As you plan next year’s corn crop and what Bt traits you want to include to control insects, you might be bombarded with a variety of protein names, events or trade names. This can make even the well-seasoned farmer go buggy.
Let’s discuss how to identify the traits. Bt proteins are derived from the Bacillus thuringiensis bacteria found in soil. Once ingested, these proteins develop a toxin that binds to the inside of the gut, killing the insect. Each protein may only control certain insect pests. For example, Vip3A will control western bean cutworm, but Cry1Ab will not.
When genetic material from these proteins is successfully inserted into the corn, it is called an event. Common events for corn rootworm genes are MIR604 or MON863. When the company makes the trait ready for sale, it is given a trade name or also called a trait package, such as Agrisure RW, Herculex CRW or Yieldgard Rootworm.
Many seed companies are creating hybrids which use two or more proteins that control the same insect, referred to as a pyramid. The function of these pyramid hybrids is to reduce the probability of an insect becoming resistant to the proteins. However, once an insect becomes resistant to one or more of the proteins in the pyramid, the ability for the other proteins to control the insect may be reduced.
Currently, the only area of Wisconsin with resistance to a protein is in the southwestern part of the state. There, western corn rootworm was found to be resistant to the Cry34Ab1/Cry35Ab1 protein, found in hybrids with the Herculex RW trait. This protein is also found in other trait packages.
The Handy Bt Trait Table developed by Michigan State University and Texas A&M University lists all the trait packages, along with the proteins that make up the traits. If you use traits, this publication should be one of the tools you use in your decision-making process.
To address this resistance issue, Bryan Jensen, University of Wisconsin-Madison entomologist with the Integrated Pest Management Program, provided advice on how to implement a resistance management program to ensure these proteins remain effective during the UW Pest Management Update meetings:
Trait selection. Use the Bt Trait Table to select trait packages that contain different proteins every two years, and preferably annually. When using these traits, always follow refuge requirements.
Crop rotation. Corn rootworm eggs overwinter and need corn roots to feed on to survive the following growing season. Rotating to a nonhost crop will disrupt the rootworms’ life cycle. It should be noted that in southern and southeastern parts of Wisconsin, western corn rootworm has been found to lay its eggs in soybeans. However, since first detected in 2002, there have been no reports of significant damage to corn following soybeans in the last decade.
Soil-applied insecticide. Planting a conventional, non-Bt hybrid with a soil-applied insecticide can be an alternative to the below-ground traits in areas with high populations. There will be an expense to retrofit your corn planter if it isn’t already equipped with the application equipment. Also, yearly calibration is needed to ensure the correct amount of insecticide is being applied. This might require you to maintain a private pesticide applicator certification to purchase and apply these insecticides.
Seed treatment. Use a high rate of seed treatment — of 1,250 milligrams per kilogram — to control low populations of rootworms.
Another key to control corn pests is to use an Integrated Pest Management approach. Corn traits that control above-ground pests, such as European corn borer, are not considered part of an IPM program because they are purchased prior to the growing season and planted before infestation levels are known. For these pests, scouting and, if necessary, an in-crop insecticide application is your best control measure if following an IPM program.
Unlike the above-ground pests, the use of traits to control corn rootworm is considered part of an IPM program. To determine if you need to use traits the following season, scout for beetles from mid-August to early September during egg laying. If the threshold of 0.75 beetle per plant is reached, a hybrid with a trait that controls rootworm is needed.
Talk with your local county agriculture Extension educator or crop adviser about how you can use corn traits in an IPM program to protect corn yields and minimize insect resistance.
Marzu is the Extension agriculture educator for Lincoln and Langlade counties in Wisconsin.