Western Bean Cutworm: Peak time to scout for egg masses

Results from week five of The Ohio State University Western bean cutworm (WBC) monitoring network has resulted in an overall increase of moths captured across Ohio. Meaning, last week (July 22 – 28) marks our peak week in adult WBC flight to date. A total of 26 counties monitored 79 traps across Ohio and resulted in a 2842 WBC adults (up from 2001 total last week) and a statewide average of 36.0 moths/trap (up from 25.3 average last week) (Figure 1). Since 2016, peak adult WBC emergence has typically occurred during the second or third week in July; however, with the cooler temperatures and increased rainfall the 2019 WBC peak week was delayed (Figure 2). If your county is averaging over 7 moths per week, now is the time we recommend scouting for WBC egg masses. You can view our scouting video here

Figure 1. Average Western bean cutworm adult per trap followed by total number of traps in the county in parentheses for week ending July 28, 2019.

Figure 2. Average western bean cutworm (WBC) per trap in monitoring counties in Ohio for 2016 (blue), 2017 (red), 2018 (green) and 2019 (purple).

Scouting and management. 

Check pre-tassel corn approaching tassel fields first – females prefer these fields to deposit eggs.

  • To scout for eggs or larvae, choose at least 20 consecutive plants in 5 random locations (scout different areas of the field that may be in different growth stages).
  • Inspect the uppermost 3–4 leaves.
  • Threshold (when to consider treatment):
    • Field corn, if >8% of inspected plants have eggs or larvae.
    • Sweet corn, if >4% of inspected plants have eggs or larvae for the processing market or on >1% of plants for fresh-market.

What you are looking for.

WBC egg masses are often found on the upper leaf surfaces in clusters. Eggs laid by WBC are round and first appear white, then tan and then a dark purple. Once eggs turn purple, they will hatch within 24 to 48 hours (Figure 3). WBC egg masses can be easily confused with stink bug egg masses; however, stink bug eggs are larger in size and are more barrel shaped with a ring of hairs around the top.

Figure 3. WBC egg mass


If infestations exceed threshold, many insecticides are available to adequately control WBC, especially those containing a pyrethroid. However, as with any ear-burrowing caterpillar pest, timing is critical. Insecticide applications must occur after egg hatch, or after tassel emergence, but before caterpillars enter the ear. If eggs have hatched, applications should be made after 95% of the field has tassel. If eggs have not hatched, monitor for the color change. Hatch will occur within 24–48 hours once eggs turn purple. To search for larval injury after it has occurred, search the corn for ears having feeding holes on the outside of the husks.               


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.