Corn Earworm Picks up Activity in Iowa

September 23, 2019
ICM News

Corn earworm  (Photo 1) is common in Iowa corn. It is a migratory pest, migrating from the southern and southeastern U.S. every summer. There are two general fights to the Midwest every summer, but the first flight is not of economic concern. Iowa’s corn is more vulnerable to infestation of the second flight, arriving in late July. Yield losses can occur in late-planted cornfields, where caterpillars can infest >50% of plants. It is more common to see corn earworm issues in sweet corn, white corn, popcorn, and seed fields. Kernel injury can promote ear mycotoxins (e.g., fumonisins and aflatoxins) and further reduce grain quality. The frass (excrement) can also be a contaminant for corn processing.

Corn earworm.
Photo 1. Corn earworm adult. Photo by Robert J. Bauernfeind,

Caterpillars feed on leaves, tassel, silks, and the whorl, but they prefer to feed on kernels. They are generally found at the ear tip and leave extensive frass inside the husk, creating soft ears (Photo 2). They can eat ~15 kernels or the top two inches of the ear tip.

Corn earworm ear injury.
Photo 2. Corn earworms are messy feeders, generally active at the ear tip. Photo by Ashley Dean, Iowa State University.

Females lay about 1,000 eggs. The caterpillars are 1 ½ - 2 inches when fully developed and range from pale green to pink to dark brown (Photo 3). Usually, the head capsule is orange and dark tubercles (bumps) run along the side of the body. Caterpillars will drop from the ear to pupate. In some cases, they chew their way out of the ear and leave an exit hole that can be confused with western bean cutworm.

Corn earworm color variation.
Photo 3. Corn earworm caterpillars are variable in color. Photo by Ashley Dean, Iowa State University.

Hybrids with Bt traits that target corn earworm (e.g., Vip3Aa20, Cry1A.105 + Cry2Ab2) provide acceptable larval suppression in Iowa. In addition to reducing feeding injury, the amount of mycotoxins in the grain can be reduced.

In 2018 and 2019, several fields planted with a pyramided Bt hybrid had severe injury with nearly all the ears infested with corn earworm. Research confirming Bt resistance of corn earworm in sweet corn on the east coast was published in 2017, but resistance is not known to occur in the Midwest. USDA-ARS entomologists are working with ISU Entomology to determine the extent of resistance of these migratory pests.

If you are unsure if your corn hybrids have Bt traits that suppress corn earworm, check out the Handy Bt Trait Table for 2019 from Michigan State University.



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, 2019. The information contained within may not be the most current and accurate depending on when it is accessed.


Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology, and Microbiology, 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...