Mississippi’s mixed-up spring weather and scattershot planting season have resulted in a somewhat abnormal situation with cotton pests, says Angus Catchot, Mississippi State University Extension professor of entomology and plant pathology.
“It appears the number of applications for thrips will be down this year,” he said at the annual meeting of the Mississippi Boll Weevil Management Corporation and the Mississippi Farm Bureau Federation Cotton Policy Committee. “I think it’s primarily due to the extended rains during the time the bulk of our cotton would have been subject to thrips infestation. Rain does a pretty good job of beating thrips off cotton.”
There have, however, been reports of “a lot of thrips issues in late-planted cotton, particularly in Tennessee and Arkansas — some of the highest pressure they have seen in a while.
Catchot cited a Delta Farm Press article detailing problems with the pest’s resistance to thiamethoxam, which began in 2010.
“Up until 2014, we continued to get pretty good control with imidacloprid,” he says, “but our preliminary results appear to indicate that product is probably losing some of its efficacy, too.
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“This shouldn’t be a surprise, since they’re the same class of chemistry. We expected this would happen, and we have bioassays and field results to back it up. Right now, it’s regional — in the hills of Mississippi, and a few isolated fields in the Delta — and it’s not consistent across a large geography. We’re past the stage now for use of the product, but next year we expect to have a lot of information from our bioassays to determine what is going on with regards to what we were seeing in some fields.”
Migrating plant bugs
Plant bugs “actually started out fairly light this year,” Catchot says, “but in the last week of June/first week of July that changed tremendously. We’ve been seeing large numbers of adult plant bugs migrating into cotton fields, with the worst area in the northwest Delta. We’ve experienced this kind of situation in some areas every year.”
But shrinking cotton acreage changes the scale of things for insects, he says. “Anytime we have big acreage declines, as from 2006 to 2007, it’s unbelievable how bad plant bugs can be. Mississippi’s cotton acreage has dropped significantly, and this year we’re down 100,000 acres or more from 2014.
“Cotton is not the source for plant bugs — rather, it’s the recipient. If the landscape is producing X number of plant bugs, large numbers of them are going to wind up in cotton."
If there is a million acres of cotton, he says, that number of plant bugs is diluted over that large universe of acres. “But if we have only 310,000 acres, as this year, we’ve still got the same number of plant bugs moving into a lot fewer acres of cotton. We expected that to happen this year, and we’ve been seeing it.”
The problem with adult plant bugs, Catchot says, is that “it’s super-difficult to evaluate how well insecticide applications are working, and when growers see high numbers in a sweep net shortly after a spray they often think the insecticide has failed.
“Actually though, the products may be working pretty well. The only way you can really tell is to look at square retention. If you spray on Monday, and you look at the field Thursday or Friday, you may have the same number or more plant bugs as when you made the application. But that’s not necessarily an indication of product failure. You’ve got to look at square retention — if that’s going down, then you might be seeing product failure, or you might just be overwhelmed by plant bugs.”
A difference this year, Catchot says, is that the big adult plant bug migration has been a bit later than usual. “Generally, I stop monitoring square retention at first bloom, but we’re telling our consultants this year to continue monitoring on into bloom a while, until the adult migration is over.
“Don’t assume, looking at sweep net numbers alone, that your products are failing. We pretty much know which products are working, and we don’t have much indication that anything has changed. Just be sure and watch square retention.”
Despite all the rain earlier, he says, “We are spraying for spider mites in a lot of areas. Two years ago, we saw a lot of failures with abamectin. That’s unfortunate, because it’s the cheapest go-to product. We began using this product at the 4 oz. rate, when it was Zephyr and before it went generic. Recently, I had a report of a 14 oz. rate that wasn’t working too well.”
But, Catchot says, “I’m not saying don’t use these cheap abamectin products. I would probably start with an application of an abamectin. Resistance hasn’t become widespread through the populations right now. But if you have a problem, don’t follow with another abamectin product, even though it may cost more to go to another class of chemistry.
“There may be a $10 swing from the cost of the cheaper abamectin product to a product in another class. But if you start seeing problems, you need to switch to something like Portal, Oberon, Zeal, or other non-abamectin product. Don’t keep applying abamectin if it is not working, because there’s no question there is resistance to that active ingredient in Louisiana and Mississippi.”
Technology field day
Catchot noted that at a technology field day, scheduled this Thursday, July 16, at 9 a.m. (registration at 8:30) at the Rodney R. Foil Plant Science Research Center on the Mississippi State University North Farm, “Jeff Gore, Don Cook, and I will, for the first time, be talking about some of the products we’ve been testing for years — things you’ve not heard about before, such as lygus cotton, thrips cotton, Bt soybeans, and other new technologies. A lot of interesting information will be presented.”
The Rodney R. Foil Plant Science Research Center is located on Hwy. 182 east just north of the main campus.