Understanding Bt Traits

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• With the number of Bt corn hybrids available in the market, it can be confusing to decide which product best fits one’s needs.
• Insect resistance to Bt traits is a serious concern, especially for pests like corn rootworm, corn earworm, fall armyworm and western bean cutworm.
• Understanding the activity and differences between Bt hybrids can help us manage target corn pests more effectively.

Bt traits provide a form of insecticidal protection to some of our crops, such as corn and cotton. With the number of Bt corn hybrids commercially available, it can be confusing to decide which product best fits one’s needs. The field history, along with reported control failures and evidence of insect resistance or cross-resistance to insecticidal traits, are all important factors in deciding which hybrid to plant. Texas A&M University regularly publishes a very useful “Handy Bt trait table” listing the various trait packages available along with target pests, resistance findings, new product information, refuge requirement etc. The latest version available was updated March 2022. This table can be found at: https://agrilife.org/lubbock/files/2022/02/BtTraitTable-March2022.pdf

Bt traits are used to control a wide spectrum of common corn pests. Target species include aboveground Lepidoptera pests such as western bean cutworm, European corn borer, corn earworm, fall armyworm, etc., as well as soil or belowground insects like corn rootworms and black cutworms. Generally speaking, trait packages containing Cry1’s, Cry2’s and Vip proteins target Lepidoptera pests, while Cry3’s proteins target corn rootworm larvae. More specifically Cry1Ab’s target is European corn borer, while Cry2Ab2 was initially targeted against tobacco budworm, corn earworm, fall armyworm and other important pests of cotton. Cry1F provides good control of corn borers, fall armyworms, black cutworms and suppression of corn earworms. About a decade ago, Cry1F was considered efficacious against western bean cutworm (WBC) but after numerous failure reports this protein is no longer considered an option to manage WBC. Currently Vip3A is the only Bt toxin providing control of WBC and it is the most consistently effective protein against corn earworm.

Insect resistance to Bt proteins is a serious concern, especially in pests like corn rootworm, corn earworm, fall armyworm and western bean cutworm (where found). Corn rootworm is an important economic pest of corn in the Midwestern states. Bt corn hybrids used to control corn rootworms have been available for over a decade, but performance issues were reported just a few years after their first commercial launch. Likewise, recent experiments conducted on Bt sweet corn have documented decreased susceptibility of corn earworm to multiple Cry proteins providing evidence of field evolved resistance of this pest.  There also have been observations of corn earworms in the Midwest feeding unimpeded on Bt-traited corn that should provide suppression.  This has also been observed in other regions out East.

Resistance management strategies have been developed to help lengthen the efficacy of Bt traits. Some of the most important ones include the refuge requirement and rotation of Bt proteins. Initially, Bt corn hybrids had one Bt trait and a refuge requirement (non-Bt corn) of 20%. Since then, research has allowed to develop hybrids containing multiple insecticidal traits and the refuge requirement is different for different hybrids and target pests (5-20%) and it can be structured (block or strips) or integrated (in bag). The adoption of integrated refuges or “refuge in a bag” has greatly reduced issues with compliance requirements. Most hybrids are now refuge in a bag. In continuous corn situations, rotation of Bt proteins is key to managing insect resistance. Rotating Bt traits can be a confusing process. This is where the “Handy Bt trait table” can be useful. Using hybrids with multiple proteins (pyramids) can slow down resistance development as insects are less likely to develop resistance to two toxins at once versus a single toxin. However be aware that if the target pest has developed resistance to one of the traits in the pyramid, the utility of the pyramid is reduced.