Soybean podworm (also known as corn earworm) are major pests of soybean in many southern and eastern states and traditionally have been an occasional pest of soybean in Southeast Missouri. In recent years the numbers of soybean podworm larvae in more northern counties of Missouri have gradually increased. This year substantial damage from podworm has been observed in some late-planted soybean fields located in both southeast and southwest Missouri, and in areas surrounding St. Joseph and Chillicothe in northwest Missouri. Although soybean podworm pupae may overwinter in the soil, most larvae come from eggs laid by moths migrating into the state during early spring or from moths produced in the state from first, second, and possibly third generation soybean podworm populations.
Several factors may be responsible for the heavy soybean podworm infestations found in some areas of Missouri this growing season. They include date of planting, wind direction during moth migration, possible prior use of insecticides and fungicides earlier in the growing season, intensity of moth flights, weather/field conditions and possibly factors that are not known. Problems with soybean podworm this summer and fall have been almost exclusively found in soybean fields planted after June 1, 2010. These late fields are most attractive to migrating moths as females prefer to lay eggs in fields where soybean plant canopies remain open. Another factor which helps determining whether infestations of podworm are successful is wind direction during migration and intensity of moth flights. Typically many insects are transported into the state or are moved about the state when winds blow from southern or southwestern directions. Damage to soybean has been substantial in some areas ranging from minor pod damage to 100% destruction of pods in some fields. Fields exhibiting the greatest damage from soybean podworm larval feeding are those that have previously received applications of an insecticide, fungicide, or both during the growing season. Biological agents such as parasites, predators, and beneficial fungal or bacterial pathogens all help to keep soybean podworm numbers low in most years. The early season use of insecticides, such as those applied for webworm, or the application of fungicides for soybean diseases have the potential to reduce these biological agents and may allow for pest populations to flourish under certain field conditions. In recent years numbers of green cloverworm, soybean podworm, and fall armyworm all have generally increased in soybean as well as other Missouri field crops. Although many factors regulate whether an insect population flourishes or declines, larval stages of the above mentioned insect pests are often regulated by beneficial fungal pathogens such as Numuraea rileyi. This pathogen causes the larval worms to stop feeding, take a upright posture on a leaf or stem, and turn the insect white to whitish-green as the fungal pathogen kills the insect from within. In some fields dead and dying infected larvae may also be found on the ground within the row.
Although it is unknown which factors are responsible for increasing pest populations in soybean observed during recent years or for the outbreak numbers of soybean podworm found this season, effective pest management decisions are necessary to protect crop yield. In most years it may not make a difference whether a foliage feeder such as green cloverworm is present in high numbers during late season as crop damage is limited.
In contrast, high numbers of pod feeders such as soybean podworm, bean leaf beetle, green stink bug, and grasshoppers may require intensive management to prevent substantially reduction of crop yields.
Additional information is available in University of Missouri Extension Guide Sheet G7110 "Corn Earworm in Missouri".
REVISED: January 5, 2012