Manage Cotton for the Unexpected Bollworm

Don’t Let worms pressure your yield.

The fellow who advised folks to prepare for the unexpected might have been a cotton grower, and it’s highly likely he was talking about worms, particularly bollworm.

Cotton growers in many areas experienced higher than usual bollworm infestations in 2016. With Bt-traited cotton planted almost exclusively in most growing regions, growers tend to count on those traits to manage worm infestations. For the most part, they still can.

Two management practices are essential to worm management:

  • Choose a cotton variety that includes a two- or three-gene Bt trait. For instance, TwinLink® is a two-gene Bt trait and TwinLink Plus is a three-gene Bt trait. Additional proteins in the traits increase the level of control and extend the durability of the trait by improving resistance management.

  • Scout your cotton to ensure worm populations are staying below the economic threshold.

As Dr. David Kerns, Texas A&M Entomologist, points out, traits should not be counted on for 100 percent control. They certainly provide a high level of control, which is why insecticide uses in cotton production is down 42 percent since 1996, when the Bt trait came to market. To answer the question of whether to treat, growers or their consultants must scout cotton fields to ensure the level of control provided by the Bt trait stays within the economic threshold.

“Field monitoring is essential because bollworm or fall armyworm populations may develop, particularly on blooms, and also appear late in the season on stressed cotton where the Bt toxin production may be compromised,” Kerns says.

In addition to increased focus on blooms and bloom tags, the economic threshold that triggers a treatment has changed.

Scouting for bollworm in cotton focuses on blooms and developing bolls.

Scouting for bollworm in cotton focuses on blooms and developing bolls. Planting varieties with traits that have more than one Bt protein, such as TwinLink and TwinLink Plus, reduces the likelihood that in-season worm treatment will be required.

“Consultants have adjusted their thresholds with changes in technology,” says Gary Schwarzlose, Bayer Technical Service Representative. “They look at percent worms and percent damage and then look at a combination of the two before they pull the trigger.”

Texas Economic Threshold for Bollworm in Bt Cotton

  • Before bloom: 30 percent or more damaged squares with worms present
  • After boll formation: 6 percent or more damaged fruit with worms present

Also, in Bt cotton, the threshold is based on worms longer than one-quarter inch. Smaller worms are more likely to be controlled by the Bt toxin.

In general, Schwarzlose says, growers and consultants tend to scout once a week. At pinhead square or if neighboring growers are reporting issues, scouting frequency could increase to twice a week.

“The biggest thing is the line of communication is opened up way more than it used to be. If a consultant sees a problem starting, he’ll call his growers and, a lot of times, other consultants to let them know what’s going on,” Schwarzlose says. “Sharing information is helpful.”

Growers can call on their local Bayer Technical Service Representative, agronomist or sales representative to consult on worm control.

Relative Efficacy of Bt traits against Caterpillar Pests

Source: Texas A&M, 2017

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