Same Chapter, Different Verse
February 10, 2009 Weed-resistance management is more of a hot topic these days than insect-resistance management. Is that because insect resistance has been around for so long that it’s almost a given? Maybe. But both can be addressed with the same strategies, just different tools.
“The first case of insect resistance in cotton was in 1916,” says Dr. Ralph Bagwell, Louisiana Extension entomologist at the LSU Ag Center in Winnsboro. “We have dealt with resistance to organic insecticides longer than I have been alive. I have been on this earth for 43 years. The point is, resistance has always been around, and will always be one of the things we have to deal with.”
Pre-Bt, the traditional approach to insect management was reactive, but Bt has given us our first preventative tool. “We have used Bt for over 10 years, and we’ve yet to see resistance develop,” Bagwell says. “Resistance developed to pyrethroids in four or five years.”
When Bollgard varieties hit the market in 1996, EPA required a nearby refuge of non-Bt cotton to reduce the probability of resistance developing. With Bollgard II, EPA will allow alternate crops and even weeds to serve as a natural refuge, except in some areas of Texas, Arizona, New Mexico and California where pink bollworms are a problem.
What Bt has done is make the tobacco budworm a non-issue. Pyrethroids have done nearly the same with the cotton bollworm. But neither Bt nor pyrethroids have much affect outside the lepidopteran family. In parts of the Mississippi River Delta, what were secondary pests have become major ones – in 2007, plant bugs were devastating.
Resistance or Exploding Populations?
There are cases to be made for resistance, or just overwhelming populations of plant bugs:
• Plant bugs have demonstrated that they can develop resistance. In 1993, Dr. Gordon Snodgrass, an entomologist with USDA/ARS in Stoneville, MS, found that plant bugs had developed resistance to pyrethroids.
• Dr. Angus Catchot, an Extension entomologist at Mississippi State University, had sweep counts of 48 plant bugs per 100 sweeps after a third insecticide application. Normal thresholds in Mississippi are 8 adults per 100 sweeps. The point is that even if a spray controlled 100% of the plant bugs in a population, bigger populations followed, and followed quickly.
And another consideration: Natural refuges are the perfect hosts for plant bug populations. Partly to blame for the Delta’s problems in 2007 was that populations built up and flourished in corn. When the corn matured, they moved into cotton that was still green. That could be both an early and late-season problem this year in the Mid-South with the increase in wheat acreage.
No matter what the conditions, plant bugs and other early season insects have to be dealt with, and some of the techniques used for herbicide resistance management apply to insecticide resistance management as well.
Growers have increasingly gone to on-seed insecticide treatments for two reasons – they are convenient, and they do a good job of holding down early season populations of thrips, aphids and plant bugs.
The problems arise when the same chemistries are used in on-seed treatments and in foliar sprays. For instance, the active ingredient thiomethoxam is the active ingredient in Cruiser on-seed treatment, and in the foliar product Centric. Imidacloprid is the active ingredient in the on-seed treatment Gaucho Grande, and in the foliar product Trimax-Pro. To complicate it even further, both thiomethoxam and imidacloprid are in the neonicotinoid class.
Obviously alternating not only active ingredients, but classes, too, is a must. A good option for the first spray, if an on-seed insecticide treatment has been used, would be an organophosphate, like Bidrin or Orthene.
Captions:
Plant bug damage.
Photo credit: University of Georgia
Plant bugs feeding on an immature boll.
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