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Extension Entomology

Dectes Stem Borers

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Dectes stem borer adults continue to emerge throughout north central Kansas.  These adults are currently aggregating mainly around field borders and most commonly on ragweed.  They will start dispersing into soybean fields within the next week to 10 days, as they do every year, to begin oviposition.  The adult females are relatively mobile and move from plant to plant inserting eggs into, or just below, the petioles of many plants.  This oviposition period may last for four weeks or more and may be spread throughout the field! This is one reason why controlling dectes stem borers with an insecticide is so difficult – timing of application.

Eggs hatch in the stem and the small larvae start feeding/boring their way to the main stem and then down this stem to the soil surface.  They usually reach the soil line in late August and larvae girdle their way around the inside of the stem, weakening the stem and often leading to lodging, especially if there are strong winds.  This lodging is responsible for most yield loss.  For more information regarding dectes stem borers, please see Dectes Stem Borer, MF2581: https://www.bookstore.ksre.ksu.edu/pubs/MF2581.pdf

For more information on soybean pest management, please refer to the KSU Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

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