Soybean Podworm in Soybean

Published: September 3, 2014

Economic damage from soybean podworm larvae is present in many Southwest Missouri soybean fields. Although soybean podworm infestations can occur in any soybean field, most at risk are late planted and double crop soybean. Conditions which favor the development of economic infestations of soybean podworm in late planted and double-crop soybean include:

1. Plant canopies which remain open after plant canopies of earlier planted soybean grow closed. Migrating corn earworm moths prefer open canopy soybean stands as egg laying sites. Approximately 6-8 weeks after egg laying occurs, soybean podworm larvae may reach economic levels in these late planted fields.
2. Elevated moth flights from within Missouri and from more Southern states 6-8 weeks prior to soybean pod production.
3. Reduced numbers of green cloverworm larvae in soybean fields. This may be due to weather conditions which limit green cloverworm populations or by the application of early season insecticide applications for earlier occuring.
4. Reduce the development of the fungal pathogen, Nomuraea rileyi which is the main biological control agent for soybean podworm in soybean. The levels of this beneficial fungus may be reduced by dry weather conditions or by applications of fungicides used to prevent fungal diseases in soybean.

Soybean podworm larvae are important pests of soybean in many southern and eastern states and traditionally have been a pest of soybean in Missouri counties bordering Arkansas, although in the past few years this pest has been found in higher numbers throughout the state of Missouri. Corn earworm larvae may feed on host plant foliage, but most often feed directly on the fruit of its cultivated hosts such as the tips of corn ears, grain sorghum seed heads, cotton squares and bolls, and soybean pods, seeds and occasionally flowers.

Although corn earworms overwinter as pupae in the soil in Missouri, additional moths migrate into Missouri each spring from more southern states. Moths are variable in color but tend to be tan with a yellow to light green tint. Moths are relatively large with approximately ½-inch wing spans. They may lay eggs throughout fields at sites where crop canopy has not yet closed. Eggs are laid singularly on a variety of field crops, although silks of late planted field corn and sweet corn are excellent hosts. Traditionally a pest during periods of hot, dry conditions when beneficial insect numbers are reduced, this pest may also flourish whenever field conditions and/or farming practices reduce beneficial insect numbers.

There are typically 3 generations of this insect produced in Missouri annually with the second and third generations being most damaging to soybean. Each female may lay an average of 1000 (500 to 3000) white to cream colored, dome shaped eggs which are laid singularly. Eggs display brown bands just prior to hatch with larvae emerging in 2-10 days depending on field temperatures. Once corn earworm eggs hatch in soybean, larvae initially feed on foliage, but prefer to feed on pod walls and consume seeds as larvae approach maturity (1-1/2-inch in length). Larvae grow through 5 or 6 worm stages and change in color with age. Newly hatched larvae are yellowish-white in color with second and third instar larvae changing to yellowish-green. Later instars found on feeding on soybean pods can range in color from green, yellow, and white to tan or reddish brown.

Regardless of color, they will generally display several dark longitudinal stripes running the length of their bodies and numerous black bumps with protruding hairs will be present on the top and sides of their bodies. The black bumps are most visible in very young larvae. In addition, this insect has 4 pairs of abdominal prolegs and 1 pair of anal prolegs. When disturbed, larvae often roll into a tight ball until the threat passes.

Late-planted or double-crop soybean are most at risk of attack by this pest. The preferred method of scouting for this pest is through the use of a shake cloth (drop cloth). Typically the cloth is laid between rows where 1-2 linear feet of each adjacent soybean row is vigorously shaken over the cloth. If an average of 1 or more corn earworm larvae are found per linear foot of row sampled, then the economic threshold has be reached. A less effective method of sampling for this pest is through the use of a 15-inch diameter sweepnet. If this method is used, be sure to angle the net opening upward as you sweep the upper 2/3 of the soybean plants in an upward motion. Each pass is considered a sweep with 25 sweeps per location at several locations in a field recommended. If you capture an average of 15 or more corn earworm larvae per 25 sweep sample, the economic threshold has been reached. Note that sweepnet samples tend to under estimate the number of larvae present. In soybean where neither sampling method can be used, treatment of the pest population is recommended if 5-10% or more of the soybean pods exhibit feeding damage from corn earworm larvae. This damage is generally seen as a hole chewed through the side of the pod to reach individual developing beans.

Several insecticides are labeled for corn earworm control in soybean and are listed below. Additionally, several beneficial insect species feed on corn earworm larvae and can help prevent the buildup of corn earworm larval populations in most years. Insecticide applications through the growing season should be used as a last resort as most insecticides substantially reduce both beneficial insect numbers. Pest alerts issued by the University of Missouri IPM Plant Protection Program use statewide pheromone traps to provide early warning of elevated corn earworm moth numbers by region. Additional information is available in University of Missouri Extension Guide Sheet G7110 “Corn Earworm in Missouri”.

Excellent images of corn earworm are available in guide G7110 or at the Iowa State University entomology photo gallery (www.ent.iastate.edu/imagegal)

Treatment of corn earworm infestations in soybean are justified when 20% or more defoliation occurs during pod fill or 1 or more larvae are present per linear foot of row or if 5 to 10% or more of soybean pods are damaged. See the following table for insecticides labeled for control of this pest in soybean.