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Podworm infestations in soybeans may be a cause for concern in 2011

Posted Monday, February 14, 2011, at 3:02 PM

In 2010, infestations of soybean podworm (corn earworm) 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. This insect is a major pest of soybean in many southern and eastern states and traditionally has been an occasional pest of soybean in "Bootheel" counties of Missouri and other counties bordering Arkansas.

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.

This situation is a concern for producers in the area as to what could be the implications for 2011. 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 examining the podworm larvae remaining in soybean fields last fall, almost 100% had been infected with a fungal pathogen and should have died within a few days. The 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 spacing 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 stessors such as nutrients, pH, and moisture will experience less problems with this soybean pest according to Wayne Bailey, University of Missouri Entomologist.

All of these factors strongly suggest that the podworm larvae present last fall 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.

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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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