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Scouting and Treatment Recommendations for Western Bean Cutworm

Scouting and Treatment Recommendations for Western Bean Cutworm
Figure 1 (Image courtesy of UNL Cropwatch)

If western bean cutworm eggs hatch when corn plants are in the late whorl stage of growth, survival will be high.

- UNL Cropwatch

The first western bean cutworm moths were captured in University of Nebraska-Lincoln black light traps June 12 at Concord, June 20 at Clay Center, June 28 at North Platte, and June 29 at Scottsbluff. Flights have been particularly heavy in central Nebraska, with a steep incline in moth numbers in the Clay Center light trap since June 29 and reports of heavy egg laying and hatching over the past week (See Figure 1). Egg masses have been reported on field corn in a variety of locations across the state, including near Grand Island, Kearney, North Platte, Brule, Holdrege, and Benkelman. As the moth flight continues and egg masses are laid in crops, it is essential to scout fields for the white to purple, dome-shaped eggs and young larvae.

Western bean cutworm moths overwinter in the soil as pre-pupae before pupating in late May and emerging as adults in late June or early July. Adults then mate and lay eggs in clusters of 5 to 200 on the top surface of the upper third of a corn plant. The eggs require five to seven days to develop, during which time the egg color changes to tan and then to purple immediately before hatching. Young larvae move vertically up the plant (see video) to feed on tassel tissue before moving back down to enter ears and feed on developing kernels (Figure 4). If larvae hatch before corn tassels are available, survival will be very low. Leaf tissue alone is an unsatisfactory food source for young larvae. Young larvae that feed on late whorl plus tassel tissue are more likely to survive and weigh more than larvae raised on pollen, silk, and pollen plus silk. Once inside the corn ear, larger larvae (Figure 4) are protected from foliar applications of insecticides.

Western bean cutworm moth
Figure 1. Western bean cutworm moth (Photos by Julie Peterson)
Western bean cutworm egg mass
Figure 3. Western bean cutworm egg mass
Western bean cutworm moth larva
Figure 2. Recently hatched western bean cutworm larvae near their egg mass
Western bean cutworm feeding in corn
Figure 4. Western bean cutworm larva feeding on corn

If western bean cutworm larvae hatch before corn tassels are available, survival will be very low.


Scouting

Scouting for western bean cutworm should begin in field corn during the first 25% of flight. Control decisions should be made shortly after the moth flight peaks, usually in early to mid-July. (See the CW article: Using Degree-Day Models to Predict Western Bean Cutworm Flights.)

When scouting for western bean cutworms in corn, you can save time by using UNL’s speed-scouting mobile app or spreadsheet. Benefits include:

  • paperless scouting with real time, in-field decision support,
  • a visual aid in identifying western bean cutworm egg masses,
  • scouting data storage so you can review information without having internet access,
  • and a reminder for when future scouting should be conducted.

The app is available for iPad, iPhone, iPod Touch, and Android devices. The free app can be downloaded from the Apple App Store or Google Play by searching “western bean cutworm.” It is also available as a downloadable Excel spreadsheet, Western Bean Cutworm Speed Scouting, EC1585.

If you are not using the speed-scout tool, check 20 plants in at least five areas of each field. Look for eggs on the top surface of the upper most leaves and larvae in the tassel or leaf axils. Western bean cutworm moths prefer to lay eggs in corn plants that are in the late whorl stage compared to those that have completely tasseled. Pay particular attention to later planted fields or those with uneven development. Western bean cutworm eggs that hatch when corn plants are in the late whorl stage of growth have a high rate of survival. The larvae are well protected in the whorl or tassel and fresh tassels provide the best food source for early instar larvae.

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