Mid-South cotton growers are overspraying Bt cotton more and more every year, says Arkansas Extension Cotton Specialist Bill Robertson, and “some technology traits are holding up better than others — but we’re having to overspray all technologies more now than we have in the past.
“Some point their finger only at resistance as the cause of the increase. But regardless of the role of resistance in this issue, application techniques have the potential to impact the efficacy of chemical control measures. We need to adjust our application techniques to maximize the efficacy of our insecticide oversprays and minimize the number of applications we make.”
When he watches high clearance sprayers running across the field, Robertson says, he’s sometimes tempted to think that sprayers have only two speeds — wide open in first gear (about 13 mph), and wide open in second gear (about 18 mph).
“Many insecticide applications go out at 18 mph, and at that speed it’s difficult to get the products down into the canopy where they need to be,” he says. “I don’t know if we can put out enough water to make an 18 mph application work effectively in cotton that has lapped the middles.”
Robertson urges growers to do as good a job as possible with applications. On high clearance sprayers, that means fine-tuning speed, droplet size, and spray volume.
MOST EFFICIENT APPLICATION
“With the assistance of Jason Davis, our spray application technology specialist, we conducted a non-replicated demonstration plot using a half-rate of a harvest aid program to give us an idea of coverage and penetration into the canopy,” he says.
“We sprayed at 13 mph and 18 mph, using with 10 gallon and 15 gallon spray volumes. We also used three different spray droplets: medium, very coarse, and ultra-coarse.”
The most efficient combination in the demonstration, Robertson says, was 15 gallons of water, very coarse droplets, and 13 mph. “When we dropped the water volume to 10 gallons, droplet size really had an impact on efficacy. If you decrease water volume, you need to use a very coarse droplet. The 15-gallon rate performed better than the 10-gallon, regardless of spray droplet size.
“When we went at 18 mph, the difference in efficacy from droplet size was greater. So if I had to go 18 miles per hour, I would recommend using 15 gallons and very coarse droplets in this situation.”
Other factors, such as boom height, can also affect application efficiency, Robertson says. “I often see sprayers going across the field with the boom raised 5 feet or 6 feet above the canopy, but spray tips are designed to run about 24 inches above the canopy. And running the rig 18 mph with a high boom decreases the application’s effectiveness even more — the insecticide just isn’t penetrating the canopy, especially when using a medium spray droplet.”
INCREASED NEED FOR OVERSPRAYS
Whereas the Mid-South normally experiences sustained cotton bollworm pressure throughout the season, North Carolina usually has only one early bollworm infestation to contend with, and growers are urged to scout for bollworms and treat when thresholds are exceeded.
But this year, cotton bollworm pressure surprised many growers in some parts of the state. “Normally, no one makes a separate scouting trip for bollworms, so in several cases this year we were a little late treating,” says North Carolina State University Entomologist Dominic Reisig. “We made more than normal oversprays; several were well-timed, some were poorly timed.”
One factor that could have influenced the increase in oversprays, he says, is a decrease in the efficacy of the Bt trait in corn hybrids against bollworms. In one Bt corn hybrid, efficacy dropped from 80 to 90 percent efficacy to only 60 percent. Combine an efficacy decrease with a corn acreage increase, and a bollworm problem was teed up in 2016.
“We went from 800,000 acres of corn to 1 million acres, and planted less than 300,000 acres of cotton,” Reisig says. “Many more bollworms migrated from all that corn into cotton, and many were probably pre-adapted to survive in Bt cotton. In July, I recommended that cotton growers scout for bollworms, and use an overspray if necessary.”
Based on cotton bollworm experiences this past season, North Carolina will probably adjust its overspray recommendations, he says. “In the past, you could spray a pyrethroid if you hit larvae when they were 1/8 inch to 1/4 inch long — big enough to ingest the insecticide, but not big enough to penetrate bolls. When threshold levels are exceeded, growers might want to consider overspraying a newer material, like Prevathon, at the small egg stage. Prevathon is taken up into the plant and moves to new tissue, and it’s softer on beneficials than pyrethroids.”
Reisig cautions growers that moth and egg identification is critical if they are considering an egg spray. Spray money will be wasted if eggs are laid by tobacco budworms and not bollworms. Bt is still really effective for tobacco bollworms, he says.
NON-ISSUE IN WEST TEXAS
Several West Texas cotton growers found cotton bollworms in their fields this year, but the insect was never a problem for the region.
“It was so late in the season when bollworms popped that they just weren’t an issue,” says Texas Extension Cotton Specialist Seth Byrd. “Overall, bollworm populations were very light in West Texas, but cotton fields in the San Angelo area and farther south had a few worm issues.”
Fortunately, West Texas cotton growers don’t experience the intense cotton bollworm pressure of their counterparts in the Mid-South and Southeast. Even so, more than half the cotton planted in West Texas has a Bt trait.
“Our growers often plant Bt trait varieties, especially the newer ones, more for their high yield potential and herbicide package than for worm control,” Byrd says.