While many cotton growers are eagerly awaiting the full commercial release of ThryvOn Technology to help manage thrips and tarnished plant bug infestations, there’s still the matter of this crop season. What are the best strategies for managing these pests before the “easy button” arrives on the market?
Scott Stewart, professor of entomology and director of the West Tennessee AgResearch and Education Center provided an insect management update for 2021 during the University of Tennessee Institute of Agriculture’s virtual Cotton Focus. He discussed the major pests of the northern cotton belt and most effective control methods according to his research.
Tobacco thrips can be a substantial pest of cotton if they’re not managed. Fortunately, according to Stewart, when it comes to thrips management, growers need to be good but not perfect to preserve yields. In a seven-year research trial, managing for thrips with just a neonicotinoid insecticide seed treatment averaged a 107 lb/a lint increase over non-treated plots. That comes out to a net value of roughly $70 per acre.
“We know there’s a significant economic advantage on average to managing thrips in cotton,” Stewart said. “You don’t always realize this return — sometimes you see no benefit — but other times it’s substantially more.”
There are a number of seed and in-furrow treatments that can reduce thrips injury, although according to Stewart’s research, they vary in their efficacy. He recommends starting with an imidacloprid base seed treatment. For growers considering an add-on treatment, Stewart says Aeris has provided some additional protection above imidacloprid alone in recent trials. AgLogic, an aldicarb pesticide applied in-furrow, has also shown to provide good thrips control.
While acephate seed or in-furrow treatments are popular in some geographies, they have not risen to the top in recent research trials in western Tennessee.
“Acephate as a seed treatment or even by itself as an in-furrow treatment is not providing particularly good control,” Stewart said. “It provides some protection, but we have pretty good information that our tobacco thrips are developing resistance to some of these insecticides, particularly acephate.”
Given the concerns over thrips resistance, foliar-applied insecticides will very often be necessary, particularly in tough weather conditions or areas with high thrips pressure.
“Usually, we recommend one insecticide application very early,” Stewart said. “At about first leaf stage is the best timing.”
In 2020 trials, Intrepid Edge and Radiant, which both contain the active ingredient spinetoram, were the top foliar insecticide treatments for thrips in Stewart’s trials. Both come at a premium price compared to Orthene, but in recent years, these products have outpaced Orthene in thrips control, which Stewart attributes to developing acephate resistance.
“Presuming you have an imidacloprid seed treatment, I think we can continue to use Orthene,” Stewart said, “but I do suggest you use a higher rate of acephate. That does seem to overcome some of the issues to resistance we’re seeing. Bidrin is an option that can also be considered.”
Stewart is also leading studies looking at the impact of plant density and insecticide seed treatment in combination on thrips injury.
“In my experience, when we have thinner plant stands we see higher thrips injury. We’re not really sure why that’s happening, but our data is confirming that,” Stewart said.
Emerging research is showing increases in thrips injury as plant populations are decreasing. Isolated plants tend to have worse thrips injury, even when a seed treatment was applied. Stewart recommends not cutting seeding rates too low to avoid potential for thrips issues.
“If you have a poor stand to begin with and then have high thrips pressure, it’s going to make it necessary to make that foliar application and maybe two to control thrips,” Stewart said.
Curious as to what level of thrips pressure you can expect for 2021? As your planting date approaches, Stewart suggests visiting the online Thrips Infestation Predictor for Cotton. This tool, developed by faculty at North Carolina State University, provides predictions for thrips dispersal and injury risk based on planting date and weather conditions in your location.
The information can be used to optimize selection of thrips management measures and prioritize scouting.
In the second part of his presentation, Stewart transitioned to plant bugs, beginning with knowing thresholds for treatment. For tarnished plant bugs the first two weeks of squaring are critical, and growers should treat upon finding eight or more plant bugs per 100 sweeps. After the third week of squaring, the treatment threshold is relaxed to 15 plant bugs per 100 sweeps, but Stewart cautions that anytime square retention drops below 80 percent and plant bugs are present, treatment should be considered.
“In my tests and trials across the Mid-South, Diamond, Orthene and Transform are consistently the top three insecticides for managing tarnished plant bug numbers,” Stewart said.
Stewart said a season-long approach to managing plant bugs, as well as stink bugs created substantial yield increases in his trials — on average a 1,250-pound increase in seed cotton compared to doing nothing.
“You can’t forget to account for stink bugs or other pests. I’m a big fan of using mixes or rotating these top-shelf insecticide products. I like mixes of products like Orthene, Diamond, Transform, and Bidrin.
“We’re seeing some neonicotinoid resistance. I think neonicotinoids still have a place, but it’s in the early squaring window. They don’t have a place after flowering at all.”
Stewart also discussed non-insecticide approaches to plant bug management.
“When we have cotton near corn, there tend to be more plant bugs in the area, so you can try to avoid that affect by blocking cotton together,” Stewart said.
“Planting early as early as practical is always recommended. For Tennessee, I think it’s very important to use early or mid-maturing varieties. You’re very likely to have to make an additional insecticide application if you’re growing more full season varieties.”
Because it’s in the stewarded launch phase, cotton with ThryvOn Technology will only be available on a limited basis to select growers during the 2021 crop season. Stewart has evaluated the trait since 2014 and feels confident that upon it’s full-scale release, ThryvOn will be a valuable tool for growers.
“It’s excellent at reducing thrips injury. In our ThryvOn evaluations, we never rated thrips injury greater than 1.0 on a 0 – 5 scale. That’s essentially unheard of with an insecticide seed treatment or in-furrow treatment for thrips.”
This technology is delaying and reducing the buildup of tarnished plant bugs. Because of that, we’re not having to make as many insecticide applications,” Stewart said.
In University of Tennessee trials, ThryvOn Technology reduced the need for foliar insecticide applications for thrips and plant bugs by half. For ThryvOn, two applications for tarnished plant bugs and stink bugs were required, while spray thresholds for thrips were never triggered. Compare that to non-traited varieties, which required on average one foliar application for thrips and three applications for tarnished plant bugs for a total of four applications.
“We are going to have to continue to manage plant bugs even after ThryvOn,” Stewart said, “but we may realize insecticide savings.”
UT Cotton Focus
The Cotton Focus presentations are being released weekly on the UT Crops New Blog. As of press time, webinars on weed control, disease management and the Cotton Trust Protocol were also available.