Western Bean Cutworm Peaking in Ohio

western bean cutworm eggs and larvae on corn leaf

Western bean cutworm (WBC) numbers for the week ending July 18 continue to increase across the state. Eight counties including, Ashtabula, Champaign, Defiance, Fulton, Henry, Lorain, Lucas and Woods have 7 or more moths per trap over the past week, indicating now is the time to be scouting for egg masses (to view our WBC scouting video click here: https://aginsects.osu.edu/news/western-bean-cutworm-video ). Overall, the statewide average was 6.2 moths per trap, which was an increase from an average of 4.6 moths per trap for week ending July 11. Below you will find our predictions and scouting recommendations. However, we do not recommend tank mixing insecticides with corn fungicide spraying. The trap numbers and GDD calculations only predict that moth flight is occurring, NOT where eggs are laid or which corn fields are infested. There would be likely be a lot of wasted sprays without scouting. Furthermore, keep in mind that WBC larvae are protected in the egg stage as well as once they enter the ear. There is a limited spraying window with WBC, which will likely be missed unless proper egg scouting has occurred.

 

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WBC Predictions
Growing degree day calculations were used to predict approximate percentage of adult WBC flight as of Sunday July 18th (Figure 2). Based on this data, counties in NW Ohio are seeing approximately 90% adult flight, whereas counties in central and NE Ohio are between 50 – 75% adult flight. For more information on calculating GDD and WBC please see the following article: https://cropwatch.unl.edu/2021/degree-days-prediction-western-bean-cutworm-flight

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Scouting guidelines
Counties with adult WBC trap counts averaging 7 or more moths per week should begin scouting for WBC egg masses in corn fields that are pre-tassel approaching tassel. Freshly laid egg masses are white and turn a purplish color as they mature.

To scout:

  1. Randomly choose at least 20 consecutive plants in 5 locations within a field (a total of 100 plants per field).
  2. Inspect 3–4 leaves on the uppermost portion of the corn plant. It is very useful to look at leaves with the sun behind them – often the shadow of the egg mass will reveal it without having to examine the leaf closely. 

Field corn should be treated with a foliar treatment if more than 5 % of inspected plants have eggs or larvae. Sweet corn should be treated if more than 4 % of inspected plants have eggs or larvae (processing market), or 1 % of plants (fresh-market).

Treatment
If the number of egg masses/larvae exceed the threshold (mentioned above), foliar applications of insecticides are available, especially those containing a pyrethroid. Timing an insecticide application is critical and must happen before the caterpillar enters the ear, but after the eggs hatch (Figure 3). If the eggs have hatched, applications should be made after 95% of the field has tassels. If the eggs have not hatched, monitor the egg masses for the color change. Newly laid egg masses will be white but turn purple as they mature. Hatch will occur within 24–48 hours once eggs turn purple.

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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.