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To Bt or not to Bt? Is that your question?

Bryan Jensen and Bill Halfman
UW Division of Extension
This undated image made available by Frank Peairs in 2007 shows a European corn borer. A warmer world likely means more and hungrier insects chomping on crops and less food on dinner plates, a new study suggests. (Frank Peairs/Colorado State University/Bugwood.org via AP)

Using Bt-hybrids to control European corn borers and corn rootworm has been a key management practice used by Wisconsin farmers since 1996 and 2003, respectively. However, low corn prices, rootworm resistance, low insect populations and, in some areas, markets paying a premium for conventional corn have led producers and crop consultants to question their cost effectiveness. This summary was developed to provide information and options that will assist growers who are considering non-Bt hybrids.

What are “Bt hybrids”?

The term “Bt hybrid” is a general term used to describe a protein bio-engineered into a corn hybrid. There are approximately 9 different proteins available for use in corn which can be separated into two categories. Those which control corn rootworms are considered below-ground Bt traits and the above-ground Bt traits control an insect complex which may include one or more of the following insects, European corn borer, western bean cutworm, black cutworm, stalk borer and true armyworm. The above-ground Bt traits can be very specific regarding the insects they control. No single trait will control all the above ground insects.

Will corn yields suffer?

There is a misconception that conventional hybrids do not yield as well as Bt hybrids. Producers should judge the merits of each hybrid individually, not as a group of one versus the other. As with any hybrid selection, you will have to do your homework. We strongly urge reviewing data from the University of Wisconsin Corn Hybrid Performance Trials as an unbiased source of hybrid performance from multiple locations and multiple years. The authors of this publication do an excellent job of guiding you through the hybrid selection process.

What do I have to do differently?

Field scouting is always important but even more so when switching to non-Bt hybrids. Scouting, in some respects, serves as a replacement for Bt hybrids and will help reduce the risk of insect damage through early detection and allow time to use a cost-effective rescue treatment if needed. When considering switching to conventional hybrids, carefully consider if and how you will be able to monitor your fields. You may want to consider what the cost would be to hire an independent crop consultant to do the field scouting. Contact your local county extension agent/educator for individuals who offer this service in your area.

Or, you may choose not to scout. Understand that there is some risk involved without field scouting. Earlier in this fact sheet we indicated that insect pressure has been low for a while, however, there is still some risk of enough insect pressure that some fields may need a rescue treatment.

If you are going to do your own scouting, there are specific field sampling techniques for European corn borers, corn rootworms and other insects that should be used to provide an accurate assessment of damage potential. They are not difficult and will ensure your time in the field will be as efficient as possible.

Information on insect identification, life cycles, economic thresholds and scouting methods can be found in the Field Crop Scout Training Manual.

Becoming familiar with these techniques before switching to non-Bt hybrids will help you with your decision. Although our primarily focus is insects, the value of field scouting goes far beyond the question of switching to non-Bt hybrids. Field scouting will provide additional information for diseases, weeds, nutrient deficiencies as well as anything else that may be going wrong (or right!) with your crop.

The following electronic newsletters are published weekly during the growing season and will provide scouting and pest management assistance. The Wisconsin Pest Bulletin (http://datcpservices.wisconsin.gov/pb/index.jsp), issued by the Wisconsin Department of Agriculture, Trade and Consumer Protection, is an excellent newsletter that emphasizes current and trending pest populations.

The University of Wisconsin-Madison Division of Extension publishes the Wisconsin Crop Manager https://ipcm.wisc.edu/wcm/ which provides both management and pest alert information. Subscriptions to both electronic newsletters are free.

Insect Management Recommendations

Become familiar with pertinent insect management practices, especially economic thresholds. It may have been several years since you’ve had to think about controlling these insects and a quick review will update you on current management practices.

Specific insecticide recommendations and economic thresholds are available in A3646, Pest Management in Wisconsin Field Crops https://patstore.wisc.edu/. This publication is updated annually. For further interpretation of these management practices consult with your local county extension agent/educator.

Bryan Jensen is associated with the Department of Entomology, UW Division of Extension while Bill Halfman is with the Monroe County, UW Division of Extension