When it comes to buying corn seed, one way to save money is to ensure you are not investing in GMO insect protection traits you do not need for your particular farm or field. Traited corn has been genetically modified to express one to five of the nine Bt proteins commercially available. The mode of action for Bt traits depends on the proteins matching up with receptors in the gut of the insect; this means that each type of protein will only affect a specific range of insect species.

Due to the complexity of available traits and the fact that certain proteins are only effective on certain pest species, it is important to understand the circumstances under which specific Bt traits are needed. A knowledge of crop rotation history, past performance of traits, and current or expected pest pressures for each field will also be important in making Bt trait decisions.

What Corn Rootworm Traits Do You Need?

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In most fields, rootworm Bt traits are not needed when growing corn after a non-host such as soybeans, wheat, dry beans, or sugar beets.

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In most of Nebraska, the western corn rootworm and northern corn rootworm are only pests in continuous corn. The female beetles lay their eggs in cornfields in late summer, the eggs spend the winter in the soil, and then larvae hatch out the following spring. If the larvae are not able to quickly find a suitable host (corn or a few species of grasses), they will die. For this reason, Bt traits targeting rootworms are not necessary when planting first-year (rotated) corn, as long as abundant, unmanaged volunteer corn populations were not present in the previous year.

Some populations of the northern corn rootworm in eastern Nebraska can have extended diapause in the egg stage, meaning that in a corn-soybean-corn rotation, the eggs will stay in the soil from the late summer of year one all the way until the spring of year three, skipping the year when a non-host is grown (such as soybeans), and attacking the rotated corn grown in year three. In rare cases, this can lead to northern corn rootworm economic injury in first-year corn. However, in the majority of fields, rootworm Bt traits are not needed when growing corn that is following a non-host. (Soybeans, wheat, dry beans, and sugar beets would all be non-hosts.)

In most fields, rootworm Bt traits are not needed when growing corn after a non-host such as soybeans, wheat, dry beans, or sugar beets.

Four types of Bt proteins target corn rootworms: Cry3Bb1, mCry3A, eCry3.1Ab, and Cry34/35Ab1. It is important to note that in multiple counties across Nebraska, greater than expected injury has been observed or resistance confirmed when single trait Cry3Bb1 or mCry3A has been planted for more than three consecutive years. (See Crop Production Clinics Proceedings Article, 2018 Corn Rootworm Management Update.) Cross-resistance between the Cry3Bb1, mCry3A, and eCry3.1Ab proteins exist. For this reason, in areas with a history of greater than expected damage to the Cry3Bb1 trait, such as YieldGard Rootworm or VT Triple Pro, it is not recommended to plant continuous corn with traits that express only mCry3A or mCry3A/eCry3.1Ab proteins (such as some Agrisure or TRIsect products).

The Cry34/35Ab1 protein, expressed either singly or in a pyramid (such as Herculex RW or SmartStax, respectively) currently works pretty well in most areas of Nebraska. However, resistance in western corn rootworm has recently been reported in northeastern Iowa. With resistance to all four commercially available rootworm-targeting Bt proteins now a reality, the need to adopt a diverse management strategy that includes crop rotation is critical.

What Western Bean Cutworm Traits Do You Need?

What European Corn Borer and Other Lepidoptera Traits Do You Need?

Although European corn borer (ECB) populations have decreased since the widespread adoption of Bt crops, they can still pose a threat to untraited corn. Fields that are planted early are most at risk for damage from first generation ECB and late planted corn is most at risk for second generation ECB. (Second generation damage is more likely to lead to yield loss.) If corn without one or more ECB traits is grown, timely and accurate scouting is the key to preventing yield loss. See this CropWatch article on how to plan for insect management in non-Bt corn.

All of the commercially available aboveground Lepidoptera Bt traits (Cry1Ab, VIP3A, Cry1F, Cry1A.105, and Cry2Ab2) are effective against European corn borer.

The Bt traits that are effective against additional Lepidoptera pest species found in Nebraska are:

• Corn earworm: VIP3A, Cry1F, Cry1A.105, and Cry2Ab2
• Fall armyworm: VIP3A, Cry1F, Cry1A.105, and Cry2Ab2
• Common stalk borer: VIP3A, Cry1F, Cry1A.105, and Cry2Ab2
• Black cutworm: VIP3A and Cry1F
• True armyworm: VIP3A only

Bt Traits as Part of an IPM Plan

For all pests, it is best to use Bt traits within an IPM framework (rotate crops, rotate traits, use trait pyramids, scout fields, follow thresholds, preserve and/or promote beneficial insects for biological control, etc.) to prevent overuse of any one technology and slow the potential rate of resistance evolution. Regardless of hybrids planted, regular scouting of corn fields throughout the growing season is important to watch for the presence of pests not controlled by Bt, or if you choose to plant non-Bt traited hybrids. This will allow you time to respond to the presence of economically damaging levels of pests, should they occur.

Finally, compliance to resistance management requirements (such as refuges) is part of a grower’s contractual agreement when buying Bt-traited corn seed. Failure to comply with these requirements could result in a grower not being allowed to buy Bt corn hybrid seed in the future. Specific resistance management information will be a part of each corn seed bag label and must be followed to help delay the development of resistance by pests.