Bt resistance in cotton worth watching

By Ron Smith

The No.1 variety consideration for cotton growers is to choose one that performs well under specific locations and field conditions. Yield and quality pay the bills.

Entomologists encourage producers to also be aware of some of the older traits (Cry1 and Cry2) as insect resistance begins to wane.

“Things have not gotten worse,” said Scott Stewart, University of Tennessee Institute of Agriculture. "And the Cry2 traits still have a place, but producers planting those varieties may need to spray for bollworms if pressure is high or sustained."

Most Tennessee producers have switched to the three-gene Vip trait, which he encourages.

“I don’t think Vip is immune to resistance. Use it wisely. Don’t abuse the technology. The most important thing we advise people to avoid resistance to Vip is not to plant that same technology in corn,” Stewart said. “We have pushed that for last three years. Corn is a big driver for resistance in cotton. Follow corn refuge guidelines, and steer away from Vip corn technology.”

Sebe Brown, Louisiana State University Extension entomologist, said Bt resistance and bollworm management recommendations remain unchanged for 2021. “We continue to recommend sprays on 20% egg lay in dual-gene cottons and 6% fruit injury in three-gene cottons. Diamide insecticides are the preferred products for controlling bollworm in cotton and we don’t recommend planting corn that contains the Vip gene.

“We need to minimize selection of bollworms on Vip proteins as much as possible,” Brown said. “The resistance gene for Vip already exists in Louisiana populations, and we need to protect this technology in cotton for as long as possible.”

Brown said more than 95% of all bollworms in Midsouth cotton funnel through corn first. Corn contains the same genes as cotton, he said, so worms are getting exposed for multiple generations.

Southwest

Bollworm resistance to early generations of Bt cotton, or the Cry proteins, is becoming “widespread throughout much of Texas and across the U.S. Cotton Belt,” said David Kerns, statewide IPM coordinator and associate entomology department head, Texas A&M AgriLife Extension.

Kerns, speaking at the Texas Plant Protection Association annual conference in December, said genetics play a role in resistance.

“Currently, we have essentially four Bt proteins available in cotton: Cry1As, Cry2As, Cry1F and Vip3Aa,” Kerns said.

He said a single gene may be driving much of the resistance issue. “Cry1A resistance appears to be incompletely to nearly completely dominant. Cry2 resistance involves multiple genes, is incompletely dominant to incompletely recessive, depending on the Cry2 dosage.”

He said Vip cotton technologies “remain quite effective,” but does point to some resistance concerns.

“We may have several types of Vip3A resistance,” he said. “The form we see in the field appears to be a low-level of resistance or may be natural variation in susceptibility. We have detected a form of Vip3A resistance, however, that renders the bollworm immune to Vip3A. This resistance is controlled by a single recessive gene.”

He said low levels of resistance to Vip have shown up in corn.

Bt varieties have a place in cotton but may require insecticides. “We saw a yield response when we sprayed Bollgard II. Prevathon and Besiege work very well and Intrepid Edge worked well last year. Pyrethroids have been highly effective in our area," he said.

Southeast

The situation appears less serious in Alabama, said Auburn University Extension entomologist Scott Graham.

“We see some resistance in corn plots in North Alabama with different Bt varieties. We can make collections out of two-gene corn easily. But we have to look hard to find a damaged boll in two-gene cotton. We have seen no resistance in Vip.”

He recommends producers plant varieties with the highest yield potential. “We will manage escaped worms if we run into them in cotton plots.  In six or seven research stations across the state, it’s been hard to find damaged bolls in any Bt varieties. We have a lot of BGII and some farmers are looking at VIP varieties on farms.

“For now, we are not recommending spraying on eggs, although some farmers are, in fields with more pressure. In 2017, some North Alabama fields needed to be sprayed. We have not heard of a field in the last two years that needed to be sprayed for bollworms.”

Graham said Alabama farmers planted 55% of their acreage in BGII in 2020, 10% in Widestrike3, 30% in BG3, and 5% in TwinLink plus. “So, we had 45% in VIP trait cotton.

Dominic Reisig, North Carolina Extension entomologist, said cotton producers planted about one-third of their acreage in Bollgard II or TwinLink in 2020. These varieties express related toxins in the Cry1A and Cry2A families.

“Fortunately, tobacco budworm remains susceptible to these toxins, but bollworm resistance is on the rise.  Levels of Cry1A resistance are higher than those for Cry2A for bollworm. That means Cry2A is doing a lot of the heavy lifting.”

Resistance levels vary across the Southeast but are typically lower than in the Midsouth. “However, they are widespread enough that we consider any bollworms resistant to these toxins and recommend protective insecticide sprays if enough eggs are present in Bollgard II or TwinLink.  Even with resistance, these varieties still have some efficacy and will slow the bollworms down in most cases.”

He said farmers planted the remaining acreage in Bollgard 3, TwinLink Plus or WideStrike 3.  “All these varieties express Vip3Aa19, as well as the Cry1A and Cry2A toxins.  So Vip3Aa19 is doing the heavy lifting here.”

He said bollworms remain susceptible to this toxin.

He recommends producers monitor fields weekly for square retention and bug numbers from squaring through two weeks after cutout. “We’ve had trouble with pyrethroid resistance lately and it’s important that growers rotate insecticides.”

Far West

Farther west, things are different, said Peter Ellsworth, University of Arizona IPM Coordinator & Director, Arizona Pest Management Center, Maricopa.

The bollworm/budworm complex "are considered minor cotton pests in Arizona. Damage is infrequent and rarely treated with insecticides in our system. The reasons are complex and relate to an ecology we don’t fully understand.

"Our landscape is dominated by Bt cotton. But we do plant a small acreage of non-Bt upland (1.7% in 2020) and a larger acreage of Pima (non-Bt, 4.8% in 2020) cotton in Arizona.

“Resistance to Cry1Ac and potentially the other proteins is present in Arizona populations,” Ellsworth said. “However, even with corn planted adjacent to cotton, we rarely see infestations, let alone survivors in the Bt cotton (2- and 3-gene versions).

“We do know historically that we have extremely high rates of 'natural' mortality of bollworm/budworms in low desert conditions in Arizona. This may be another reason why we rarely see any insecticides deployed for bollworm/budworm control.”

Ellsworth said Bt resistance is an important issue, but "I emphasize the importance of not overreacting to occasional observations of square and boll injury in cotton. When insecticides are needed, which is rarely, I encourage using fully selective options that will conserve the natural enemy complex. The best options are diamines.”