Scott Stewart wants growers and consultants to remember that thrips control matters, especially with some potential issues with thrips management raising questions among researchers.
“I’ve looked at a lot of data in Tennessee, Arkansas and Mississippi,” said Stewart, University of Tennessee Extension entomologist, speaking to a group of consultants in Memphis. “Consistently, we are getting a minimum of 100 pounds yield increase by managing thrips, and, in some cases, you might be able to argue that it’s 200 pounds. It’s too big of a risk to not manage thrips.”
Over the past eight to 10 years, thrips control in cotton has shifted almost entirely to insecticide seed treatments. Long-time cotton growers may always lament the loss of Temik, but the seed treatments have brought their own positives to cotton production.
“We are heavily reliant on seed treatments, primarily imidacloprid and thiamethoxam at a rate of .375 mg of active ingredient,” said Stewart. “Historically, if you look at the data, Temik was better than the seed treatments at reducing the numbers of thrips and providing longer residual control. But it wasn’t reflected in yield. The seed treatments have done just as well at protecting yield, and historically, the difference between thiamethoxam and imidacloprid has been negligible.”
Although thrips management is still in good shape, Stewart points out that some potential control issues have surfaced over the past few seasons – issues that he and other entomologists are continuing to examine closely in their research work. The focus is on three potential reasons for diminished performance – potential plant damage and interference from preemergence herbicide applications, possible degradation of the active ingredients by soil microbes, and decreased susceptibility of thrips to the seed treatments.
“During this timeframe, growers started battling glyphosate-resistant pigweed and increased their use of preemergence herbicides,” said Stewart. “We started seeing more herbicide injury to young plants, which hurt the vigor. And if we also had thrips hurting the vigor, we were getting a double whammy in the field.
“There are definitely times when preemergence herbicides hold back the cotton. They can cause delay and stunting. If you get thrips on top of that, there’s definitely going to be some negative impact.”
The possibility that soil microbes have evolved to the point where they can metabolize the seed treatments and lessen their effectiveness is not unprecedented, pointed out Stewart. It’s happened before with Temik and with several herbicides.
To find out, soil samples were collected and examined. The results to date show that the microbes are having some impact on the active ingredients, but not enough to affect performance.
The real concern lies with diminished performance, and that’s where the majority of testing will focus this year.
“These seed treatments are neonicotinoid insecticides, and those chemistries are used on virtually every crop,” stated Stewart. “Almost 100 percent of our cotton and corn acres in the Mid-South and probably 50 to 75 percent of our soybeans are getting treatments. We are using these products a lot, and there is potential for selection pressure and resistance.
“It’s not likely that thrips have developed resistance, since there are so many hosts for them,” he added. “But whatever it is, it seems to be consistently impacting thiamethoxam more than imidacloprid.”
The primary problem seems to be with tobacco thrips, which make up roughly 81 percent of the thrips population in cotton. Testing was somewhat limited last year, says Stewart, but will expand this year throughout multiple Cotton Belt states.
“We’ll all be involved in trying to figure out how widespread and severe this problem may be,” he said.
So how should growers approach thrips control this year?
“It’s hard to fix a thrips problem after you have it,” stated Stewart. “You have to be proactive.”
He suggests using imidacloprid as the primary seed treatment and adding a foliar application at the 1-2 leaf stage. Imidacloprid still provides enough protection that one foliar application should be sufficient in most circumstances.
For growers who use thiamethoxam, Stewart suggests treating the cotton proactively with a foliar insecticide. Plan for at least two scheduled applications – the first 4-5 days after emergence, followed by the second application a week later. A third application may be needed if thrips populations are high.
“You’ll have to be very aggressive,” he said. “If you are making multiple applications, you might want to consider using Radiant for the second application. And there are a couple of reasons why.
“If you start with acephate, it’s inexpensive and works pretty well on tobacco thrips. But it leaves the Western flower thrips, and they’re harder to kill with acephate. If you come back with Radiant, you can take out those Western flower thrips and are less likely to flare a problem like spider mites or aphids.”