Will 2010 Problems with Corn Earworm in Soybean Lead to 2011 Problems?

Published: October 19, 2010

Over the past 4-5 years numbers of corn earworm (soybean podworm) have gradually increased in many of Missouri's soybean fields. Larvae may feed on numerous crops including the pods, seeds and occasionally flowers of soybean. This insect is a major pest of soybean in many southern and eastern states and traditionally has been an occasional pest of soybean in Missouri's "Boothill" counties and other counties bordering Arkansas. In 2010, infestations of podworm larvae could be found in many late planted fields throughout southern and western Missouri. Most fields with economic podworm infestations were almost exclusively 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. Wind direction and the intensity of moth flights during moth migration both help determine the location and intensity of developing podworm larval infestations. Another factor which may allows podworm populations to increase to economic levels is the application of early season foliar insecticides and fungicides. Early spraying for webworms in 2010 may have contributed to the reduction in beneficial insects allowing podworms to flourish during late summer and fall. Additionally, use of fungicides early season often limit the development of beneficial fungal pathogens such as Nomuraea rileyi, a major pathogen responsible for control of late season podworm larvae in most years. In 2010 economic podworm infestations were most severe in soybean fields located in southwest Missouri and in the northern areas of St. Joseph and Chillicothe. Most fields received moderate damage although in the St. Joseph and Chillicothe areas some fields exhibited total yield loss due to all pods being consumed by larvae.

Many producers have asked if the high numbers of larvae found in their fields will overwinter and be a source of future podworm problems in 2011. Although podworms may overwinter in the soil as pupae in southern and central regions of Missouri, a majority of mid-season and late-season podworm larvae come from moths migrating into the state during late summer from more southern locations. In most years the level of the fungal pathogen Nomuraea rileyi will substantially reduce numbers of podworm before they are able to pupate in the soil. In surveys conducted during the past two weeks by Ben Puttler (Emeritus extension assistant professor) almost 100% of podworm larvae remaining in soybean fields have been infected by this pathogen and will die within a few days. This pathogen will overwinter in the soil and will again emerge as a strong biological control agent in 2011 if conditions are favorable. In addition, those producers who (1) plant early and use narrow row spacings resulting in quick plant canopy closure (2) do not spray unnecessary insecticide and fungicide foliar applications resulting in reduced numbers of biological control agents, and (3) limit plant stressors such as nutrients, pH, and moisture will experience less problems with this soybean pest. These factors strongly suggest that the podworm larvae present during the past few weeks will have limited effect on the number of podworms found in 2011. The most effective methods of determining whether podworm populations are elevated is through monitoring of soybean podworm (corn earworm) moth flights during June–August and frequent scouting of soybean fields throughout the season, but especially during flowering and pod fill growth stages.

Some identifying characteristics of podworm moths and larvae are as follow. Moths are variable in color but tend to be tan with a yellow to light green tint. Moths are relatively large with approximately 1-1 ½-inch wing spans. They may lay eggs throughout fields at sites where crop canopy has not yet closed. 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. 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. There are typically 2-3 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, somewhat transparent, dome shaped ribbed 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 to11/2 -inch in length). Larvae grow through 5 worm stages and change in color with age. Newly hatched larvae are yellowish-white in color with second and third instar larvae changing to yellowishgreen. 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 4 pairs of abdominal prologs (middle of larva) and 1 pair of anal prologs (back end of larva). When disturbed, larvae often roll into tight balls until the threat passes.

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)