Bt technology has its advantages and disadvantages, but it is best to consider factors such as timely planting and pest pressure when dealing with bollworms.

Scott Stewart, University of Tennessee Department of Entomology and Plant Pathology and UT Extension, and his graduate student, Dawson Kerns, discussed Bt cotton and resistance at the virtual Milan No-Till Field Day.

Bt cotton resistance

"Bt cotton has been genetically modified to express proteins from the bacteria Bacillus thuringiensis," Kerns said. "These proteins are toxic to a lot of caterpillars, but the primary targets for Bt technologies are bollworm, tobacco budworm, and in other parts of the world, pink bollworm."

Bt cotton expresses various Cry toxins, which are Cry1Ac, Cry1F, Cry2Ab, or Cry2Ae, as well as the Vip toxin, which is the Vip3A toxin also expressed in the newer Bt traits.

These widely adopted traits are oftentimes used along with the herbicide-tolerant GMOs. Some Bt technologies that are used today include Bollgard 2 and TwinLink, which are the two gene traits, and there is also WideStrike 3, Bollgard 3, and TwinLink Plus, which express the Vip toxin.

"These traits have high efficacy against most pests," Kerns said. "With bollworms, a significant pest in cotton, the Cry toxin doesn't work as well. Typically, bollworms become a problem after first bloom, so it's a later season pest. Unfortunately, we have field-evolved resistance to the Cry proteins, such as Cry1Ac and Cry2Ab, and this results in the need for supplementary insecticides, especially with the two gene cotton such as Bollgard 2.

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"Also, the Bt traits expressed in Bt cotton are like the ones expressed in Bt corn, so this leads to resistance. When they feed on corn, bollworms, also known as corn earworms, can select for resistance to the traits expressed in Bt cotton."

In recent years, the two-gene Bt technologies like Bollgard 2, the original WideStrike technology, and TwinLink were still providing substantial protection.

"We've seen more need to have to make insecticide applications for bollworms more frequently," Stewart said. "Looking at data from the Jackson station, we had good bollworm pressure in recent trials, showing the percent increase in yield from making an application of an insecticide, like Prevathon, onto cotton. The biggest yield increase was on the non-Bt trait, almost 30%, but with the older technologies, the two gene technologies such as TwinLink, Bollgard 2, or WideStrike, we're seeing a significant increase in yield percentage-wise compared to non-treated plots."

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All these technologies, under certain circumstances, could require spraying for bollworms.

"In Tennessee, however, we’ve not yet seen a need to make an application for bollworms in the three-gene technologies like Bollgard 3 or WideStrike3," he said. "There's going to be a need to spray these technologies on some occasions. Three-gene technologies containing the Vip trait may completely replace the older Bt technologies in two to three years. These technologies are going to be a little more stable and less likely to need an insecticide application, at least soon, but we are concerned about resistance. That's why scouting is important."

Spraying Bt cotton

Cheaper products can work on bollworms in the right circumstance, such as having relatively light pest pressure. Also, if it is very late in the season and you don't need to protect the crop very long, using cheaper insecticide options like a pyrethroid + acephate combination could work well.

"However, what we've been seeing consistently throughout the Mid-South is that the diamide insecticides, such as Prevathon and Besiege, are quite a bit better, particularly if you have heavy pressure and you need extended residual control," Stewart said. "The pyrethroid insecticides are typically only going to provide four or five days of residual control.

"These diamide insecticides, depending on the rate, may provide two to three weeks of residual control, and it usually takes one spray of the Bt technology to get adequate bollworm control in Tennessee."

Stewart said to remember to have realistic expectations.

"It's not realistic to make an insecticide application one time and expect to see no damage," he said. "Some crop damage, even when using Bt technologies, is inevitable."

Start early

Remember to manage your crop for earliness.

"This is something we've recommended for a long time, but try to plant the crop as early as possible," Stewart said. "Use early to mid-maturing varieties that mature before the bollworm moth flight kicks off."

The corn earworm or bollworm moth picks up as the season progresses. The moth flight that comes out of corn typically the last week of July or August causes cotton the most problems.

If a crop is maturing early August versus late August, it avoids a lot of potential corn earworm damage.

"The Bt corn technologies don't always do a good job of killing bollworms unless it has the Vip trait," he said. "It does delay the development of those larvae, so with the Bt corn technologies, it's probably even more important to get an early maturity cotton variety because the moth flight from corn is occurring a little bit later."

Bt cotton has tremendous value to the cotton producer, but it's not perfect.

"A grower can choose to grow non-Bt cotton," Stewart said. "If they do so, they need to be good managers of worms and use the right insecticides, and, of course, use the right variety. With the newer technologies with the Vip trait, I'm less concerned about resistance. However, we are concerned we'll develop resistance over time to that Vip trait as well. The Vip trait, just like the Cry1 and Cry2 traits, is being implemented in corn. We're discouraging farmers from using the Vip trait in corn, because it doesn't seem to have a lot of value in corn, but it selects for resistance to corn earworms (bollworms).

"I think it is time to start switching to these three-gene technologies, assuming they yield well. You don't want to pick a bad variety just because it has three-gene technology. Nonetheless, if you have a good three-gene technology, you might want to consider switching because it very well could save you at least one application of a diamide insecticide."