The three most important things to remember for this season's insect wars are scouting, scouting and scouting, according to Extension specialists in the Missouri Bootheel and west Tennessee.
Most insect experts stress that it's impossible to predict pest populations from year to year. But they do know that boll weevil eradication and Bt cotton are reducing sprays for boll weevils (in the latter stages of eradication) and for the budworm/bollworm complex, respectively, thereby reducing adventitious control of secondary pests.
Two secondary insects emerged as damaging pests in west Tennessee in 2001 — stinkbug and plant bug.
“The stinkbug will feed on quarter-sized bolls or larger,” said Craig Massey, Extension area specialist with the University of Tennessee. “But where they're really becoming a factor for us is in the middle of July. Using targeted chemicals and technologies and getting away from the broad spectrum insecticides are allowing these pests to emerge.”
Plant bugs, mostly tarnished plant bugs with a few clouded plant bugs, are also damaging cotton, according to Massey. “A lot of farmers try to get by without putting a pinhead (insecticide application) on and letting boll weevil eradication sprays take care of them.
“In a few places, we can handle some plant bug damage early, but we don't need to let square retention slip below 85 percent. We also see plant bugs kick back up late in the year and cause some damage.”
“Secondary pests, particularly stinkbugs, caused more damage in the last two years than in any previous years,” said Gary Lentz, research entomologist at the West Tennessee Experiment Station, in Jackson, Tenn. “We don't know for sure whether it's due to reduced spraying or overwintering survival.
“Scouting for stinkbugs takes extra work,” Lentz added, “but if you walk out in a field, turn around, and see stinkbugs all around you, you probably don't need to sample very much. It got to that point last year.”
Growers should also look beyond their own fields for signs of a problem. “If we're seeing a lot of brown stinkbugs in Arkansas wheat (as has been reported this year), and this is the overwintering generation, we are pretty safe in saying there is going to be a significant stinkbug population in the Mid-South again, this year,” Lentz said.
Massey says an over-spray study he's conducting indicates changes might be necessary in scouting techniques and thresholds for secondary pests.
The initial study was to determine the benefits of over-sprays of a broad-spectrum insecticide (a pyrethroid) for cotton bollworm in Bt cotton. But worm populations decreased every year of the study, prompting Massey to shift focus toward the benefits of over-sprays on secondary pests.
The over-sprays were made in June, July and August, roughly around the middle of each month.
“Every time we made an over-spray, we increased yield,” Massey said. “The last two years, we had the biggest yield enhancement in July and August when stinkbugs were damaging bolls. We probably knocked back some plant bugs and a few worms or spider mites that were out there, too.”
The yield increases were 120 pounds to 200 pounds, versus a check where no over-sprays were made. The increases more than offset the cost of the chemical used, according to Massey.
“We need more research and study on our thresholds on transgenic cotton, especially for stinkbugs. And we need more of a multi-pest threshold. Sometimes we may go in with a material to take care of the spider mites, but it won't do anything for the plant bugs.”
Square retention is a big part of that threshold, according to Massey. “A cotton field with 90 percent retention and 4 percent plant bugs is not the same as 80 percent retention and 4 percent plant bugs.”
Scouts should cover a wider area, according to Massey, and more trapping should be done in adjacent areas where there are non-crop hosts.
The bottom line? The type nuances learned in trapping and scouting traditional pests like worms and weevils need to be learned for plant bugs and stinkbugs as well.
New thresholds in transgenic cotton should focus on multi-pests and a combination of insect scouting and plant mapping, according to Massey.
The pest spectrum in the Missouri Bootheel has yet to undergo radical changes in the pest spectrum because boll weevil eradication is still in the beginning stage and growers are only slowly increasing Bt cotton acreage.
But growers should be prepared for year-to-year changes in pest populations, according to Michael Boyd, state Extension entomology specialist at the University of Missouri's Delta Center in Portageville, Mo.
“The only way you know you have a problem is to scout for it,” Boyd said. Once the Bootheel “gets through the eradication program and indeed continues to plant more Bt cotton, I think we'll have more problems with the bug complex, including more localized problems with stinkbugs.”
In 2001, the plant bug displaced the boll weevil as the number one damaging pest in the Bootheel, Boyd noted. The weevil slipped to No. 5 in 2001, “primarily due to the winter of 2000-01. We had several episodes of really intense cold temperatures in conjunction with some wet weather. That really hammers most types of overwintering insects.”
Subsequently, plant bug numbers, too, “depend a lot on what kind of winter we have and how much growers have to spray for boll weevils and bollworms.”
Boyd and Bootheel growers have found that plant bugs “are a lot more persistent than what we think. They have so many alternate weed hosts. They can move in and out of cotton fields much more readily than boll weevils.”
Lentz says producers should be prepared to handle anything Mother Nature throws at them. Each year will be different. Few scenarios are a given, according to the entomologist.
With the success of boll weevil eradication, “we're not spraying cotton nearly as much in the early season,” Lentz noted. “Those early sprays without a doubt did contribute to some plant bug and stinkbug suppression.”
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