Western bean cutworm and spider mites in field corn

Assefa Gebre-Amlak
Extension Specialist, Colorado State University Extension

Western bean cutworm (WBC) moths began to emerge in most part of northeastern Colorado. According to the historic pheromone trap data, the moth flight will continue until middle of August in Colorado. The peak population of the moth usually occurs between the last week of July and first week of August.

Western bean cutworm is a pest of both corn and dry beans. In dry bean, pheromone traps may be used to monitor moth populations and make treatment (

Control is expected with only those Bt corn hybrids containing the Herculex I or Herculex Xtra events. For corn hybrids that do not contain these events, fields should be scouted for this pest; good control will be difficult once the larvae move into ears.

In non-Bt corn, management decisions of western bean cutworm are based on monitoring egg masses on corn leaves. Eggs from the moth are deposited in clusters on upper surface of leaves. Upon hatching, young western bean cutworm larvae, move to one of the two places on corn plant, depending on the stage of the plant. If the corn has not tasseled, larvae will feed on pollen in the developing tassel. If tasseled, larvae will feed on silk in the ear and once the ear is formed, the larvae will feed on developing kernels.

Chemical control is justifiable if 8 percent or more of the plants have egg masses or small larvae in the tassels and the crop is at least 95 percent tasseled. If tasseling is much less than this, the economic threshold should be raised to as fewer larvae are likely to reach the ears.

Scouting for eggs of western bean cutworm in corn is recommended the next three weeks in Colorado. Effective insecticide products for both corn and dry beans are found in the High Plains IPM Guide: ( Many of the insecticides registered for western bean cutworm have been associated with spider mite outbreaks, so fields should be monitored for mites after a treatment is made. It is important to monitor Banks Grass Mite (BGM) in corn if dry hot conditions prevail during the growing season. Webbing and discoloration on leaves are often first signs of mite infestations.

BGM builds up on the plant from the bottom up. Chemical treatment is justified when there is visible damage in the lower third of the plant and small colonies are present in the middle third of the plant before hard dough stage. For effective products for both species of spider mites management and detailed economic threshold, check the High Plains IPM guide (


See more