Last week we noticed emergence of northern corn rootworm adults in South Dakota (Figure 1, NCR). Typically, the emergence of these primarily underground corn pests is simply an indicator of what population levels are and what the potential for future root injury might be. However, in some cases, adult corn rootworms can also cause issues themselves by feeding on corn leaves, silks, tassels, and also developing kernels.
Figure 1. Northern corn rootworm (NCR) and western corn rootworm (WCR) adults. Courtesy: Adam Varenhorst.
The majority of issues arise when corn rootworm adults begin to clip silks during pollination (Figure 2). Sampling for silk clipping should begin right around the onset of the silking stage. Since we are past the onset, scouting in areas where rootworm adults are being observed should begin if they haven’t already.
Figure 2. Northern corn rootworms adults feeding on corn silks. Courtesy: Marlin E. Rice.
The best time to scout for corn rootworm adults is from midmorning to late afternoon. If corn rootworm adults are observed in a field, scout five random plants from 10 locations throughout the field. The number of corn rootworm adults present should be recorded. The threshold for corn rootworm adults on corn plants is an average of five or more beetles per plant during the first week or so of pollen shed. If the threshold is reached, a foliar applied insecticide is recommended to reduce the populations.
Another way to monitor for corn rootworm adult activity in a corn field is to monitor for silk clipping. During pollen shed, scout five random plants from 10 locations throughout the field. For each plant, measure the length of the remaining silk that is protruding from the ear of the selected plant. If the silks are clipped to within ½ inch of the ear tip on 25-50% of the total number of scouted plants, foliar insecticide application is recommended. It is important to remember that yield will not be affected if silk is clipped after pollination has occurred (brown silks present).
If thresholds are reached, please refer to the current edition of the South Dakota Pest Management Guide: Corn.