Originally Submitted: August 5, 2022
We have been observing a lot of corn rootworm adults in South Dakota, most of them being northern corn rootworms (Figure 1-A). The adults of both the northern and western corn rootworms generally emerge in mid-July and remain active until the first hard frost. The emergence of these primarily underground corn pests can provide some insight into field population levels and what the potential for future root injury might be.
The adult populations can be monitored by using yellow sticky cards. The sticky cards should be placed in several locations in the fields during July and scouted and replaced weekly through August. If the average captures from a field exceed two rootworm adults (of either species) per trap per day for a week, the economic threshold has been exceeded. At this point, future management of the rootworms is advised for the next time corn is planted into the field.
Corn rootworm adults can also cause issues if they begin feeding on silks (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 it hasn’t already.
In some situations, adult corn rootworms can also cause issues themselves by feeding on corn leaves, silks, tassels and developing kernels. 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 different 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 is to monitor for silk clipping. During pollen shed, scout five random plants from 10 different 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 of an inch of the ear tip on 25 to 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).