Sap Beetles in Corn: Are they Pests?

September 23, 2022
ICM News

Sap beetles are a relatively common insect in cornfields, typically seen each year around harvest. People usually notice sap beetles (and other ear-feeding pests) while doing pre-harvest yield checks. Adult sap beetles are usually less than ¼ inch long and oval. Most are dark colored and sometimes have orange or yellow spots (Photo 1). Sap beetles can be distinguished from other beetles in corn by their antennae, which have a knob at the end. Larvae may also be found on corn ears. The larvae are small and white with a light brown head, and they turn yellowish as they mature.

Sap beetle.
Photo 1. Picnic beetles are a type of sap beetle often found in cornfields. (Photo by Joyce Gross, UC Berkeley.)

But are sap beetles pests of corn?

The answer is a bit complex. Sap beetles are scavengers, typically feeding on decaying plant matter, over-ripe fruit, and plant sap. In corn, sap beetles are almost always a secondary pest, meaning they begin feeding on corn ears only after the ear is already damaged. They typically hollow out kernels, usually at the ear tip.

Most often, sap beetles will colonize corn ears where caterpillars were feeding, such as corn earworm. Because of this, we rarely consider sap beetles to be an economic pest of corn, but its association with other harvest issues can lead to concern. The damage from any ear-feeding insect creates a wound where ear rots and mycotoxins can develop (Photo 2).

Ear molds.
Photo 2. An example of ear molds that can develop after insects feed on the ear. (Photo by Meaghan Anderson.)

If you are experiencing high numbers of sap beetles feeding on ears:

  • Consider harvesting those fields early to shorten the time sap beetles are feeding on corn or prevent the development of ear molds prior to harvesting;
  • Change combine settings to try to blow out beetles and damaged kernels;
  • Monitor grain for the presence of sap beetles, especially if grain will go directly from the field into a grain bin for storage. Insecticide treatments may be necessary to prevent beetles from feeding on stored grain.
  • If grain will be dried, make sure to use a high temperature drying operation to kill beetles before storage. Anecdotes from last fall suggest some sap beetles can survive low temperature drying operations.

Sap beetles that are not removed with the crop during harvest will seek sheltered sites (e.g., wooded areas or plant debris) for overwintering soon. Make sure to monitor grain for presence of sap beetles or ear rots prior to storing or marketing.

For subsequent growing seasons, you can reduce the risk of sap beetles a few ways. Consider a corn hybrid with effective Bt traits for corn earworm and other ear-feeding caterpillars to minimize feeding by secondary pests. Additionally, some corn hybrids are more likely to have exposed ear tips than others. Minimize ear tip exposure by choosing a hybrid where the tip of the ear is not as exposed.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 23, 2022. The information contained within may not be the most current and accurate depending on when it is accessed.

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Authors: 

Ashley Dean Education Extension Specialist I

Ashley is an education extension specialist for field crop entomology at Iowa State University. She coordinates the Iowa Moth Trapping Network, develops educational resources for field crop pests in Iowa, and aids in the research efforts of the

Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated pest management (IPM) for field crops. Dr. Hodgson's current extensio...

Meaghan Anderson Field Agronomist in Central Iowa

Meaghan Anderson is a field agronomist in central Iowa and an extension field specialist at Iowa State University Extension and Outreach. Educational programming is available for farmers, agribusinesses, pesticide applicators, certified crop advisors, and other individuals interested in...