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Sprays may be needed with some Bt cotton in 2019

“Our data clearly shows that if you get pressure in Bollgard II you’re going to have to spray it.”

Winter meetings can be a mixed bag for farmers: Sometimes they hear about things they really would rather not know such as the development of resistance in bollworms or corn earworms to some of the older Bt cotton traits and pyrethroids.

That may have been the case when producers and industry members attended the Winter Production Meeting held by the University of Tennessee Extension offices in Fayette and Hardeman counties at Lone Oaks Farm near Middleton, Tenn., Jan. 22.

“It’s very clear we do have resistance to some of the Bt traits,” said Dr. Scott Stewart, Extension entomologist at the West Tennessee Research and Education Center in Jackson. “Specifically, we have resistance to Cry 1 and Cry 2 (genes in Bt cotton). It’s Cry 1F that’s in Widestrike and Cry 1A that’s in Bollgard and Bollgard II and Cry 2 in Bollgard II and Twin-Link.

“The long and short of that is that we’re having to spray for worms more,” said Stewart. “I think y’all know that we’re starting to see more worm damage, particularly in our Bollgard II. The short answer to that is don’t grow it anymore, switch to the newer technologies.”

The longer answer is growers will still need to plant varieties containing the older technologies for a variety of reasons, including the availability of seed and the reluctance to put the whole farm in a new, unfamiliar crop.

“Tyson talked about Bollgard III and Widestrike 3,” said Stewart, referring to Dr. Tyson Raper, Extension cotton specialist for the University of Tennessee, who also spoke at the meeting. “That is a solution, no doubt about it.

“You have to have varieties that are available. Right now there are going to be a lot of people growing Bollgard II next year. If you’re growing Bollgard II in 2019, you need to be prepared to manage bollworms.

“Our data clearly shows that if you get pressure in Bollgard II you’re going to have to spray it. I’ve looked at Bollgard III and Widestrike 3 for several years now, and at least in my locations, I’ve seen no benefit to treating those technologies. They provided excellent worm control.”

(In some other locations, researchers have had a different experience, he said. “There have been occasions where there has been a need to spray each of those technologies, but I feel fairly confident we would not need to spray Widestrike 3 and Bollgard III this coming year.”

Both of those technologies contain the VIP or vegetable insecticidal protein trait, and there is no documented resistance to the VIP trait that Stewart is aware of.

Because of the development of resistance to pyrethroids in the corn earworm or bollworm, growers have limited choices when it comes to spraying the older Bt cotton varieties. Most will have to rely on the diamide class of insecticides such as Prevathon and Beseige, which are relatively expensive.

“We’re fortunate in Tennessee that we’re in a one-spray scenario,” said Stewart. “Go farther south from here and they’re having to pull the trigger twice on bollworms in Bt cotton. You can imagine how crazy they are when they have to pay for that on top of the technology fee for Bt cotton.”

As farmers prepare for planting, Stewart said they should be aware of the need for a seed treatment on their corn seed. Studies across the Mid-South have shown that such treatments, generally at the lower of the two recommended rates, produce higher stand counts and yield increases most years.

“What’s interesting in this data set is there’s not one location where at that location there was a significant difference in yield,” he said. “But they all had the same pattern – when you combined them, there was as much as a 25-bushel yield response from not putting something on the seed for insect control. That’s the take home message.”

In general, he said, growers can get by with the lower rates. “Now there are scenarios where something unusual happens. Last year I was in Weakley County, and the farmer just had an unusually severe wireworm infestation. He had Cruiser 250, and it wasn’t good enough in that scenario. He lost a third of his stand and had to replant.”

Planting behind cover crops, late burndown, planting in fields with a lot of green matter are situations which call out for use of a seed treatment.

“If you already have ordered your seed, the only option left to you is to run something in-furrow,” he said. “Some farmers put something in with their pop-up fertilizer like bifenthrin. That might be a good option for you.”

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