Corn Rootworm Beetle Emergence and Feeding in Corn Fields

Corn Rootworm Beetle Emergence and Feeding in Corn Fields

Eileen Cullen (Extension Entomologist)

Because corn rootworm beetles can reduce yield by silk pruning, it is important to scout cornfields before and during early pollination. This is especially important now under drought conditions also adversely affecting pollination by disrupting the ‘nick’ or synchronization of pollen shed and silk emergence.

Corn rootworm beetle emergence is underway and beetles are being reported feeding on corn leaves and silks. Western corn rootworm beetles are more common in Wisconsin, but you will find northern corn rootworm adults as well.

When corn rootworm beetles emerge before silking, they feed on the green tissue of the corn leaf surface in fields where they emerged. This results in leaves with green tissue scraped away.

Rootworm beetles prefer silks and corn pollen as a food source. Corn growth stage in a particular field in relation to surrounding fields will influence how attractive a field is to corn rootworm beetles.

Entomologists talk about a “donor field”, a “receiver field”, or a “neutral field”. These terms relate to the timing of pollination. A field that pollinates later than surrounding cornfields will be attractive to beetles (receiver field) from fields that have finished pollination (donor field).  If most fields on a farm or in a local area pollinate around the same time (neutral fields), this lessens the chance of mass movement by beetles into cornfields.

Check cornfields before 70% of the field has silked, and count the number of beetles per plant on 10 random plants from five areas of the field.  Record the level of silk clipping (in inches from the tip of the fresh silk to the husk).

Treatment with a foliar insecticide is warranted when the following conditions occur:

  • 5 or more adults per plant,
  • Pollination has not yet occurred or is less than 50% complete, and
  • Silks are clipped to ½-inch or less

Joe Lauer, UW-Madison Corn Agronomist, has posted several new articles containing drought management information for corn growers, including Corn Management Decisions During Drought Depend Upon Pollination Success, which explains how to determine if an ear has been pollinated.

It is important to scout cornfields now for corn rootworm beetles and silk clipping. Conferring with Joe Lauer this morning about corn pollination and drought stress, we agree this is a critical period (July 10 to August 1) to protect silks in the event that rain arrives in time. After pollination, there is no risk or need to treat for corn rootworm beetles.


Hammond, R.B., A. Michel and J.B. Eisley. 2009.  Corn Rootworm Management. The Ohio State University, Publication FC-ENT-0016-09

IPM Field Crop Scouting Manual. 2010. Integrated Pest Management Program, University of Wisconsin-Extension Cooperative Extension Service.

Wright, R.J. and L. Meinke. 1999. Managing Corn Rootworm Adults. University of Nebraska Corn Rootworm Guide