Insecticide Resistance

Cotton bollworms have been a major problem for Oklahoma producers. (Photo by David Kern)

By Everett Brazil, III

The cotton bollworm and corn earworm have been a problem for producers in years past, but modern transgenic cotton and corn varieties contain specialized traits designed to eliminate the pests from the field. Growers have recently seen a resistance in the insect; however, leading industry leaders and researchers to recommend scouting and spraying to contain the insect.

A trait, known as Bt, was discovered in a soil-borne bacteria, Bacillus thuringicnsis (Bt), which is hostile to the insect through a protein that damages the digestive system.

“The protein binds to the lining of the gut and essentially makes an ulcer, so gut contents will leach into the body of the insect, and it becomes septic,” said David Kern, Texas A&M University professor and Texas Integrated Pest Management (IPM) coordinator.

The transgenic trait was originally released under the Monsanto BollGard system in 1996, but is no longer available due to a compromise of the trait, as the insect is completely resistant to it. BollGard II is found within Monsanto Roundup Ready Flex cotton and corn varieties.

Other companies have also created similar traits, such as TwinLink, through Bayer Crop Science, and Widestrike, through Dow AgroSciences. Widestrike is featured in the Enlist cotton system.

BollGard II features two traits – Cry1AC and Cry2AB. TwinLink features Cry1AB and Cry2AE. Widestrike III includes Cry 1AC and Cry1F.

Researchers have noticed insect resistance the past three years, especially 2017, Kern said. TAMU researchers began a study to quantify incidences of the insect starting in 2015, and continuing into 2016 to 2017, where they saw larger populations.

Most of the research was performed in East Texas and South Texas. Very little data was gained from West Texas or the Panhandle. Central Texas and the Gulf Coast had some of the largest populations.

The insect seems to be a larger problem in cotton than corn, as the insect tends to have limited damage in the corn crop.

“There is not much of an issue in corn because even before there were transgenic traits in corn, they’ll only get it in the top of the ear,” said Tom Royer, Oklahoma State University IPM coordinator.

Those low-quality kernels will be blown out the back of the combines, saving the quality of the crop, Kerns said.
The insect is more detrimental to cotton, as it feeds on the bolls and squares, which create the bolls, leading to yield loss. Without the transgenic traits, cotton producers could see a larger loss due to the insect.

“It depends on the year and the infestation, but you can get any amount of injury. You can get a complete crop loss with enough pests,” Kerns said. “On average, it wouldn’t be uncommon to get a 25 percent reduction in yield.”
However, it seems that the rate of resistance comes out of corn, due to earlier maturity rates.

“The same toxins, modes-of-actions, are the same in corn and cotton. They’re getting exposed to these toxins in corn. We think, probably corn is driving the resistance,” Kerns said. “Typically, corn develops before cotton, at a state where it’s attractive to the worm, so the generation starts on the corn, and it’s selective for it, so the pupae move to cotton, and it’s already selected for it.”

Many current traits are already compromised, meaning the insect is resistant to it. That includes BollGard and Widestrike traits.

The good news is that a new trait is emerging on the market, VIP3A, and will be released in BollGard III, Widestrike III and TwinLink Plus.

However, most current varieties do not contain the trait, and most that do, are in limited varieties.
While the insect is mostly found as far as Central Texas, it is believed to be making inroads in Oklahoma cotton and corn fields. If producers start seeing populations in their fields, the best weapon is scouting, and insecticide applications if large enough populations are discovered.

“They’ll need to scout, particularly if they don’t have the VIP3A trait,” Kerns said. “Our recommendation, right now, if they are detecting 6 percent injury with worms present, that justifies the insecticide applications.”