Julie Peterson – University of Nebraska Extension Entomologist
This month a group of six extension entomologists from Michigan State University, Purdue University, The Ohio State University, Cornell University, and Pennsylvania State University published an open letter to the seed industry describing poor performance of Cry1F Bt corn against western bean cutworm (WBC) in their states in 2016. The Cry1F protein is present in products such as Herculex I, Herculex XTRA, AcreMax, and SmartStax. In their letter they strongly encouraged a change in the labeling for these products, in particular a removal of the designation of “control” for this pest from the Cry1F protein. (View the complete letter.)
Western bean cutworm larva feeding in corn ear.
Nebraska Extension Entomologist Julie Peterson talks about the efficacy of the Cry1F protein against western bean cutworm in this Market Journal video segment.
While the letter reports problems with field performance of Cry1F against WBC occurring for the first time in the eastern Corn Belt in 2016, crop consultants and farmers in Nebraska have been dealing with similar issues for several years.
Prior to about 2000, WBC populations were primarily found in western Nebraska, northeastern Colorado, and eastern Wyoming; since then the extent of the population has moved east to the Great Lakes region, New York, and southern Ontario. When the Cry1F trait was first introduced to the market in 2001, other Lepidoptera such as European corn borer, were the primary target. The Cry1F trait was marketed as providing only about 80% control of WBC.
While it continues to be effective against other labeled pests, such as European corn borer, fall armyworm, and black cutworm, recent research has shown that its effectiveness against WBC has decreased in some areas. (See more in a July 8, 2016 CropWatch article.) In regions where problems have been observed (particularly southwest and central Nebraska), it is recommended that fields with Cry1F products be scouted for WBC and insecticide treatment be considered when 5%-8% of plants in a field have egg masses or larvae.
Seed Selection, Scouting and Treatment Options for 2017
As seeds are now being selected for 2017, it is important to consider that Cry1F is not the only type of Bt protein being marketed as providing some protection against western bean cutworm. Products that express the VIP3A protein, such as Agrisure, Viptera, and Leptra, provide effective control and should not need to be treated, although it is always advised to inspect Bt cornfields to ensure adequate efficacy. See the Handy BT Trait Table for a list of commercially available Bt corn hybrids and the proteins they express.
For corn hybrids that do not provide Bt control of western bean cutworm, consider applying an insecticide if 5%-8% of the corn plants have egg masses or larvae. If an insecticide treatment is warranted in corn, it should be made when 95% of the plants in a field have tasseled. This application timing increases the chance that larvae will be exposed to the insecticide, resulting in better control.
WBC Scouting App
When it comes time to scout, try the Western Bean Cutworm Speed Scout app which can help you determine whether WBC populations have reached economic thresholds. 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 in Nebraska Extension publications, Western Bean Cutworm Speed Scouting, EC1585.
Aerial application and chemigation have provided good control for this insect if applied before larvae enter the ear. (Note: Sprayer drops should be above the canopy or have nozzles that allow sufficient application of product onto the tassels.) Carbamates (Sevin), organophosphates (Chlorpyrifos), spinosyns (Spinosad), methoxyfenozides (Intrepid), diamides (Prevathon), and many pyrethroids (51 products) are labeled for WBC control. Consider rotating mode of action in areas where pyrethroids have been heavily used for western bean cutworm and western corn rootworm control. The potential for resistance to common active ingredients such as bifenthrin is a concern.
A list of registered insecticides, rates, preharvest intervals, and grazing restrictions is available in the Insect Management section of the most recent UNL Extension Guide to Weed Management in Nebraska with Insecticide and Fungicide Information (EC 130).