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Agronomic Crops Network

Ohio State University Extension


Corn Rootworms and Fireflies

Male Western Corn Rootworm

Most of us remember a connection between fireflies and corn rootworm hatch. This connection may not be absolute and could have changed since we’re now using different production practices than in the past. Northern Corn Rootworm AdultWe also know a lot more about corn rootworm biology.  In Ohio, western corn rootworms are by far the most common (although you may see some northern corn rootworms). Adult rootworms lay eggs in the late summer and these eggs typically hatch the following June.  We also know that peak egg hatch (i.e., 50% of the total hatch) occurs between 684 to 767 accumulated growing degree days (base 52oF).  Our map below shows that much of Ohio has reached or exceed these GDD. If rootworms are in your field, chances are they have hatched and begun their feeding.

Accumulated growing degree days (base 52°F) through June 20, 2021. Orange numbers indicate areas where 50% percent of corn rootworm larvae will likely have hatched with red numbers indicating conditions that are past this GDD range. Data courtesy of the Midwestern Regional Climate Center (

Over the next few weeks, corn should be inspected for damage.  Prioritize your scouting in non-rotated corn (i.e. 2nd, 3rd-year corn, or more), and corn without below-ground Bt traits or insecticidal seed treatments.  However, some rootworm populations have also adapted to infest 1st-year corn, as well as to overcome virtually all Bt traits (important note—we have NOT detected any Bt-resistant rootworms in Ohio yet).  See our corn rootworm fact sheet ( for more information on inspecting corn roots for damage.  If any damage is detected on Bt roots, please contact us or your local extension educator, because it might be an early indication of resistance in Ohio.

 Western Corn Rootworm feeding on weeds.

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.