Figure 1. Corn earworm/soybean podworm moths are robust, light brownish-tan with a 1” to 1 1/2” wingspan. They have a prominent dark spot on each forewing. They tend to be active during the evening and dark hours but can be commonly seen in fields during the day. Photo by Ronald Smith, Auburn University, Bugwood.org.

Figure 1. Corn earworm/soybean podworm moths are robust, light brownish-tan with a 1” to 1 1/2” wingspan. They have a prominent dark spot on each forewing. They tend to be active during the evening and dark hours but can be commonly seen in fields during the day. Photo by Ronald Smith, Auburn University, Bugwood.org.

Updated: October 23, 2024
By Emily Zobel

Podworms in Soybeans

The second generation of corn earworms/podworms/headworms (Helicoverpa zea Boddie, Figure 1) will soon emerge from corn fields and other crops. On soybeans, the podworms may feed on foliage, flowers, and fruit. Female moths will lay eggs over the entire plant, but caterpillars prefer to feed on tender vegetative terminals and/or flower clusters. Fields in bloom to pod-set stage with an open canopy and stressed soybeans during this second-generation flight typically reach the highest populations and are at the most risk. Due to our early summer weather this year, more fields might be at risk compared to last year. Podworm populations can be spotty, so one field may have high podworm populations while the field next to it may have low podworm populations. The only way to know what is happening in a particular field is to sample it. You can sample pod worms using either a sweep net or drop cloth.

Thresholds are generally around 2-3 worms per 15 sweeps. NC State University (NCSU) has a nice podworm threshold calculator that helps you customize your threshold based on treatment cost, row width, and price of the bean into consideration: https://bit.ly/NCSUwormthershold. Davis Owen, Delaware Extension Entomologist, recommended applying Besiege or Elevest at their low rates last year to control soybean podworms. Pyrethroids are not guaranteed to provide good efficacy.

Even though fields in the bloom stage are at risk for high populations of podworms, it is generally recommended not to spray during this stage and instead wait for the end of flowering or early pod set to treat. Soybean plants produce more flowers than they can convert into pods. A study conducted by NCSU has found that soybean plants can compensate for this loss of flowers due to caterpillar feeding damage. No yield losses were recorded even with caterpillar levels three times higher than the economic threshold for podding-stage soybeans. Also, young caterpillars tend to have a high mortality rate. They are often difficult to reach with chemical controls while feeding within flower clusters.

Continue to scout during pod growth and fill, as serious yield loss can occur when large caterpillars coincide with soybean seeds that have achieved almost full size.

This article appears in August 2024, Volume 15, Issue 5 of the Agronomy News.

Agronomy News is a statewide newsletter for farmers, consultants, researchers, and educators interested in grain and row crop forage production systems. This newsletter is published once a month during the growing season and will include topics pertinent to agronomic crop production. Subscribers will receive an email with the latest edition.

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