OMAHA (DTN) -- As Bob Dylan would put it, the bollworms, they are a-changing.
Also known as the corn earworm, the cotton bollworm has spent the last decade steadily evolving resistance to most of the corn and cotton Bt proteins on the market that target it.
Now, entomologists across the South are reporting increased feeding and survival in Bt corn and cotton fields expressing the Vip3A protein, once believed to be the final Bt stronghold against this pest. Texas scientists are testing some of these suspect bollworm populations for resistance.
In the past few years, researchers have documented resistance in the bollworm/earworm to some of the older Bt Cry proteins found in three-gene cotton varieties (Bollgard 3, TwinLink Plus and Widestrike 3) and pyramided corn hybrids (Viptera, Leptra and Trecepta). That has put tremendous selection pressure on the Vip3A protein in these varieties and hybrids, entomologists told DTN.
To add to that pressure, earworms feed on corn first before subsequent generations move into cotton fields as bollworms, intensifying the risk of resistance development, said David Kerns, an Extension entomologist and IPM coordinator at Texas A&M, where he has documented two consecutive years of unexpected feeding in Vip3A-expressing corn and cotton.
"We're selecting for resistance in corn, and when the next generation goes to cotton, we're again selecting for resistance there," he explained.
"It doesn't bode well for the longevity of three-gene cotton," noted Gus Lorenz, a University of Arkansas Extension entomologist. "There is just so much pressure on [Vip3A] technology, especially in corn, which is the preferred host for Helicoverpa zea."
Syngenta, the developer of Vip3A, said the company is not aware of any performance issues in VIP cotton and corn, but it is sensitive to pressure the trait is under in the southern states.
"As the trait developer of Vip3A, Syngenta is aware of the unique challenges related to H. zea in the southern United States," the company told DTN in an emailed statement. "To date, only the Vip3A trait offers excellent control of corn earworm (cotton bollworm in cotton) in both corn and cotton...As of late July 2019 there have not been reported performance issues related to corn earworm with any Syngenta products."
The academic entomologists agreed that three-gene cotton and Vip3A-expressing corn hybrids are still controlling bollworm/earworm in the field, but said that growers should be aware that cracks are starting to show in that technology, the entomologists said.
"There is some survival in VIP corn we did not see in the past," said Angus Catchot, Extension entomologist with Mississippi State University. "So it is something that we're watching."
THE FATE OF PAST BT TRAITS
The addition of Vip3A to cotton and corn was a welcome relief for growers in the South. There, the bollworm/earworm had been chipping away at older Bt traits for several years.
In 2016, university entomologists documented bollworm resistance to Cry1Ac, one of two Bt traits found in Bollgard II and dual-gene Widestrike cotton varieties. The next year, resistance was documented against Cry2Ab, the second protein in Bollgard II, which left those varieties vulnerable to increased bollworm damage.
Widestrike varieties were also compromised, since the second protein in them, Cry1F, only supplies suppression, not full control, of the bollworm. Likewise, dual-gene TwinLink varieties contain two proteins, Cry1Ab and Cry2Ae, which are so closely related to the proteins found in Bollgard II that Bt-resistant bollworms can also survive on them, Kerns noted.
Helping to drive these problems was the fact that many of these same Bt proteins are used in Bt corn varieties that are sometimes grown alongside Bt cotton.
Early generations of corn earworm moths lay their eggs in those Bt corn fields. The hatched caterpillars feed on the Bt corn and any survivors develop into a new generation of adult moths, which then move to Bt cotton fields and lay eggs there instead. Entomologists have long noted that the situation is a recipe for the development of resistance.
To combat this growing resistance to Bt traits, seed companies added Vip3A to cotton varieties, creating three-gene varieties in Bollgard 3, Widestrike 3 and TwinLink Plus. But they have also added the same protein to pyramided corn varieties, such Viptera, Leptra and Trecepta -- setting up the same potential dynamic that led to resistance against the Cry proteins.
"The fact that the Cry proteins that we've used for many years have become less effective has essentially left the Vip3A gene sitting out there by itself," explained Catchot. "Now that we're introducing both VIP cotton and corn, that leaves Vip3A alone to tote the load -- and I'm not sure it can do that without the help of those Cry proteins."
THE FUTURE OF VIP3A
Kerns was among the first of the southern entomologists to document unexpected earworm feeding in Vip3A corn and cotton varieties last year. This year, the feeding has continued, and Kerns and his team are working to test these Vip3A survivors for resistance.
"In one case, we made a collection of large larvae out of VIP cotton and we're going to test them," Kerns said. "We don't know if it is a resistance issue or an issue with expression of the Vip3A gene in the plants they're feeding on --- or a combination of those things."
In particular, the corn earworm seems to be surviving in Leptra corn, most likely because the other Cry toxins in those hybrids supply only suppression against earworm (Cry1F) or are compromised by resistant earworm populations (Cry1Ab).
Initial testing has shown that these populations of bollworm/earworm are capable of surviving a dose of Vip3A, but more work needs to be done before researchers can officially use the word "resistant," Kerns explained.
"We need to do full bioassay to put a number on how resistant these survivors are compared to a known susceptible insect," he said.
In the meantime, Syngenta's statement said the company is encouraging growers to help maintain the lifespan of the trait.
"In order to protect the durability of the Vip3 trait, Syngenta and other Bt technology developers have been advocating that growers implement Best Management Practices (BMPs), which include scouting fields for insect pests and appropriate insecticide applications when economic thresholds are met," the company statement said. "These BMPs, when combined with the proper refuge, can help slow down insect resistance to Bt traits that are currently available to growers."
Emily Unglesbee can be reached at: Emily.firstname.lastname@example.org
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