Gear Up: Western Bean Cutworm

Bryan Jensen, UW Extension, UW-Madison Integrated Pest Management Program

It is that time of the year to start thinking, if not scouting for Western bean cutworm.  Degree days suggest the moth flight is underway, although not peaking, in southern Wisconsin.  Female moths will seek late vegetative corn or corn in the early stages of pollination.  Eggs are laid in clusters each having several eggs/mass.  After hatch larvae will disperse w/in and across rows and will begin to feed on corn pollen or move down to silks when available.  Larvae will enter at the ear tip and feed on developing kernels causing direct yield loss and perhaps create a site for ear molds to grow on damaged kernels.

Scout fields for eggs and larvae by monitoring a minimum of 20 consecutive plants in each of 5 locations.  However, larvae will be difficult to find because of their small size.  Because of the long adult flight period several visits per field may be necessary.  Eggs are typically laid on the upper leaf surface and on leaves in the top 1/3 of the plant.  If scouting on sunny days, the egg mass can be easier to spot because of the shadow it creates on the leaf when backlit by the sun.  After hatching larvae will consume their egg shell making then difficult to spot.

When eggs are first deposited they are white and eventually turn a dark purple color just prior to hatch.  Treatment is suggested when approximately 5% of the plants have an egg mass.  Keep in mind the flight period is extended and you should try to time the foliar insecticide for first egg hatch.  Our foliar insecticides will not kill eggs nor will they kill larvae which have entered the ear.

Fresh Western Bean Cutworm Eggs
Photo credit: Frank Peairs, Colorado State University, Bugwood.org

Mature Western Bean Cutworm Eggs
Photo credit: Frank Peairs, Colorado State University, Bugwood.org

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