Field trials conducted in the past two years in Alabama, Georgia, North Carolina, South Carolina and Virginia are providing valuable insights into how Southeastern cotton producers can battle thrips infestations.
“We’ve been doing identical trials on a number of different fronts looking for ways to mitigate our thrips infestations,” says Michel Toews, University of Georgia entomologist.
“In the Southeast, we seem to have more than our fair share of thrips, and we seem to get everything from cosmetic damage to definite economic impacts.
“Delayed maturity is very common in our untreated plots, and in some cases, we see stand loss under severe conditions in the northern part of the Southeast.”
The loss of aldicarb or Temik is driving the need to identify alternatives that growers can use, says Toews.
“We’ve shown conclusively that our damage is much worse on conventional-tillage in early planting windows,” he says.
Research has shown that neonicotinoid seed treatments are inadequate in terms of preventing economic damage under very high-pressure situations, he says. “We’ve also shown that if you don’t have a neonicotinoid treatment under there, it can really look bad. So we’re recommending to our growers that such a treatment is absolutely essential in everything we do,” says Toews.
Research also has shown that repeated foliar applications can lead to secondary pest outbreaks, so they should be avoided if at all possible. Growers should limit foliar application for thrips to just one during the best time possible, he recommends.
“We’ve looked at the efficacy of different foliar insecticides in trying to identify different chemistries that’ll be effective. Also, we know that a fast-growing plant is less susceptible to thrips injury, so we wanted to increase the speed at which the plant would be growing.
Pop-up fertilizer helps
“That’s when we identified pop-up or starter fertilizer in combination with a single shot of Orthene as an effective treatment.”
Several response variables were measured, including adult immature counts, plant height, the number of true leaves, and dry plant biomass. Visual ratings were made at 14, 21, and 28 days after planting.
“Historically, immature thrips correlate best with the amount of damage we see. And we’ve seen other research that showed that dry plant biomass, at 42 days after planting, very closely correlates with the number of immature thrips on the plant as well as the amount of biomass on the roots.”
Trials were conducted across five states using the same seed, says Toews.
Insecticide trials were sprayed twice with high rates of Benevia (not registered yet for use on cotton), Dimethoate, Orthene, Karate, Lannate, Radiant and Vydate. Starter fertilizer trials were conducted in both irrigated and dryland conditions.
“This is base fungicide-treated seed only with no seed treatments and two shots of each of these compounds at their highest labeled rate.
“We’d like to see damage at a rating of less than three, and the only compound that scored less than three was Benevia.
“When rating the number of true leaves, two treatments really didn’t hold up as well, including Karate, which is the pyrethroid, as well as the untreated.
“Last year, we showed that the pyrethroid treatment scored worse than the untreated in a number of cases, and this past year’s data doesn’t look any better statistically. We seem to be knocking out the beneficial insects with pyrethroids, but we’re not killing thrips.
In terms of dry plant biomass, our top-performing products are Benevia, Orthene, Radiant and Vydate, keeping in mind that this is two shots of the product.”
Yield is not a very good response variable for assessing the effects of thrips on cotton, says Toews, because all of the damage from thrips is very early, and these trials were conducted under good fertility and irrigation.
“Cotton plants have an amazing ability to compensate during the season for whatever damage has occurred.
“Thrips is just one stressor on a plant, and all of these plots have been managed according to Extension recommendations.
Stresses are cumulative
“I would not tell a grower he’ll be able to compensate for thrips damage because we can’t predict things like rainfall, nematodes and plant fertility. All of these stresses are cumulative, and we want to minimize the ones we can. Thrips can definitely be mitigated.”
In terms of the rate trials, there is the base fungicide-treated treatment, Cruiser, and then Cruiser-treated seed in addition to a single foliar spray, he says.
“In the case of Radiant, the Dow label recommends that you apply it with an adjuvant, so we’ve got it with and without an adjuvant. These trials are getting a single foliar application at optimal timing under ideal conditions. We think first true-leaf is the ideal timing under most conditions.
All of these treatments, with the exception of just the base treatment, are underneath the three rating. This is very strong evidence that the best recommendation is to put on that neonicotinoid and supplement the control it provides us.”
The best-looking plants were treated by Benevia at 20.6 ounces. This is probably higher than the labeled rate would be, says Toews.
“With Radiant, our better-looking plants came from using the adjuvant. We nearly always see a benefit from using the adjuvant with that particular product.”
When rated for true leaves, the strongest treatment was Benevia at 20.6 ounces.
In the starter fertilizer trials, researchers looked at irrigation versus dryland. Ten gallons of 10-30-40 was applied in a 2-by-2, which is 2 inches below and 2 inches outside the furrow.
“Starter fertilizer under irrigated conditions, for the second consecutive year, has shown benefits for us. We have a bigger, more robust plant when we give them that pop-up fertilizer. The best timing, in terms of the foliar application, is at first true-leaf.”
These trials, says Toews, are showing options that can be used to improve thrips management in cotton.
“In this particular trial, for the second year in a row, starter fertilizer with a seed treatment and a foliar shot at first true leaf is my best treatment, and that has held up amazingly well.
In dryland, he adds, the benefits of starter fertilizer are lost. “If the conditions aren’t optimal, you can’t take advantage of everything that’s in the soil.
“We saw the same thing last year. It’s more of a low-yield environment without irrigation. We definitely see the impact of foliar treatments on dryland, and first true-leaf is our best bet. Sometimes in dryland situations, we see more benefit to the second leaf, but that’s because the plant is slower growing.”
Starter fertilizer also doesn’t give as much of a benefit on heavier soils as compared to sandier soils, says Toews.