Corn Rootworm Beetles and Silk Clipping 

Bryan Jensen- IPM Program

This could be one of those years when it pays to scout corn fields for silk clipping caused by Western and Northern corn rootworm beetles. Both species are attracted to pollinating fields and are capable of clipping silks prior to fertilization resulting in poor ear fill. The concern this year would be over variable planting dates. During “normal” years, the majority of our corn acreage pollinates over a relatively short period of time. Although beetles are attracted to pollinating fields their numbers are diluted over large acreage and few field have problems. Those fields, or areas of fields, which do have problems are those which pollinate well before or after the majority of acres have pollinated. Adult rootworm beetles are mobile and will migrate to these fields in numbers potentially high enough to cause economic silk clipping. 

This growing season is anything but normal. We’ve had variable corn planting dates, replanting dates, delayed development, etc. This situation could cause localized or even more general concerns about silk clipping. Crop advisors should begin checking for adult beetles before 70% of the plants are in the process of silking. Count the number of beetles on 10 random plants in five separate areas for a total of 50 plants. Record the number of beetles per plant and the number of plants with silks clipped to 1/4 inch or less. In addition, record the number of plants that haven’t begun to silk, the number with fresh silk and the number with brown silk. Typically, it will take an average of 5-6 beetles per plant and silks being clipped within in ¼ to ½ inch of the tip of the husk. 

As of this writing it is unknown what the beetle populations will be or the outcome and timing of crop development. However, do keep an eye out for high risk fields.