UNL CropWatch April 23, 2010: Avoid Cutworm Damage in Sugarbeets after a Winter Wheat Cover Crop
April 23, 2010
Recently, I was asked whether an insecticide could be tank mixed with a herbicide to kill off the winter cover crop while protecting against cutworm damage in sugarbeet? This is an important question, considering the season-long impact of these early-season management decisions. To answer this question, first let’s consider the biology and ecology of the pest.
Figure 1. Army cutworm (Photo by Jim Kalisch)
Cutworms make up a number of species of moth caterpillars that can be very damaging to many crops in Nebraska. In fact, nearly any spring-planted crop following a winter wheat cover could suffer cutworm damage. In western Nebraska, the most damaging cutworms include primarily the army cutworm (Figure 1), and very rarely other cutworms, such as the pale western cutworm, darksided cutworm, and variegated cutworm. The army cutworm easily is the most destructive cutworm to crops such as sugarbeets in the early growing season in the Panhandle.
In late September and October, army cutworms lay 1,000 to 3,000 eggs directly on bare soil, such as in newly planted winter wheat. Eggs will hatch over an extended period in the fall, resulting in varying sizes of caterpillars feeding and developing as long as temperatures allow. Come April, large larvae can sometimes be abundant in winter wheat fields. In fact, there is a treatment threshold of four or more cutworm larvae per foot of row when planning for a cash crop of winter wheat. (See this May 1, 2009 CropWatch article for more details.) In sugarbeets, treatment is warranted with just one cutworm per 20 row feet. Because newly emerged beet seedlings are so small, these large cutworm larvae in the spring can destroy many sugarbeets.
Preplant Treatment Recommendations
If your winter wheat is being used as a winter cover crop, consider the potential for cutworms before you plant your summer crop. The following recommendations may protect your spring sugarbeet seedlings from cutworm damage:
- Ensure that you have sufficient herbicide coverage of your cover crop. Untreated areas of the field could serve as a reservoir for cutworms that might eventually move onto emerging seedlings.
- If cutworms are present in the winter wheat cover crop this spring, consider tank mixing a pyrethroid insecticide with your herbicide. Follow the herbicide and insecticide labels to determine if the compounds are compatible and if the application is allowable by law.
If insecticide treatment is warranted, these recommendations may protect your sugarbeet seedlings from cutworm damage.
Please stay tuned to CropWatch for future insect management information.
Jeff Bradshaw, Extension Entomologist, Panhandle REC, Scottsbluff
Bob Wright, Extension Entomologist, Lincoln