Cotton bollworms building resistance to Bt traits

Ed Bynum, Texas AgriLife Extension entomologist, cautions cotton growers in regions with large corn acreage to be on the lookout for bollworms building resistance to the Bt traits found in genetically engineered corn and cottons.

Scientists have found that the cotton bollworm, which is also known as the corn earworm to corn growers, has developed resistance to the first generation Bt trait, Cry1A, that’s been added to corn and cotton. More recently bollworms are starting to develop resistance to the second-generation trait, Cry2Ab.

“We have the same traits in our corn and our cotton, which means that these plants are adding a lot of selection pressure for worms to develop resistance to the Bt traits,” Bynum explained at the recent Top of Texas Cotton Conference in late January in Pampa, Texas. In areas where both corn and cotton are predominant crops, the bollworm will feed predominantly on corn first before moving over to sorghum and then cotton host plants.

“But, as we increase our cotton acreage in areas where we have a lot of Bt corn already growing, we may see more and more of them moving into our cotton,” he said. “The bollworm really loves cotton when it’s flowering.”

Bt traits in both corn and cotton

The damage that the bollworm inflicts on cotton can be devastating to the yield and quality of the crop. The larvae chew into the base of bolls and hollow out locks. In older bolls, the larvae may just damage the boll’s surface, which can lead to infections from other organisms. The larvae also feed on the squares.

When the first cottons that were genetically engineered to produce their own Bt toxins they were introduced to the marketplace and farmers were enthusiastic. Bt, or Bacillus thuringiensis, is a bacterium found in nature that is toxic to insects when ingested. This bacteria produces a crystal-shaped protein (Cry toxin) that kills insects. They could reduce the number of foliar insecticide applications and still preserve their yield.

In the late 1990s, scientists figured out how to engineer plants to produce their own Bt toxins to build in protection from insects. The first generation Bt cottons used the Cry1A genetic trait, Bynum explained. This trait is probably more commonly known to growers under the commercial name “Bollgard.”

Then bollworms started developing resistance to that Cry1A genetic trait, so scientists introduced the second generation of bollworm targeting traits in their cotton—Bollgard II. It “stacked” the Cry1Ac and Cry2Ab Bt traits in the same plant. Other companies got on board the stacking system, and that’s when farmers got the WideStrike trait package, which stacked the Cry1Ac and Cry1F traits, and then the TwinLink cottons, which stacked the Cry1Ab and Cry2Ae traits.

“And then there’s the third generation of stacking,” Bynum said. We have Bollgard III, which added the Vip3A trait to the stack. And other companies added this Vip3A trait into its genetic offerings under the WideStrike 3 and TwinLink Plus labels.

While these traits were being introduced in “stacks” in cotton varieties, they were also being introduced in corn hybrids.

“Over some time now, we’ve had the Cry traits out there for longer periods of time, so the bollworm, which is the same as the corn earworm, has begun to develop resistance to the Cry traits,” Bynum explained. The Vip traits, he continued, work similarly but are different traits. And for now these Vip traits are still showing some measure of control of the bollworm.

That’s the good news. The Vip traits, even when stacked with Cry traits, are still holding off bollworms.

Nature always seems to find a way though, and that’s what scientists are finding now. Bynum said that because the cotton bollworm/corn earworm moths can move back and forth from cotton fields to corn fields, they are naturally selecting for resistance to these traits. That’s reducing the traits’ efficacy.

“So, in areas like here in the Texas Panhandle where we have corn and cotton production in the same area, farmers should be aware of the problems we are seeing with resistance in the boll worm populations,” Bynum said. He presented lab studies that show resistance to the older Cry traits is building in regions with both corn and cotton production.

While cotton bollworms/corn earworms aren’t supposed to overwinter this far north, Bynum said, the population of moths migrates during the growing season. And that’s what concerns entomologists. Those populations farther south in Texas that have been identified as resistant to the Cry Bt traits may migrate farther north and bring their appetites with them.

Scouting for worms

In corn it’s not economical to apply insecticides to prevent corn earworm infestations, Bynum explained. But, in cotton, if those moths begin to move out of corn and into cotton to lay their eggs, cotton growers may be able to catch them with an insecticide application.

There’s a catch, though, Bynum warned. If a grower chooses to apply a pyrethroid insecticide, which is a less expensive group of insecticides and still shows control of the bollworms, they could wind up with a bigger train wreck.

The cotton bollworms/corn earworms will feed first on corn, then move to sorghum if it’s available, and then cotton.

“The population levels you have at the time the crop is susceptible is what’s important,” Bynum said. “If you get an infestation they can damage the fruiting for the cotton bolls and squares.” He recommends treatment at the 6 percent level of fruit damage.

“The threshold of 6 percent fruit damage appears to work because that gives the Bt trait a chance to work,” Bynum said. “If you see worms larger than one-quarter to one-half inches in size, that means the Bt is not providing the protection as it should.” Farmers really need to target the smaller larvae with any insecticide treatments, because the larger they grow, the further down into the canopy they will move and become harder to kill with an insecticide application.

“You want to get ahead of them and don’t let them get too heavy,” Bynum said.

And remember, the cotton bollworm/corn earworm can survive over winter on many host plants. So applying an insecticide in the hopes of getting rid of next year’s bollworms could actually cause more selection pressure and build an even more resistant population.

“You could wind up with a bigger trainwreck,” Bynum said.

What’s on the horizon?

Companies are looking toward newer technology to replace the older Cry traits and build upon the success of Vip traits, Bynum said, but these are still eight to 10 years down the road. So farmers now need to just be vigilant in their cotton and corn fields to get ahead of any infestations as best as they can.

Some companies are working on RNA interference, or RNAi, technology. This technology helps scientists discover the function and roles of key cotton genes for traits like resistance to insects.

“Unfortunately, that won’t be the cure all for everything,” Bynum cautioned. “You would still need the Cry or Vip protein in the plant to give you a good level of protection.”

For now, if growers see populations of Bt-resistant bollworms/earworms, they need to report those fields to their seed company representatives and inform Extension entomologists so that they can make a field visit and perform a toxin check.

“Choose your varieties based on yield potential, and if the yield is equivalent, then let the technology weigh in on your decision,” Bynum said. “Manage the variety based on the technology you have.

“And if you’re growing a non-Bt cotton variety, then you’ll probably have to make an insecticide application,” he added.

Jennifer M. Latzke can be reached at 620-227-1807 or [email protected].