Agronomy Talk


Published on Monday, April 20, 2020

Western bean cutworm (WBC) is a relatively new pest to field corn in the Midwest. While WBC is native to North America, it has primarily been a pest of specialty crops up until the early 2000s. Like European corn borer and earworms, WBC is part of the Lepidoptera family of corn pests, meaning they resemble caterpillars. Even though they look much like corn borers and earworms, their feeding and life cycle is quite different.


Western bean cutworm has only one generation per year in the corn belt. Moths begin to emerge from the soil at the end of June/early July and then migrate to fields where they lay their eggs on the upper surface of corn leaves. Female moths prefer fields that are closest to pollination as their larvae feed on pollen after emerging from their eggs. Once hatched, the larvae will migrate towards the base of the corn leaves and eventually to the ear itself. Unlike earworms, which are cannibalistic, there may be multiple cutworms on each ear. The larvae will undergo six growth stages (instars) before reaching the maximum size. At the end of the season, the larvae will then drop to the soil and emerge as moths the following spring. 


The primary issue with WBC is that the openings they create to the ear act as avenues for ear molds to develop. Early-hatched larvae will enter the ear through tiny holes that are difficult to identify. However, the later-hatched larvae that may have spent more time feeding on pollen will then enter the ear, causing severe damage with their aggressive feeding through the husk. Many ear molds such as fusarium, diplodia, etc., will rapidly develop at these infection points. 


Scouting for WBC is critical because, unlike many other pests, bio-tech traits are relatively ineffective at managing them. Therefore, we need to know when the larvae are present so that we can apply an insecticide at the appropriate time. Follow these steps when scouting: 

  1. Set pheromone traps like the one shown below to monitor moth emergence and when they start flying into fields. These traps are very affordable and easy to use.
  2. Once moths have been caught, begin scouting for eggs. To scout for eggs, identify 20 consecutive plants in five separate places throughout a field and look for egg masses on the upper leaf surface of the top four leaves of each plant.
  3. To calculate your results, determine how many plants have egg masses. If eight of the total 100 (8%) plants observed have masses, then treatment may be warranted. 


As previously stated, biotech traits have shown to be relatively ineffective at WBC control. The one exception is Agrisure Viptera®. If you have not planted a hybrid that contains the Agrisure Viptera trait, then chemical treatment may be needed if the WBC population hits the threshold. Timing is critical with this application because it is only effective if the larvae are hatched and still on the outside of the leaf surface. Apply an insecticide when 95% of the silks have emerged within a given field. If a large number of eggs hatch before tassel, consider treatment when 80% of the eggs have hatched. Scouting will be critical to the success of this treatment! 



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Author: Ben Puestow

Categories: Agronomy, Agronomy Talk


Ben Puestow

Ben Puestow

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