Scout for Emerging Western Corn Rootworm Beetles

Western corn rootworm
Figure 1. Western corn rootworm

Scout for Emerging Western Corn Rootworm Beetles

Western corn rootworm beetles began emerging in southeast and south central Nebraska during the last week in June. Beetles typically emerge somewhat later in northeastern and western Nebraska.

Beetles emerging before silk emergence may feed on corn leaves. They feed by scraping the surface tissue, leaving a white parchment-like appearance. Once silks emerge, they become the favored food. The earliest silking fields in an area often are most heavily damaged because beetles will move to them in search of green silks.

There are no thresholds for silk-clipping damage based on beetle numbers because damage levels are not correlated well with beetle densities. Usually an average of 5-10 beetles per ear is required to seriously affect pollination. Severe silk feeding (silks clipped to less than ½ inch from the ear) at 25%-50% pollen shed may indicate a need to apply insecticide. Silk feeding after pollination is complete does not affect yield potential.

See the 2016 Guide to Weed Management in Nebraska with Insecticide and Fungicide Information (EC 130), for insecticides labeled for adult rootworm control.

Japanese beetles also prefer to feed on corn silks.  See the article in last week’s Crop Watch for Japanese beetle thresholds for silk feeding damage.

western corn rootworm damage
Figure 2. Corn leaf damage caused by western corn rootworm