Integrated Crop Management

Variegated cutworms delay alfalfa

As noted [1] in the May 28, 2001, Integrated Crop Management newsletter, alfalfa fields should have been scouted for variegated cutworms following the first cutting. Variegated cutworms are now causing a delay in alfalfa regrowth in scattered locations across Iowa. Some cutworms are nearly full grown at 11/4-11/2 inches but others are only 5/8-3/4 inch in length. These smaller cutworms will continue to feed for another 21-24 days, depending on weather conditions. Large areas of fields or areas that were under windrows that have not regrown should be scouted for variegated cutworms. Some cutworms hide in the soil or leaf litter around the crown of the plant but they also may be found clinging to the top of the alfalfa stem.

If variegated cutworms are found defoliating alfalfa stubble and preventing regrowth, and most of the larvae are less than an inch in length, consider spraying the field if the economic threshold is exceeded (see May 28, 2001, Integrated Crop Management article [2] on pages 92-93). If most of the variegated cutworms are 11/2 inches in length, they have done most of their damage and should be finishing feeding; no control would be necessary in this situation.



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[3]
Three variegated cutworms resting in the shade of alfalfa stems.


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[4]
Variegated cutworms have yellow spots along the midline on the back.


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[5]
Alfalfa field with large areas defoliated by variegated cutworms. Green areas show more normal regrowth where cutworm populations were lower.

This article originally appeared on page 114 of the IC-486(14) -- June 18, 2001 issue.


Source URL:
http://www.ipm.iastate.edu/ipm/icm//ipm/icm/2001/6-18-2001/variegcw.html