Thrips are the Number One insect problem on cotton in Virginia, causing the loss of 1,586 bales in 2001 over 103,000 infested acres, says Ames Herbert, Virginia Tech Extension entomologist.
“Thrips are, far and away, our primary insect pest problem,” said Herbert at the Southeast Cotton Conference in Raleigh. “As we move more into Bollgard cotton, these numbers probably will go down. Currently, the bollworm-budworm complex is our second most important pest, followed by stink bugs.”
Researchers in Virginia conduct numerous trials each year in an effort to find the best treatment for thrips control in cotton, he says. “We've conducted a lot of field trials in the past five years, looking at a number of treatments. This past year alone, we had nine trials with 66 treatments. These aren't all different insecticides. In some cases, we're looking at the same insecticide at different rates and different application timings,” says Herbert.
A summary of all treatments reveals a substantial increase in pounds of lint per acre over the untreated controls, he says. “We saw an increase of about 340 pounds of lint over the controls. In other words, had you not treated for thrips, you would have lost an average of 340 pounds of lint. That's about one-third of our total yield, and that makes it a serious problem.
“On average, you'll see a range of 50 pounds more of lint with a treatment that's not very good to 600 or 700 pounds more with the best treatment. It's not uncommon to see that range or impact. In looking at yields at the end of the season, we've determined that even moderate thrips control may not be good enough in Virginia, much less poor control,” says the entomologist.
Researchers in Virginia have looked at a broad range of thrips treatments, says Herbert.
“We've looked at in-furrow insecticides at planting time in granular or liquid formulations. We've also looked at seed treatments, either commercially applied or hopper-box treated. We've looked at liquid foliar sprays — alone or in addition to in-furrow treatments — banded, broadcast and at different timings, either late cotyledon or first true leaf or second to third true leaf.”
Researchers also have looked at a wide range of products over the years, he adds.
Cover crops have been used in the Virginia trials, says Herbert, because sandblasting was damaging the cotton seedlings. Plots also are planted in Roundup Ready varieties to minimize problems with volunteer peanuts in the peanut rotations, he says.
Tobacco thrips, he says, are the main thrips species that cause problems in Virginia cotton. “It's important to know this because all insecticides aren't equally effective against all species.”
Plant mapping is used in some cases, notes Herbert, to try and pin-point the causes of cotton yield reductions in untreated plots.
“We looked at the best protected versus the least protected plant to show the biggest difference between treated and non-treated plants. We treated with five pounds of Temik in-furrow followed by a foliar band of Orthene 75S. Even as earlier as July 1, we were seeing a difference.
“The untreated cotton is taller, but it has fewer squares. If we follow it through the seedling, we see the same trend. That's a big part of why we get yield reductions in Virginia — you get less fruit. And a lot of the fruit that is set on the untreated plant is further out and further up, and it doesn't mature or it's smaller on the plant.”
Herbert looked at all 220 thrips treatments over five years to try and determine the best approach for Virginia cotton producers. “We didn't necessarily look at the treatment by product, but rather by the approach. Insecticide treatments resulted in numerically higher lint yields compared with untreated controls 99 percent of the time.
“Those differences were ‘statistically different’ 78 percent of the time. In other words, it's not a matter of whether you should be treating for thrips in Virginia — it's a matter of which system you'll use.”
Foliar treatments improve the performance of in-furrow or seed treatments about 75 percent of the time, he says. This includes having Temik or a seed treatment in place and following with a foliar application, he adds.
“And the lint increase from this treatment averaged about 70 pounds. It's economical at that point, and the treatment more than offsets the relatively inexpensive foliar applications.”
Foliar treatments alone for controlling thrips appear to be “too little, too late,” says Herbert. “We can do this on peanuts and get away with it. But in our trials, foliar sprays alone rarely resulted in yields that were equal to in-furrow or seed treatments. Foliar treatments generally work best if applied at late cotyledon or first true leaf versus later in the season.
“It's a management quandary. Many producers want to put out Orthene tank-mixed with Roundup, and they want to apply Roundup as late as possible, allowing more weed seeds to germinate. But in terms of thrips control, it's not worth the wait. Those thrips are working on the cotton plant every day the treatment is delayed.”
The results of trials looking at the effects of banding versus broadcasting for thrips control have been varied, says Herbert.
“We haven't come up with one system that seems better than another. We've generally found that it's more dependent on the rate you're using and the coverage you're getting. You can get good control with a broadcast system. But you can't take a band rate, put it out as a broadcast and expect to get the same results.”
In terms of seed treatments, researchers in Virginia have looked mostly at Gaucho 480, he says.
“By itself, at eight ounces per hundredweight of seed, it rarely resulted in yields equal to five pounds of Temik. That's pretty much what researchers have seen in other states. But, we've seen good results when a foliar application is used along with the seed treatment. We generally see better control with a foliar band.”
Pyrethroids have shown some promise in controlling thrips, he continues. Some growers, he says, are using products such as Karate and Baythroid in lieu of Orthene.
“We have some concerns about the early season use of pyrethroids for thrips control, in terms of developing resistance. However, we have yet to see a lot of resistance development.”