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Which Bt Traits Do You Need to Buy?

November 19, 2018

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.

For additional reference, the Handy Bt Trait Table has an excellent summary of corn traits, pest efficacy, and refuge requirements.

What Corn Rootworm Traits Do You Need?

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?

Western bean cutworm (WBC) will lay their eggs in corn fields regardless of whether they have been rotated, although overall, populations tend to be highest in regions with more continuous corn. Pest pressure also tends to be highest in areas with sandy soils and when crop phenology matches up so that corn is in the late whorl to early tassel stage during peak moth flight. Recently, Nebraska has seen its highest WBC populations in the southwest and central parts of the state; however, the Clay Center location in south central Nebraska recorded the state’s highest moth flights in 2018 (Light Trap Data).

Two types of Bt proteins have been marketed against western bean cutworm: Cry1F and VIP3A. The Cry1F protein is present in products such as Herculex 1, Herculex XTRA, AcreMax, and SmartStax. When first introduced to the market, Cry1F provided approximately 80% control of WBC. However, recent research has shown that its effectiveness has decreased in some areas, such as parts of southwest and central Nebraska, in the last 10 years. In addition, field-evolved resistance to Cry1F has recently been confirmed in Ontario, Canada. These reports have led to the recent removal of western bean cutworm from the list of controlled pests on the label of Cry1F products. Not having a certain pest on a product label means that the company cannot guarantee control of that pest by that trait; however, pest populations likely vary in their susceptibility by location. In regions where reduced effectiveness of Cry1F has been observed, it is recommended that fields with Cry1F products be scouted for WBC and insecticide treatment considered if thresholds are exceeded.

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 more in past CropWatch articles, Nebraska Perspective on Efficacy of Cry1F Bt Corn Against Western Bean Cutworm and Scouting and Treatment Recommendations for Western Bean Cutworm).

In areas where reduced effectiveness of Cry1F has been observed, such as southwest and central Nebraska, fields with Cry1F products should be scouted for western bean cutworm and insecticide treatment considered if thresholds are exceeded.

Due to reports of resistance and poor field performance, seed companies have removed western bean cutworm from the list of controlled pests for Cry1F Bt products, including Herculex and AcreMax.

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.

Source: University of Nebraska CropWatch