COLLEGE STATION, Texas — Cotton farmers across the state could face a growing bollworm problem this season, said Dr. David Kerns, Texas A&M AgriLife Extension Service entomologist and statewide integrated pest management coordinator at College Station.
For 15 years Bt transgenic cotton varieties have done a good job preventing bollworm damage across much of the U.S. cotton belt, but things are changing and not for the good, Kerns said.
Over the past few years, Kerns said, there have been increasing numbers of instances where cotton bollworms were largely unaffected by genetically altered Bt cottons meant to stop them.
“It kind of comes and goes with the first instances noticed in 2010,” he said. “But these outbreaks have gotten progressively worse, with last season being the worst we’ve seen.”
To address the problem, Kerns now heads a research project with the Cotton Technology Stewardship Committee, a group comprised of seed companies with vested interests in cotton.
The project involves collecting worms from damaged cotton and other crops, establishing lab colonies and running laboratory assays on worms from the various colonies to determine how they respond to varying concentrations of Bt toxins in their diet.
In 2016, their work revealed larvae from many of the colonies survived after eating diets treated with concentrations of Bt toxins that previously killed them. Kerns said the responses of larvae from several colonies prove bollworm larvae from many areas in the southern states are now resistant to several Bt toxins. High levels of resistance were seen to Cry 1Ac, which was the first Bt toxin available in Bt cotton and corn to combat bollworms/corn earworms – common names for the same species.
Kerns and colleagues also detected bollworm resistance to the Cry2 toxins, the second toxins introduced into Bollgard II cotton in 2003, and TwinLink in 2014. Widestrike uses a combination of Cry1Ac and Cry1F technologies. Resistance to that toxin package was seen as well.
“Our 2017 data collected throughout the southern states indicated a very large shift with all bollworm populations exhibiting high resistance to the Cry1Ac technology and about 70-75 percent of these populations also showing resistance to the Cry2 toxins,” Kerns said.
“So, if you lose those two toxins, then really the only technology that’s even out there and available is the Vip3A toxin, which is pretty new, so it has not been grown extensively.
“That said, the occurrence of resistance doesn’t mean the worms are immune, so even with resistance, large bollworm infestations are usually required to cause a problem.”
Kerns said they haven’t found any resistance to Vip3A in cotton bollworms, but the toxin often doesn’t work as well in cotton as it does in corn. And stressed cotton may not express enough Vip3A to provide desired control. So under the right conditions, cotton can still suffer damage, even with the Vip technology.
“Essentially, we have bollworm resistance to all the Cry proteins – all of them,” Kerns said. “Those trying to ride this Vip technology exclusively to protect their crop from bollworms are setting their crop up for a very dangerous scenario. That’s just bad management, so producers should pick their cotton varieties based on something that’s going to yield on their farm.”
Kerns said using tried and true integrated pest management methods, starting with scouting fields for bollworms, is important in areas plagued by Bt resistant bollworms, particularly if planting cotton without the Vip technology.
“If you don’t have the Vip technology and you’re in a high bollworm pressure area, then you definitely need to be scouting these fields for bollworms and treating them with effective insecticides as needed.” Pesticide treatments can be tricky too, Kerns said, as there is much concern about pyrethroid resistance in bollworms, which is rampant in some areas.
“We don’t have a really good idea on how common it is in Texas yet, but we do get a lot of complaints,” he said. “I have treated grain sorghum infested with these worms and control was not very effective.” Kerns said in areas with pyrethroid resistance, Prevathon or Besiege, which contain a diamide insecticide, are the most common products of choice. They are effective, but expensive. They do have long residual activity, but applications must be applied when larvae are small, he said.
“If you allow bollworms to get very big and they move into the plant’s leaf canopy, you won’t get them even using this very good chemistry, because you won’t be able to get the product down there where they are feeding,” he said. “Timeliness is of the utmost importance. We have a lot of producers who say: ‘Well, I’m going to wait to see if the technology works,’ after seeing a lot of small worms. They wait too long and then they’re unable to control them.”
Kerns said it was not unusual to see as much as 40 percent crop loss from bollworms in affected areas last year. The problem is exacerbated because while corn earworms/cotton bollworms are the same pest, they feed on two very different crops.
“What we think is probably happening is we’re selecting for earworm resistance in corn, because these same technologies with the same mode of actions are in both crops,” he said. “The worms that survive the technologies are the resistant ones. The next generation moves into cotton, which is planted later than corn, so they are already genetically selected for resistance.”
Corn producers are supposed to plant refuge acres of non-Bt corn, another integrated pest management strategy, Kerns said. The idea is to produce susceptible moths, the sexually mature stage of the worms, to cross with the resistant moths to dilute the resistance so it doesn’t carry forward.
Unfortunately, those acres often aren’t planted for a variety of reasons, one being the Bt varieties usually produce more corn, he said.
“I know some people do what they are supposed to do, but there’s a lot who don’t, and that’s just adding to the problem of selecting for resistant moths in cotton,” Kerns said. “It affects corn too, and while ear feeding in corn isn’t as damaging as boll feeding in cotton, it can open entry points for fungal growth and the development of mycotoxins, which can affect animals and humans that consume the corn.”
Developing new bollworm-resistant technologies is very expensive and takes years to be tested and approved, Kerns said. In the meantime, he urges producers in bollworm-prone areas to be vigilant in their field scouting.
He also advised producers to become familiar with available pesticides and to use them wisely when necessary to effectively control bollworms while preserving as many beneficial insects as possible. And finally, he urged corn producers to plant their Bt refuge acres.
Kern’s colleague, Dr. Charles Allen, AgriLife Extension entomologist at San Angelo, said bollworm damage to Bt cotton in Texas has predominantly occurred in fields east of Interstate 35 through 2017.
“The danger is we know insects are resistant in some areas,” Allen said. “No colonies from west of I-35 have been established in Kern’s lab and tested, so we do not have a ‘read’ on them. In fields west of I35, we have only seen a few instances where the Bt toxins did not work properly.
“I believe Dr. Kern’s advice to scout Bt cotton and spray it if worms survive at above damage threshold levels is good advice no matter which side of I-35 the field is located,” he said. “At this point implementing integrated pest management measures to control resistant worms is about the only way we have to put some lipstick on this ugly hog.”
Texas A&M AgriLife Extension San Angelo
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