Many Missouri soybean producers are once again experiencing problems with soybean podworm (corn earworm) in soybean. Soybean podworm numbers have gradually increased in Missouri soybean fields over the past 5 years. Although podworm traditionally has been a major pest of soybean in many southern and eastern states and an occasional pest of soybean in Missouri's "Bootheel" counties and other counties bordering Arkansas, in 2010 and 2011 economic infestations of podworm expanded their range into more northwestern and north central counties of Missouri. Most fields experiencing problems with this pest in both years were planted after June 1.
In 2011, soybean podworm problems have been found mainly on late planted and double crop soybean fields. In some fields, flowers were fed upon by larvae earlier in the season with much defoliation and pod damage now occurring. These late planted fields are more attractive to migrating moths as females prefer to lay eggs in fields where soybean plant canopies remain open. Wind direction and the intensity of moth flights during moth migration help determine the location and intensity of developing podworm larval infestations. Similar to 2010, a majority of soybean podworm damaged fields are located in the southeastern, western, and some north central counties of Missouri. However, soybean podworm infestations may occur anywhere in the state if field conditions and soybean growth stages are attractive to soybean podworm moths.
Moth identification is difficult as they vary in color, but tend to be tan with a yellow to light green tint. Moths are relatively large with approximately 1-1.5-inch wing spans. Eggs are laid singularly on several field crops, although silks of late planted field corn and sweet corn are most attractive to ovipositing moths as are soybean fields prior to closure of plant canopies. Each female may lay an average of 1000 (500 to 3000) white to cream colored, somewhat transparent, dome shaped ribbed eggs. Eggs display brown bands just prior to hatch with larvae emerging in 2-10 days depending on field temperatures. Larvae initially feed on foliage and sometimes soybean flowers, but prefer to feed on pod walls and consume seeds as larvae approach maturity (1 to 1.5 -inch in length). Larvae grow through five 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 feeding on soybean pods can range in color from green to yellow 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. In addition, this insect has four pairs of abdominal prologs (middle of larva) and one pair of anal prologs (back end of larva). Green cloverworm, which is also green in color, can be differentiated by possessing three pairs of abdominal prologs. When disturbed, soybean podworm larvae often roll into tight balls until the threat passes, whereas, green cloverworm larvae often thrash about violently when disturbed. There are typically two or three generations of soybean podworm (corn earworm) produced in Missouri, annually, with second and third generation larvae being most damaging to soybean.
Traditionally a pest during periods of hot, dry conditions when beneficial insect numbers are reduced, this pest also may flourish whenever field conditions and/or farming practices reduce beneficial insect numbers. In most years, a fungal pathogen (Nomuraea rileyi) will substantially reduce numbers of podworm before they are able to cause substantial damage to soybean and before they mature and pupate in the soil. This beneficial fungal pathogen is often present early in the season in soybean fields where the pathogen also attacks the larvae of green cloverworm. If green cloverworm numbers are few early, then the presence of the fungal pathogen late in the season is reduced and survival of soybean podworm larvae is increased. Another factor that may allow podworm populations to increase to economic levels is the application of early season foliar insecticides and fungicides. In most years, these types of pesticide applications may have minor impact on potential soybean podworm numbers. However, if soybean podworm moth flights are heavy within the state during July and August, soybean canopies are still open, and the fungal pathogen is reduced by spraying of foliar insecticides and fungicides, then the potential for severe damage to soybean pods by podworm is greatly increased. To reduce the risk of podworm damage soybean producers are encouraged follow these recommendations when possible.
The most effective methods of determining whether podworm populations are elevated is through monitoring of soybean podworm moth flights during June–August and frequent scouting of soybean fields throughout the season, but especially during flowering and pod fill growth stages. Scouting during these periods of plant growth should occur at least twice per week as podworm larvae can quickly cause extensive damage to flowers and pods when larval number are elevated. Direct observation of soybean plants, use of a shake sheet between rows, and even sweep nets samples are all methods used to determine podworm numbers. Direct observation and shake sheet samples will provide the best estimate of larval numbers. Typically a white cloth or paper is spread between soybean rows. A 1 foot section of plants are grasped on each side of the shake cloth and vigorously shaken over the cloth. Dislodged larvae can be identified and counted. Although economic thresholds are variable for this pest, in Missouri we use an economic threshold of 1 or more larvae present per plant or 1 or more larvae present per foot of row to be an action level where control is justified. If most larvae are 1 to 1.5 inches in length then control may not be necessary as larvae of this size will soon stop feeding and pupate.
If control of podworm is justified, select from the list of labeled pesticides on page 151. For optimal control of soybean podworm larvae use sufficient water with pesticide to provide good coverage of foliage.
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 Sheet G7110 or at the Iowa State University entomology photo gallery (www.ent.iastate.edu/imagegal)
REVISED: September 26, 2011