Insecticide sprays may still be needed for three-gene cotton

Cotton producers never know what to expect from insects. One year, such as in 2018, populations of cotton bollworms, for example, can be higher than normal in some areas. In years like 2019 and even in 2020, they can be lower than normal.

In either case, growers must be prepared with a combination of Bt technology, such as Bollgard, Twin Link or Widestrike cotton varieties and effective insecticide sprays to keep such pests from causing significant yield reductions.

“The stars on this graph actually indicate the newer technologies that have that Viptera trait,” said Scott Stewart, professor of entomology at the University of Tennessee Institute of Agriculture. “These are the newest ones that are hitting the market. It shores up control considerably, and you don’t see any significant increase by spraying those technologies.”

The graph, which Stewart displayed during a video stop on the online Milan No-Till Field Day, showed how researchers have been able to increase yields by spraying the older, two-Bt gene cotton varieties, such as Bollgard II, Twin Link and Widestrike, with insecticides such as Prevathon.

The non-Bt cotton in the study generated the greatest increase with the insecticide spraying, while the older Bt cottons had increases that varied, but weren’t always statistically significant. The researchers saw almost no increase when they sprayed the new, three-gene cotton varieties.

See, Resistance to Bt complicating cotton insect control

“That’s not always going to be the case,” said Stewart. “I think all of these technologies under the right circumstance and enough insect pressure could require spraying, and there’s always going to be a need for spraying these technologies occasionally. But it will be more common with things like Bollgard II.”

The Widestrike technology is no longer being sold, and Twin Link is rapidly being replaced with the Twin Link Plus cotton. “Presumably, we’ll also see Bollgard II disappear from the market, and we’ll go to Bollgard III,” he said. “Those three-gene technologies are probably going to be more stable and less likely to need an insecticide treatment.”

Stewart also displayed a graph that illustrated the impact of low cotton bollworm populations on the three-gene technology and one Bollgard II variety. The slide was taken from a 2019 study of the Bt cottons and a non-Bt variety.

“I wanted to show this slide to make a point that pressure matters,” he said. “In the right circumstances with light pressure, sometimes these technologies even in the presence of resistance aren’t necessarily going to need spraying.

“You really have to know what your pressure is and how long it’s been persisting out there in the field, and that’s why scouting is very important.”