GENET archive


PLANTS: Rootworms survive Bt corn in Illinois (USA)

                                  PART 1

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SOURCE: The News-Gazette, USA

AUTHOR: Anne Cook


DATE:   12.07.2007

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URBANA - The stacked corn traits farmers pay big bucks for aren’t keeping rootworms from munching on their favorite food.

But, on the positive side, genetically modified corn stops another pest, corn borers, in its tracks, and the technology produces towering plants that produce excellent yields, said University of Illinois entomologists Mike Gray and Kevin Steffey.

Gray, Steffey and their student assistants this week started digging up corn plants in UI fields to look at their root system health, an annual ritual for these scientists who study pests that prey on the state’s largest crop.

Gray said they’ve discovered some surprising differences in their 25-acre test plots near South Race Street. He said project overseer Ron Estes called their attention to the surprising difference in height between one transgenic variety that contains proteins engineered to kill both corn borers and rootworms, and its close relative without the traits.

The second surprise is that rootworms did significant early damage to transgenic varieties.

Gray said they started digging up roots to evaluate damage in July as well as August for a very specific reason - a severe storm that swept through in July a few years ago and flattened a lot of corn fields, a sure sign rootworms have been at work in the ground.

”There was more damage than expected,” he said. ”That was an opportunity presented to us. We decided to do a second set of evaluations to compare roots treated with soil insecticide to roots of transgenic varieties with stacked traits for rootworm and corn borer protection.”

Technology incorporated into plants to make them lethal to insects relies on Bacillus thuringiensis, a bacterium that expresses a protein that breaks down the digestive system of insects when they ingest it. Gardeners use the same technology to kill tomato worms.

But Steffey said the technology introduced in 1996 that works so well for corn borers, killing about 99 percent of the beetles that eat it, doesn’t work as well on rootworms, technology introduced in 2003.

”You don’t get the expression in the roots that you get in the leaves,” he said, adding that many companies don’t emphasize that fact when they’re selling their stacked hybrids to farmers.

Gray said studies show Bt slows rootworms down and kills some, but ”you get survivors.”

He said these issues are becoming increasingly important because farmers like the growing habits, drought tolerance and especially the yields of the hybrids with stacked traits.

”In 2007, 40 percent of the corn planted in Illinois was the stacked Bt varieties,” Gray said. ”That compares to 19 percent in 2006. That’s doubled in a year.”

Farmers who plant varieties genetically modified to kill insects agree to plant 20 percent of those fields with a refuge - a non-genetically-modified corn. The goal is to dilute the gene pool of insects who might be developing resistance.

Gray hopes farmers aren’t tempted by favorable market and agronomic factors to ignore that agreement to cash in on current market conditions.

”We have some concern that producers might not be looking favorable at refuges,” he said. ”They’ll get reduced yields, and prices are favorable so there are a lot of economic incentives not to do it.”

Field studies done at four locations in the state show the yield difference between hybrids with stacked traits and close non-B relatives can be as high as 50 bushels per acre.

”With $3 corn, that’s $150 per acre,” Gray said. He said agronomists think about the damage and loss of valuable tools that kind of short-sighted thinking could cause. ”We’re looking at the future,” Gray said.

He said his student assistants will dig roots again early in August, and he and Steffey will present yield results and rootworm protection information at the College of ACES annual Agronomy Day, to be held Aug. 16.

                                  PART 2

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SOURCE: Public Library of Science, USA

AUTHOR: PLoS ONE 2007 (7), by Cristina A. Faria et al.


DATE:   11.07.2007

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Concerns about possible undesired environmental effects of transgenic crops have prompted numerous evaluations of such crops. So-called Bt crops receive particular attention because they carry bacteria-derived genes coding for insecticidal proteins that might negatively affect non-target arthropods. Here we show a remarkable positive effect of Bt maize on the performance of the corn leaf aphid Rhopalosiphum maidis, which in turn enhanced the performance of parasitic wasps that feed on aphid honeydew. Within five out of six pairs that were evaluated, transgenic maize lines were significantly more susceptible to aphids than their near-isogenic equivalents, with the remaining pair being equally susceptible. The aphids feed from the phloem sieve element content and analyses of this sap in selected maize lines revealed marginally, but significantly higher amino acid levels in Bt maize, which might partially explain the observed increased aphid performance. Larger colony densities of aphids on Bt plants resulted in an increased production of honeydew that can be used as food by beneficial insects. Indeed, Cotesia marginiventris, a parasitoid of lepidopteran pests, lived longer and parasitized more pest caterpillars in the presence of aphid-infested Bt maize than in the presence of aphid-infested isogenic maize. Hence, depending on aphid pest thresholds, the observed increased susceptibility of Bt maize to aphids may be either a welcome or an undesirable side effect.

