The key to insect control in cotton production is often staying one step ahead of resistance, according to Roger Leonard, an entomologist at the LSU AgCenter in Winnsboro, La.
Leonard, speaking at the 2010 Beltwide Cotton Conferences in New Orleans, La., noted, “About every 20 years, the cotton industry comes close to a near disaster in insect pest management in regard to the development of insecticide resistance.”
Leonard cited pyrethroids as the most recent example. “The value of the insecticide was high when it was introduced in 1977. But since 1990, we have seen a dramatic and consistent increase in the survivorship of the tobacco budworm. At that point in time, we surpassed the critical level when we began to see field control failures occur.”
Leonard noted that prior to Bt cotton, Louisiana cotton producers “were averaging about five insecticide applications per season targeted at the Heliothine complex (which include cotton bollworm and tobacco budworm). However, the high number of applications could have exceeded 10 to 12 in a given year.”
“Fortunately, after 20 years of discovery and development, in 1996, we had the first commercial Bt cotton plants in the field. The success of Bt cotton could not be questioned at that point in time. We were at a level where cotton producers were leaving the industry because of the lack of effective control tools for these insects.”
Bt technology, and cotton producers, subsequently enjoyed a period from the late 1990s to early 2000s of “very low inputs for control of bollworm and tobacco budworm. Over the last five years, we’ve averaged less than 1.5 percent yield loss to these pests. For many years, Heliothines have been the most destructive pest in cotton. In the last two years in Louisiana, the Heliothines have been surpassed by plant bug.
“I don’t think anyone can argue with the success of the Bt technology based on grower adoption, not only in Louisiana, but across the entire Cotton Belt,” Leonard said. “Louisiana has hit near saturation in planted acres of Bollgard, Bollgard II and WideStrike for the last three or four years, planting over 90 percent of the cotton acreage to these traits.”
Few technologies are bulletproof, however. It didn’t take very long before weaknesses of single trait Bt cotton to appear including its limited control of beet armyworm and fall armyworm.
Technology companies responded to this with additional traits and/or stacked insecticidal traits, noted Leonard. “In 2005, WideStrike traits were brought on line, and we’re going to see additional traits in 2010 and 2012. Coming on line are the VipCot and TwinLink technologies. All of these were put in place to broaden the spectrum of control and add value by reducing insecticide applications for more target pests.”
Leonard noted that a direct comparison of the number of sprays on Bt-traited cotton versus non-Bt cotton indicates a difference of only one application over the period from 1999 to 2008.
Larger differences in the number of sprays occurred in the earlier years of the study period, but Leonard stressed this doesn’t mean the value of Bt cotton has decreased in recent years.
“It’s just that we have saturated our acres with Bollgard, Bollgard II and WideStrike cotton varieties. So it’s harder to find those areas that are non-Bt, and average sprays are decreasing as we continue to see that saturation.”
Leonard says that in general, the registration of Bollgard “decreased insecticide applications for bollworm by about 50 percent. The stacked proteins are going to decrease that by another 50 percent. We will not see an elimination of applications in the future, but we will continue to see this decrease occur.”
Leonard says that despite their high level of control by Bt cotton, “Heliothines will always remain as pests. In areas where we have planted a high percentage of non-Bt cotton across the Mid-South, it won’t take very long for tobacco budworm to re-emerge as a primary pest and cause significant yield loss.”
Leonard noted that today, most of the foliar insecticide applications and costs in cotton are coming from control of non-target pest management. This is to be expected, but Leonard says the pest spectrum will improve as more pests are incorporated into trans gene technologies.
“It’s important to remember that although these technologies have high upfront costs, they do provide a predictable level of crop protection. It’s not reactive anymore, and it’s a wonderful risk management tool to reduce the possibility of severe yield losses from those target pests.
“But I do have some concerns. Competition within the industry is critical to manage upfront costs. More traits, or companies that are involved, will greatly benefit cotton producers in trying to determine which of these technologies best fit their needs.”
Leonard added that he has concern “about value-added components such as seed treatments being placed on traited cotton seed by the basic manufacturers. Are they really value-added to the producer, or to the industry?”
Leonard says germplasm development is crucial to producer profits as Bt technology tries to stay a step ahead of resistance. “This season is a transition year, where we will be moving from the single-traited cottons into the stacked-gene cottons. It will be the last year we will be allowed to plant varieties like DP 555 BG/RR. Presently, seed companies are searching diligently trying to find replacement varieties for that cultivar, and hopefully we’ll see some of these in the near future.”
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