Begin Scouting for Stalk Borers

May 31, 2024
ICM News

Stalk borer is an occasional pest of corn, but it can be persistent in some fields, especially those fields near perennial grasses that serve as overwintering sites (fence rows, terraces, and waterways are typical sources). Tracking degree days is a useful way to estimate when common stalk borer larvae begin moving into cornfields from their overwintering hosts. Foliar insecticide applications are only effective when larvae are migrating and exposed to the insecticide. Start scouting corn for larvae when 1,300-1,400 degree days (base 41°F) have accumulated. Most of central and southern Iowa has reached this benchmark (Figure 1); therefore, scouting for migrating larvae should begin now to make timely treatment decisions.


Figure 1. Degree days accumulated (base 41°F) for stalk borer in Iowa (January 1 – May 29, 2024). Map courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.

This encyclopedia article details stalk borer identification, sampling, and management and provides information on high-risk fields. Stalk borers tend to re-infest the same fields, so prioritize scouting fields with a history of stalk borers, paying close attention to field edges. Finding “dead heads” in nearby grasses or weeds is an indicator of stalk borers in the area (Photo 1). The larvae are purple with white stripes and a dark saddle across the middle of the body (Photo 2). They are not highly mobile and typically only move into the first four to six rows of corn. Young corn is particularly vulnerable to severe injury; plants are unlikely to be killed once they reach V7 (Photo 3).


Photo 1. Smooth bromegrass with dead heads (center) from stalk borer tunneling into the stem. Photo by Ashley Dean.

 


Photo 2. Stalk borer larva. Photo by James Kalisch.

 


Photo 3. Stalk borer larvae can shred corn leaves and destroy the growing point. Photo by Marlin Rice.

No rescue treatments are available for stalk borer but tracking degree days and scouting the field can determine whether larvae are present and migrating. Insecticide applications are only effective while larvae are migrating to corn; once larvae have bored into the stalk, an insecticide will not reach them. Economic thresholds are based on market value and plant growth stage and may be found in the stalk borer encyclopedia article.

Watch this video to learn more about scouting and management for stalk borers in corn:

https://www.youtube.com/watch?v=QTZMP1qJHlM

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 May 31, 2024. 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 Agriculture and Natural Resources Extension Specialist II

Ashley is an education extension specialist for field crop entomology at Iowa State University. She coordinates the Iowa Moth Trapping Network, the Regional Corn Rootworm Monitoring Network, and the Iowa Pest Alert Network. She also develops educational resources for field crop pests in Iowa and ...

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...