The historic 2019 growing season includes challenges from increased crop insects such as resistant western bean cutworms that likely have the highest potential to adversely affect corn yields.

Insecticide treatment time for western bean cutworms has passed because the worms are protected by the ear husks, so corn ear damage may be observed throughout fall harvest.

In nontreated, severely infested fields, there may be cases of two or more larvae per ear consuming up to 50 percent of kernels. Research has shown that one larva per plant at dent stage usually reduces yields by 3.7 bushels per acre.

Bt (Bacillus thuringiensis) traits have been used in most standard corn hybrids for almost two decades to provide plant resistance to European corn borer, black cutworm, western bean cutworm and other chewing insects.

Despite efforts to extend insect resistance development by using practices such maintaining a 5 percent refuge in fields, the Cry1F Bt trait — Herculex, AcreMax and SmartStax — has provided less insect control the past few years as more western bean cutworms have developed immunity to this trait.

In Nebraska Extension insect trials, VIP3A proteins — Agrisure Viptera and Leptra — still provide nearly 100 percent control of western bean cutworms.

A reminder for crop scouts and corn growers: The VIP3A Bt trait won’t prevent adult cutworms from laying eggs on corn leaves. Insect larvae are killed when feeding on the corn tassel.

A free Bt trait table for cutworms in U.S. corn production is available at https://www.texasinsects.org/bt-corn-trait-table.html.

Nebraska Extension West Central Entomologist Specialist Julie Peterson says corn breeding resistance provides far better control than relying strictly on insecticide applications during growing season. However, resistance issues can make it more difficult to know which traits will provide adequate control and when a rescue insecticide treatment is needed.

Best management practices for western bean cutworms begin with cornfield scouting that, generally, should start during the first 25 percent flight of adult moths.

Nebraska Extension provides free online moth updates based on UV insect light traps at the Concord, Clay Center, Mead and North Platte research stations. The traps capture a wide range of insects for monitoring.

Insect counts are posted daily to weekly from mid May through September at https://entomology.unl.edu/fdcrops/lightrap.

Cutworm flight patterns and corn development are approximately two weeks behind this year, compared to 2018.

Fields still should be monitored for treatment thresholds at the first 25 percent moth flight, based on insect flight counts and/or growing degree days.

When 5 percent-8 percent of corn plants have egg masses or larvae that have changed from white to purple in color, insect treatment in corn may be warranted. The best time is when 95 percent of plants in a field have tasseled.

Aerial applications are best timed while the larvae are in the upper tassel region. It takes 10-14 days from egg hatch to when larvae enter corn ears.

The free UNL app is available for iPad, iPhone, iPod Touch and Android. Search for “western bean cutworm” in the Apple App Store or Google Play. It also is available as an Excel spreadsheet download.

The app can incorporate degree day models to predict western bean cutworm flights and then calculate potential treatment thresholds.

Cutworms have one generation per year, with moth emergence generally starting in early July. The date may be predicted by calculating growing degree days — start accumulations May 1 — and using a base air temperature of 50 degrees. Populations vary from year to year, but greater populations tend to occur every six to eight years.

Get more information at Extension offices, including NebGuide G2013 “Western Bean Cutworm in Corn and Dry Beans,” and at CropWatch.unl.edu.

Todd Whitney is a University of Nebraska-Lincoln Extension educator specializing in cropping systems and water whose focus region is Phelps, Gosper, Harlan and Furnas counties.

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