Western Bean Cutworm Egg Masses and Larval Hatch- Scout Field and Processing Sweet Corn Fields

Western Bean Cutworm Egg Masses and Larval Hatch- Scout Field and Processing Sweet Corn Fields

Eileen Cullen, Extension Entomologist

Careful field scouting in field corn and processing sweet corn for western bean cutworm egg masses and small larvae should begin now.

Economic threshold determination and treatment decisions are much easier to make at the egg mass and small larval stages before larvae have moved to silks and into the ear via silks or through the side of the husk, after which point treatment will not be effective.

Egg masses have been reported to me this week on processing sweet corn in the central sands area near Hancock in fields that are just pre-tassel and at tassel. Field corn crop consultants have also reported sighting first egg masses over the past week in southern and central WI field corn.

WDATCP and cooperators are running 164 western bean cutworm pheromone traps in Wisconsin since late June/early July. The trap network will continue to monitor the annual flight through peak emergence and to the end of the flight. WDATCP reports a cumulative total of only 186 moths thus far, compared to 7,631 moths at the same time last year when the flight was 50-75% complete statewide.

Check the WDATCP Wisconsin Pest Bulletin regularly for percent moth emergence based on trap captures and western bean cutworm degree-day accumulations in your area. The Wisconsin Pest Bulletin is issued weekly, usually on Thursdays, with the latest trap catch abundance and distribution and an accounting of western bean cutworm degree-day accumulations.

Below are some key field scouting points and economic thresholds to remember for non-Bt corn fields, refuge corn acres, and processing sweet corn fields. Not all Bt field corn hybrids contain the Cry1F or Vip3A traits for western bean cutworm control. Check your Bt hybrid if you need to determine whether or not it has western bean cutworm included in the spectrum of insects controlled.

Please refer to the July 6, 2011 WCM article ‘Western Bean Cutworm Moth Flight Has Begun’ for further review and photos. Much of that information is repeated here as an alert to WCM readers that egg mass detection and larval hatch are underway and fields should be scouted now.

  • Using the western bean cutworm phenology model, scout corn no later than 1,320 degree days (base 50F) when 25% of the moth population will be in flight. Oviposition will intensify at 50% (peak) moth emergence (1,422 degree days). This has occurred in some areas (Beloit, LaCrosse, Madison). Hancock and Wausau are between 25% and 50% moth emergence.
  • Western bean cutworm pheromone traps placed at or near your corn fields is the best indicator of when to start scouting. Begin scouting as soon as 1 moth is captured in your trap or nearby reference trap.
  • Although the 2011 pheromone trap captures are lower than 2010 and this certainly implies that we could have less infestation pressure overall, there is no correlation between numbers of western bean cutworm moths in a pheromone trap and severity of infestation in a corn field. Even if your traps are catching only a few moths per week, begin scouting for eggs and larvae.
  • Moths have been captured in southern and central Wisconsin as far north as Waushara and Wood Counties, and west to Monroe County. Please check the WDATCP Wisconsin Pest Bulletin regularly for updated moth captures and peak flight information throughout the 160 trap pheromone network.
  • Adult female moths are most attracted to corn just before tasseling and lay eggs on the upper leaf surface, primarily on upper leaves, but also near the ear zone.
  • For field corn, the economic threshold of 5% field infestation with egg masses and small larvae is recommended.
  • Processing sweet corn threshold is 4% field infestation with egg masses and small larvae.
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