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Corn Rootworm Egg Hatch and Scouting
  • Corn rootworms (CRW) are one of the most important insect pests of corn in the Midwest with the potential to cause significant yield losses and increase lodging.
  • As of June 13th, depending on where you are across the Midwest, corn rootworm egg hatching has started, and larval feeding is underway.
  • Even though low CRW populations have been observed in recent years, scouting is key to prevent, correct and predict the potential for CRW damage.

Corn rootworms (CRW) are one of the most important insect pests of corn in the Midwest with the potential to cause significant yield losses and increase lodging. A compilation of field research trials suggests that around 15% yield reduction can occur for each nodal root consumed by CRW larvae (Tinsley et al. 2013). Female CRW beetles lay eggs in the soil in late summer, those eggs overwinter and resume development the following spring. CRW development is driven by soil temperatures and it’s measured in degree days. Peak CRW egg hatch occurs between 684 and 767 accumulated degree days (52°F base soil temperature). As of June 13th, depending on where you are across the Midwest, corn rootworm egg hatching has started, and larval feeding is underway.

Scouting should start about two weeks after peak hatch but can also be done later in the season to evaluate the extent of root damage. Although in general the potential for corn rootworm damage in late plantings may be low, all corn fields should be scouted. To evaluate the number of larvae present, carefully dig and remove a 7” x 7” cube of soil around the base of one plant in at least ten locations of the field. Place and crumble the soil over a dark background and examine for CRW larvae. Alternatively you can immerse the sample in a bucket of water and if CRW larvae are present, they will float to the surface. Finding an average of two or more larvae per plant using the soil crumbling method or eight or more using the float method may indicate the potential for economic damage. To determine the severity of root damage, evaluate root injury around mid-late July. Dig one plant in each of ten randomly selected areas and wash the roots to remove the soil. Using the Iowa State node injury scale, rate the roots from 0-3 where 0 = no damage and 3 = three or more nodes have been eaten. Scout high risk fields first. These include fields planted with hybrids without CRW Bt traits, fields with a history of CRW Bt trait failures, fields on continuous corn and fields with high CRW beetle counts the previous year.  

Where to sample first? High risk fields should be sampled first. These include:  
  • Fields planted with hybrids without CRW Bt traits 
  • Fields with a history of CRW Bt trait failures 
  • Fields on continuous corn 
  • Fields with high CRW beetle counts the previous year 

Corn rootworm resistance to Bt traits was first documented in 2009 and cross-resistance was confirmed a couple of years later. Scouting provides information about the performance of CRW Bt traits and can help diagnose resistance development in a field.  Regarding CRW adult counts, they can help predict the potential for CRW larval damage the following season. Even though low CRW populations have been observed in recent years, a few hot spots have been consistently identified across surveyed areas. Scouting is key to prevent, correct and predict the potential for CRW damage.  Keeping field records across years will help evaluate the performance of our current CRW management program and adjust our CRW control strategies accordingly.

References: 

Tinsley, N.A., R.E. Estes, M.E. Gray. 2013. Validation of a nested error component model to estimate damage caused by corn rootworm larvae. Journal of Applied Entomology. 137(3):161-169

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Corn Rootworm Egg Hatch and Scouting

06/16/2020