Now is the Time to Scout for Western Bean Cutworm!

Adult western bean cutworm (WBC) numbers continue to rise for the week ending July 25. Counties currently experiencing high WBC trap counts are primarily located in Northern Ohio (Figure 1). This past week, 11 counties were at the egg mass scouting threshold including: Ashtabula, Defiance, Fulton, Geauga, Henry, Huron, Lake, Lorain, Lucas, Trumbull and Williams. The statewide average for WBC moths also increased, more than doubling from the last week average (6.0) resulting in 14.3 moths per trap. Overall WBC numbers are higher than what we observed in 2020. It is unclear if WBC moths peaked for the week ending July 25, or if the numbers will continue to rise. Regardless, now is the time to get out and scout for egg masses. Continue reading below for guidelines on how to scout.

Figure 1. Average Western bean cutworm adult per trap (in white) followed by total number of traps monitored in each county (in blue) for week ending July 25, 2021. Map developed by Suranga Basnagala, Ohio State University, using ArcGIS Pro.

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  (Figure 2) 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).

View our scouting video here: https://aginsects.osu.edu/news/western-bean-cutworm-video

Figure 2. Western bean cutworm egg mass on corn leaf.

Treatment
If the number of egg masses/larvae exceed the threshold (mentioned above), foliar applications of insecticides are available, especially those containing a pyrethroid. We do not recommend tank mixing insecticides with corn fungicide spraying; this could result in a lot of wasted sprays without scouting. Timing an insecticide application is critical and must happen before the caterpillar enters the ear, but after the eggs hatch. 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. Timing spray applications is critical for WBC. Without proper egg mass scouting, the window of opportunity may be missed.  

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.