Since 2006, when growers first began treating limited acreages of Mississippi soybeans for bollworms, the pest has increasingly become a problem, says Angus Catchot, associate Extension professor of entomology and plant pathology at Mississippi State University.
“We didn’t have a lot of bollworms in soybeans in the Mississippi Delta in the early 2000s,” he said at the annual meeting of the Mississippi Agricultural Consultants Association. “We sprayed almost no bollworms in soybeans until 2006, but now we’re on an incline where we’re spraying more and more — last year we treated around 750,000 acres an average of two times. The acreage is growing every year, and we’re seeing more pyrethroid-resistant bollworms.”
The last two years were somewhat unique, Catchot says, in that populations were sustained over most of the season.
“Normally, we have well-defined peaks and valleys in our moth trap catches, but that wasn’t the case in 2010 and 2011. Last year, I first started getting calls about bollworms in soybeans the third week of June. At end of September, we were still constantly getting calls about bollworms in late-planted beans. It never quit. We dealt with eight to 10 weeks of constantly high pressures, which is very unusual.”
There have been various suggestions as to reasons for the sustained flights, Catchot says, “but we don’t really know for sure what’s behind it
“Most of these worms — about 98 percent of them —will come through corn at some point. We estimate 40 percent of our acreage is planted to VT3P varieties containing two Bt genes for lepidopterous pests. Some of the worms coming through this corn are delayed seven to nine days, compared to non-Bt corn or regular YieldGard corn.
“So, even if you planted all your corn at the same time, with all these different technologies there is a span of a couple of weeks when worms will be coming out of the corn at different times because the Bt is slowing them down. When you start adding in variations in planting dates, you get these periods of long emergence.”
As bollworm numbers have increased, control problems have also multiplied, Catchot says.
“Since we started spraying soybeans for bollworms in about 2007, with just over 100,000 acres, I could have counted on one hand the number of control failures we had. Even in 2008, there were very few failures, and most of those we could explain.
“Now it seems seven of every 10 calls we get are for some kind of control failure where pyrethroid insecticides are used. That doesn’t mean you’re not getting any control — but that it’s not acceptable control.”
Bollworms harder to control
Treatment threshold is 9 bollworms per 25 sweeps, he notes. “If you’re getting 12 bollworms per 25 sweeps and you make a pyrethroid application, then get 6 bollworms per 25 sweeps, you may think, ‘Well the pyrethroid got me below the threshold.’ But, you only got 50 percent control. And if you’re catching 20, or 75, or 100 bollworms per 25 sweeps, 50 percent control isn’t very good.”
That bollworms are getting harder and harder to control with pyrethroids is an established fact, Catchot says.
“We came out with a recommendation to add acephate to a pyrethroid, and I still think that’s a solid recommendation. Acephate has pretty good activity on adult moths, even at the half-pound rate. I’ve seen a lot of dead moths behind the half-pound rate, although I don’t know how much it’s going to help on larvae — last year, we had control issues even with that mix.
“One of the things I want to emphasize is that we have good products that will work with this pest. We had good results last year with Belt, a new product, and Steward, which has been around for a long time and has been an anchor in our control programs.
“But it can be expensive to apply these pesticides — price of the product alone can range from $12 to $18 per acre.”
But, Catchot says, growers may be letting dollar signs stand in the way of an effective control program.
“We’ve become so accustomed to pyrethroids being so cheap, we tend to measure every new insecticide by what pyrethroids cost. If you can apply a full rate of a pyrethroid for $2.50, and a new product costs $8 to $10, you think, ‘Oh Lord, I’ll never use that.’
“This year, I’d be thinking about coming out of the gate with these products that we know will work and provide a good residual. We are getting better control, and extended control, with these products.”
Also, Catchot says, tobacco budworm numbers have been increasing in soybeans, “and pyrethroids aren’t going to work on that pest.”
Grain sorghum headworm complex
Although grain sorghum acreage in Mississippi was small last year, only about 50,00 acres, he says, “It seems we got calls on every field for the headworm complex, mostly bollworms.
“I do a lot of grain sorghum tests every year, as does Jeff Gore, assistant research professor at the Delta Research and Extension Center, and we’ve seen basically 50 percent control of bollworms. Every year, in our tests, we’ve been seeing control slipping further in grain sorghum.
“There are a couple of options: Lannate, which has been a standard, and Belt, which was only recently labeled for grain sorghum. A lot of Belt was used last year; it worked extremely well in soybeans and pretty well in grain sorghum, although it didn’t get us below threshold in a lot of situations. I don’t know why. For the most part, it did a pretty good job in grain sorghum, but it wasn’t the slam dunk that it was in soybeans.
“In years like 2011, with the kind of pressure we had with the headworm complex, particularly bollworms, I don’t know that we necessarily have a slam dunk option. If you have a tight head sorghum variety, you won’t get the control you get with a loose head variety, simply because of the coverage factor.
“One of things I tried last year in grain sorghum tests was to apply a residual such as Belt when making a midge shot with a pyrethroid, thus saving on application cost and aiming for residual control of bollworm eggs that hatched five to seven days after the midge application.
“It actually worked very well and definitely looked better than applying Belt on established threshold populations of large worms. We will look at this further. The downside of this system is that you’re assuming you’re going to have a worm problem, so you may be applying an expensive product to control a problem that may not occur.”
Pyrethroids continue to provide “decent control” of bollworms in Bollgard II cotton, Catchot says, “because we’re still targeting very small worms and hatching eggs of worms already weakened from the Bt, whereas in other crops such as soybeans and grain sorghum we’re often going after worms that already have attained some size, which compounds the control problem.
“Typically, in the Mid-South, pyrethroids are being co-applied with organophosphates for plant bug control, which also corresponds with hatching bollworm eggs. The tact is that if the pyrethroids ever stop working in cotton, we probably will have to start spraying the more expensive materials on a crop where we’re already paying a tech fee for lepidopterous pests.
“But data from Ryan Jackson, Gus Lorenz, Roger Leonard, Philip Roberts, and Scott Stewart suggest that growers may be leaving money on the table by not stepping up their worm control programs in Bollgard II and Widestrike cotton varieties. Yield increases, using two applications of Prevathon or Coragen — one about the third week of squaring, the second three weeks later — have ranged from 150 to 350 lbs.
“You might get some flak from your growers about over-treating, but these data show very clearly that there are situations where additional control measures are needed to maximize yield in these two gene varieties,” Catchot says. “Currently, our research efforts are going to focus more on identifying exactly when those situations exist.”