Lack of bollworm control in two gene Bt cotton is increasingly common. Entomologist and Extension experts suggest examining threshold levels as well as where you are in the season before making an expensive spray application.

During the virtual 2021 National Conservation Systems Cotton and Rice Conference, a roundtable discussion amongst experts revolved around learning to live with insect pests.

Worm thresholds

"It is widely acknowledged that we see a lot more worms now," said Angus Catchot, Extension Entomologist at Mississippi State University. "I do not think we have full-blown resistance or no control with the two gene cotton, such as Bollgard 2, but we do know at times we see less than desirable control in the two gene cotton."

As a result, the experts have modified their thresholds to address bollworm in two gene cotton. Jeff Gore, Associate Research and Extension Professor at Mississippi State University, addressed their recommendations for the 2021 season from a threshold standpoint to control worms in cotton.

"As Angus mentioned, we are dealing with resistance to the two gene products, the Cry proteins," Gore said. "From a management standpoint, we are spraying a lot more for worms, so we do have to adjust our thresholds a little bit. Historically, on any Bt cotton, we have always recommended not spraying on worm eggs, but we have had to adjust that the last couple of years.

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"Now, on any of the two gene products, our thresholds are 20% of plants with at least one egg on it. If you look at a hundred plants and 20 of those have at least one egg, that will trigger a necessary spray. If you find five eggs on four plants, you end up with 20 eggs, but that is not 20%. It is 20% of the plants infested with eggs."

On three gene cotton such as Bollgard 3, WideStrike 3, and TwinLink Plus, there is currently no recommended egg threshold.

"In the future, if things change, we will have an egg threshold for three gene cotton if we start seeing resistance to that Vip Bt protein," Gore said. "Right now, the three gene cotton is holding up well, so we are sticking with our damage threshold, which is 6% damage."

What to spray?

Products that work for worm control potentially vary from region to region, but in Mississippi, diamides work well when dealing with worms that are above the threshold, according to Catchot.

"We are mostly relying on Prevathon or Besiege," said Scott Stewart, Director of the West Tennessee Ag Research and Education Center for the University of Tennessee. "There are a few circumstances late in the season when you do not need the residual control, and you might be able to get by with Orthene plus Bifenthrin. From my data in Tennessee, the alternative works well for about five or six days, and then you start to see the difference.

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"When this first started happening, some people, who used Orthene plus Bifenthrin, had to go back and respray. One of the challenges is you do not know what the future holds for you when you make an application, so oftentimes, a second spray is necessary. However, that is not cost-effective. It is better to start with Prevathon or Besiege to get that extra residual control."

Stewart also said that, for Tennessee, worms are not as problematic as in other regions.

"Our worms typically move in late enough that we can get by with one spray," he said. "Further south is where the pressure is heavier and the season is longer, and two applications or more is more prevalent, which is hard on the pocketbook."

Late-season worm application

Catchot and Whitney Crow, Assistant Extension Professor for the Mississippi State University Delta Research and Extension Service, helped answer the question, Is there an economic benefit for making a late-season worm application?

"Frequently, in Mississippi, we are in a two-spray environment and even occasionally three, but oftentimes, at the second or third application, we are hitting close to cut out at that point," Catchot said.

"One of the questions we tried to address a lot within the last two years to help save a grower some money is to go back to some of the fundamental economics of cotton. What is that fruit bringing to the table at that point in the season? Is there an economic benefit from making a late worm application?"

Crow pulled together some recent data about the way cotton fruit distributes across a plant, which Darrin Dodds generated before he became the Head for the Department of Plant and Soil Sciences at MSU. From the data, she said that first and second position cotton bolls are the most critical as far as contributing to yield.

"Once we start getting later in the season, the top of the plant or the upper portions are contributing less to the overall amount of yield," Crow said. "Sometimes, we need to take a step back and look at where our damage is and how much is occurring.

"Using a couple of different factors, we estimated that roughly seven to nine bolls per 10-row feet, per acre, is what is equating to about 40 pounds of loss to cover the cost of a diamide application. Once we get later in the season, we need to be considerate of where this damage is occurring since third position bolls are going to be contributing less than other positions. Take into consideration the number of damaged bolls and where they are on the plant to know if there is enough damage happening to warrant the cost of a diamide application. You may be better off using something like acephate plus Bifenthrin, depending on the amount of residual you may need."

Catchot also added that he has recommended in the last couple of years to use acephate for very late season applications.

"Everyone I talked to who used just acephate at that point in the season seemed to be extremely pleased with the application," Catchot said. "The top part of the plant just does not contribute much to what goes into the basket. A lot of times, you are throwing good money away on an expensive application."