treat/no-treat decision. The early instars will typically do some leaf feeding before direct cutting of plants. A rough threshold for corn is 3-6% of the plants being cut. Thus, if a field is at 6-10% with leaf feeding (allowing for some natural mortality), and you know that black cutworm is present, a treatment is likely warranted. Of course, other factors regarding the potential value of the crop (fresh-market sweet corn, processing, seed corn, etc.) may also play a role. In addition, cutworm moths are often attracted to weedy areas (or weedy waterways) in a field (as are armyworms). This may result in patchy infestations that can be addressed with spot treatments of insecticide, vs. the entire field.

Control (Insecticides and Bt corn): Unfortunately, many granular insecticides applied at planting (primarily for rootworm control), will not always provide a high level of cutworm protection (data not shown). In addition, for sweet corn, rootworm insecticides are not often used. We have therefore summarized data reflecting the “rescue” (broadcast/foliar) treatment approach for cutworm protection of seedling corn. Table 1 summarizes the efficacy of several insecticides labeled for sweet corn. As indicated, control will rarely approach 100%; however, one application of a given material should provide economic control. Based on these data, it appears that there is a fairly broad window for timing the application, following initial cutworm infestation. In addition, there is no consistent pattern/response when using high vs. low rates (e.g., see Pounce data) of pyrethroids. Our conclusion is that the mid-rate for a pyrethroid is probably going to provide economical control. Final selection of an insecticide should be based on cost. Given the performance of the pyrethroids at mid-rate ranges, Lorsban may be more expensive, depending on your crop production region. Final note re: field corn: research has shown that the Bt corn hybrids based on the Cry1Ab toxin of Bacillus thuringiensis (most common Bt on the market), will not provide a high level, or consistent level of cutworm control.

For more info. on cutworms, see:
MN updates (K. Ostlie)
http://www.mnipm.umn.edu/bugweb/bugbase/cutwormnetwork/default.htm
Iowa (M. Rice)
http://www.ipm.iastate.edu/ipm/icm/2000/5-8-2000/cutworm2000.html

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Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@tc.umn.edu
Jeanne Ciborowski, IPM Program, Minnesota Department of Agriculture, jeanne.ciborowski@state.mn.us
Cindy Tong, Department of Horticulture, University of Minnesota, c-tong@tc.umn.edu
Production Editor: Suzanne Wold, Department of Entomology, University of Minnesota, woldx018@tc.umn.edu

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Last Revised June 6, 2001.
The University, including the Minnesota Extension Service, is an equal opportunity educator and employer.©2001 Minnesota Extension Service, University of Minnesota. All rights reserved. Contact copyright@extension.umn.edu for information on reproduction or use of this material.