Western bean cutworm a growing threat

    Colorado State University Extension pheromone trap-based monitoring is showing the emergence of the western bean cutworm moth in northeastern Colorado. According to historic data, the moth population will peak between the third week of July and first week of August in Colorado. This insect attacks both corn and dry beans.
    Eggs of western bean cutworm are deposited in clusters on the top surface of leaves. Following hatch, young western bean cutworm larvae move to one of the two places on the corn plant, depending on the stage of development. If the corn has not tasseled, larvae will feed on pollen in the developing tassel. If the corn has tasseled, larvae will feed on silk in the ear. Once the ear is formed, the larvae will feed on developing kernels and cause direct and indirect damage to kernels.
    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 the next three weeks. Good control will be difficult once the larvae move into ears.
    Monitoring for this insect depends on the number of corn plants with egg masses. 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, as fewer larvae are likely to reach the ears.
    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.

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