                                  PART 3

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SOURCE: University of Illinois Extension, USA

AUTHOR: The Bulletin No. 15, byAaron Hager


DATE:   06.07.2007

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We’ve all heard (repeatedly) that change is inevitable. Some of us who are engaged in public service professions are further advised that (a) diversity is good, and (b) we should embrace change as an avenue toward new understanding. While these clichés are favorites of some administrators and daytime talk show hosts, they also have (some) relevancy to weed management in agronomic crops. For instance, if we rely heavily on a limited number of tools to control weeds in row crops, the weeds ultimately will adapt and change in some manner to better enhance the probability for long-term survival of the species. These adaptations and changes come about because of the genetic diversity inherent within most weed species. While the changes that result from intense selection are sometimes difficult to manage, they often lead us (in the long term) to improved understanding of weeds and how to better manage them.

Evolved resistance to herbicides is a phenomenon that highlights the genetic diversity of plant species we consider weeds. Repeated selection, brought about by relying on a limited number of herbicide sites-of-action, causes a ”shift” in the population of a particular species by effectively eliminating all susceptible members and leaving only those that can survive in the presence of the herbicide. Herbicide-resistant weeds have plagued Illinois farmers and weed-control practitioners for decades, and there does not appear to be an end to the problem anywhere on the horizon.

Across the United States, glyphosate-resistant weeds have increased both in acreage infested and number of weed species with resistant populations. Two species recently labeled with the moniker ”glyphosate-resistant” are waterhemp and giant ragweed, initially identified in Missouri and Ohio, respectively. While Illinois farmers have dealt with glyphosate-resistant populations of horseweed (a.k.a. marestail) for several years, these other glyphosate-resistant summer annual weeds have been confined outside the state’s borders. However, as the well-worn cliché reminds us, change is inevitable.

Weed scientists at the University of Illinois recently have worked with an Illinois waterhemp population that has demonstrated greatly reduced sensitivity to glyphosate. It is altogether likely that we will label this population ”glyphosate-resistant” pending completion of additional research. Greenhouse experiments have revealed that many plants survived treatment with 3 lb per acre of glyphosate, equivalent to about 85 fluid ounces of a 4.5 lb per gallon glyphosate formulation. Subsequent field research has confirmed greenhouse results, indicating many (but not all) waterhemp plants survived treatment with glyphosate at rates exceeding the maximum labeled in-crop application rate. Indeed, a few plants survived treatment with 12 lb ae glyphosate. Additionally, our field research has suggested this population is also resistant to ALS-inhibiting herbicides; results have been less clear with respect to the population’s response to foliar-applied triazine and PPO-inhibiting herbicides.

As indicated in a previous issue of this Bulletin, (Issue 12, ”Postemergence Herbicide Applications in Soybean: Pull the Trigger or Wait for Rain?”), we have been notified of several other instances from across Illinois in which waterhemp was not controlled by an initial application of glyphosate. It is altogether possible that other Illinois waterhemp populations are demonstrating a response to glyphosate similar to the population we have been working with during the past several months. If these populations are to be retreated, the following suggestions and observations are offered for your consideration.

(1) There are only four herbicide active ingredients that control waterhemp postemergence in soybean: glyphosate, acifluorfen, lactofen, and fomesafen. Our previous research has indicated the vast majority of Illinois waterhemp populations demonstrate resistance to ALS-inhibiting herbicides, thus diminishing the effectiveness of these products. We are unaware of any new herbicide active ingredients for postemergence use in soybean that are being developed by the herbicide manufacturing industry.

(2) If glyphosate will be part of the respray, you may want to increase the application rate to the maximum allowed for a single in-crop application. Why? If the waterhemp survived the initial glyphosate application for reasons other than resistance to glyphosate, a higher application rate will be needed to control the (now) larger plants.

(3) What about tank-mixing other products with glyphosate? The choices are limited, and the results are very difficult to predict. In our field research this season, we did include tank mixes of glyphosate with acifluorfen, lactofen, and fomesafen and were able to increase waterhemp control over glyphosate alone. However, these treatments were applied to waterhemp plants between 1 and 6 inches tall, and we anticipate reduced control when plants exceed 6 inches in height. Also, we know some Illinois waterhemp populations are resistant to PPO-inhibiting herbicides. Tank-mixing a low rate of 2,4-DB with glyphosate has generally proven ineffective against large waterhemp.

Unfortunately, there is no easy or simple remedy to this problem. We have a great deal of work to do before we better understand why this waterhemp population is not controlled with glyphosate. We will keep you updated as we learn more.



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