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Extension Entomology

Soybean Update – Thistle Caterpillars and Bean Leaf Beetles

–by Dr. Jeff Whitworth and Dr. Holly Davis

                Thistle caterpillars are becoming more evident around north central Kansas as they increase in size and their feeding becomes more visible.  These larvae are the result of eggs deposited by painted lady butterflies that migrated back into the state about two weeks ago.  These larvae will pupate in a couple of weeks and the adults will emerge soon after.  There will probably be even more in the next generation.

 

 

Round and/or oblong holes in seedling soybeans are indicative of adult bean leaf beetle (BLB) feeding.  Remember, these young plants are very resilient at overcoming up to about 50% defoliation in these early vegetative stages.  It takes approximately seven adult BLB/row ft. to achieve that level of defoliation.  However, adult BLB usually don’t feed for more than a few days after locating the seedling soybeans.  While this feeding can cause considerable concern because of the highly visual holes, it typically does not result in much stress to the plants, especially under good growing conditions.

 

For more information on bean leaf beetle biology and management, please see Bean Leaf Beetle:  https://www.bookstore.ksre.ksu.edu/pubs/MF2824.pdf

For more information relative to all soybean pests, please see the KSU Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

 

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