Since bollworm is a major pest in cotton for the Midsouth, adequate control of this pest is important to overall yields and profit. Research shows that it may be time to make the switch to three gene cotton to help control bollworms.
Gus Lorenz, Distinguished Professor and Extension Entomologist for the University of Arkansas System Division of Agriculture, discussed cotton insect management, during the Arkansas online Cotton Production Meeting.
What provided bollworm control?
Getting control of bollworms is important. If you must make two applications of a foliar insecticide for controlling worms, the expense can add up quickly.
"I think it is time to make the transition to three gene cotton if you haven't already," Lorenz said. "We've been looking at the worm applications we're having to make on dual gene cotton. We looked at a lot of three gene cotton this past season, and it held up extremely well in comparison to the dual gene.
"Part of the problem with dual gene cotton is there is too much overlap with the Cry1Ac gene, which is the common protein used in Bollgard, Bollgard 2, Bollgard 3, WideStrike, and WideStrike 3."
This same gene is also used in corn. There are a few differences in the Bt protein in cotton and corn, but there is not enough difference that, if they develop resistance for one, they're going to develop resistance for both.
"In the past with the dual gene WideStrike, we lost control quickly with the Cry1F gene, and now, all the three gene cotton has the same Vip Bt protein, and they're also putting it in corn," he said. "We're seeing this issue being driven by the corn because the Bt genes they've inserted into corn are similar to those in cotton, and we're beginning to see less and less control with the dual gene cotton over time."
From 2018 research with thresholds at 6%, the research team looked across all technology such as WideStrike, WideStrike 3, TwinLink, TwinLink Plus, Bollgard 2, and Bollgard 3.
Looking at trends from the sprayed and unsprayed in the 2018 research, the trend shows how the spray component helps despite which Bt gene is used.
"This tells me that even with the 3 genes we're going to be running out of time with this technology if we don't watch out, but we're seeing a significant yield difference between unsprayed and sprayed," Lorenz said. "The obvious trend is that foliar sprays are helping maintain the yields.
"We see across the data from 2019 and 2020 that all the three gene products are providing a good level of control. However, there are instances where it isn't holding as well, and again it makes me wonder how long this technology is going to hold for us. Also, in 2020, a lot of farmers throughout the state had to make two spray applications on their dual gene cotton."
Insecticides for control
The researchers looked at various insecticide treatments, such as Intrepid, Brigade, Acephate, Besiege, and Prevathon, in seven locations in four states that provide a level of control for bollworms.
"Diamides, like Besiege and Prevathon, are providing a level of control to get the population below the threshold level," Lorenz said. "After four trials, we looked at the percent control at different rates of products. From this data, it is apparent that the 20-ounce rate of Prevathon or a 10-ounce rate of Besiege compared to the lower rates provided a much better level of control, particularly past 14 days after application.
"People are always looking to save a buck, and the level of control with 14-ounces of Prevathon compared to the 20-ounce mixture is almost as good; however, the control drops off when you get out to about two weeks. That's when the control falls off with those reduced application levels, so going from 20-ounces to 14-ounces can make a big difference in the level of control you get between the two if bollworm pressure is high."
The team's 2021 bollworm thresholds regardless of the technology will be to treat when their populations exceed eight larvae per hundred or 6% injury of any kind.
"After bloom with dual gene cotton, we're going to recommend treatment when we get to 20% to 25% egg-lay or 6% fruit injury of any kind," he said. "Regardless of the size of the larvae, you need to spray. We didn't have as bad of an egg lay this past season, but if you're treating on eggs with a diamide, I would not make an additional application any sooner than 12 to 14 days. Making double applications seven, eight, 10 days apart is not enough time to see how the application is working. We need to be patient and let the technology and the insecticide work for us."
On the three gene cotton, their thresholds are the same, but they are not going to make applications based on egg lay.
"We want to see how the three gene cotton is continuing to hold up for us since it's held up well for us so far," Lorenz said. "On the dual gene cotton, we're going to do all the same things we've been doing, but we want to give the three gene Bt cotton a chance to work.
"If you're sticking with the dual gene cotton, then target to spray early and catch up on the eggs if you get to 20% to 25% egg lay. The question is, 'Is it time to make the transition to three gene cotton?' I think that's a good question. I'm no rocket scientist, but I can tell you from my perspective it's high time for us to make that transition."